BIO 101 - General Biology 1 PDF

**CELL STRUCTURE AND ORGANIZATION** **Definition of a cell**: A cell is the simplest unit of a living organism. Cell is the basic unit of structure and function of all life. **Definition of cell biology** The study of structure, function, molecular organization, growth, reproduction and genetics of the cells, is called **cytology**(Gr. Kytos=hollow vessel or cell; logous=to discourse) or **cell biology**. Much of the cell biology is devoted to the study of structures and functions of specialized cells. **Cytology versus Cell biology** Cell biology has been studied by the following three avenues: **Classical cytology**with only light microscopically visible structure of the cell. **Cell physiology**: biochemistry, biophysics, and functions of the cell. **Cell biology**: interpreted the cell in terms of molecules (macromolecules e.g. nucleic acid and proteins) **History of Cell Biology** **Aristotle**(384-322 B.C.) and **Paracelcus** concluded that "all animals and plants, however, complicated are constituted of a few elements which are repeated in each of them." They were referring to the macroscopic structures of an organism e.g. roots, leaves and flowers common to different plants, or segments and organs that are repeated in the animal kingdom. Many centuries later, magnifying lens were invented which led to the world of microscopic dimensions. **Da Vinci**(1485) recommended the uses of lenses in viewing small objects. Swiss biologist, **Conrad Gesner** (1516-1565) published results of his studies on the structure of a group of protistscalled***foraminifera*** and which was so detailed; this could only have been possible with the use of a magnifying lenses. Cell biology continued to develop which led to the development of optical lenses and to the combination of these lenses in the construction of the compound microscopes (Gr.,micros= small; skopein=to see) **Cell Theory** In 1838, a German botanist **Mathias Jacob Schleiden** (1804 -- 1881) put forth the idea that cells were the units of structure in the plants. Along with a coworker, a German zoologist, **Theodor** **Schwann**(1810 -1882) applied Schleiden's thesis to the animals. **Exception to cell theory** Some certain organisms do not have true cells; all true cells share the following three basic characteristics: i. A set of genes which constitute the blueprints for regulating cellular activities and making new cells. ii. A limiting plasma membrane that permits controlled exchange of matter and energy with the external world. iii. Metabolic machinery for sustaining life activities such as growth, reproduction and repair of parts. t059292a **Animal Cell** An animal cell typically contains several types of membrane-bound organs, or organelles. The nucleus directs activities of the cell and carries genetic information from generation to generation. The mitochondria generate energy for the cell. Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free in the cytoplasm. The Golgi apparatus modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food. The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm. ![t059293a](media/image3.png) **Plant Cell** Plant cells contain a variety of membrane-bound structures called organelles. These include a nucleus that carries genetic material; mitochondria that generate energy; ribosomes that manufacture proteins; smooth endoplasmic reticulum that manufactures lipids used for making membranes and storing energy; and a thin lipid membrane that surrounds the cell. Plant cells also contain chloroplasts that capture energy from sunlight and a single fluid-filled vacuole that stores compounds and helps in plant growth. Plant cells are surrounded by a rigid cell wall that protects the cell and maintains its shape. **[DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS]** **FEATURES** **PROKARYOTIC** EUKARYOTIC 1\. Size Mostly between 1- 10µm in size Mostly between 10 -- 100µm 2\. Multicellular forms Rare Common, with extensive tissue formation 3\. Cell wall Cell wall present in most, not in all Only in plant and fungal cells cells 4\. Plasma membrane Plasma membrane is present Present 5\. Nucleus Absence of nucleus Presence of nucleus 6\. Nuclear membrane Absence of nuclear membrane Presence of nuclear membrane 7\. Chromatin with Chromatin with histone is absent Presence of chromatin matin with histone histone 8\. Genetic material Circular or linear, double stranded Linear double stranded DNA, DNA genes frequently interrupted by intron sequences especially higher Eukaryotes 9\. Nucleoli & mitotic Absence of nucleoli & mitotic appar- Presence apparatus -atus 10\. Plasmids Plasmids are common Rare 11\. Cellular organelles Mostly absence of cellular organellesPresence of cellular organelles, ribosomes except ribosomes that are present in 70s ribosomes in 80s 12\. Respiration Many strict anaerobes (oxygen fatal) All are aerobic, some facultative anaerobes. **[TYPES OF CELLULAR ORGANELLES]** 1. Pasma membrane and Cell wall 2. Endoplasmic Reticulum (ER) 3. Golgi apparatus/complex 4. Lysosomes 5. Mitochondria 6. Plastids (chloroplasts and vacuoles) 7. Nucleus 8. Chromosomes \(11) Cilia & Flagella \(12) Vacuoles \(13) Peroxisomes &Glycoxysomes **FUNCTIONS OF CELLULAR ORGANELLES** (1). **[PLASMA MEMBRANE (& CELL WALL)]**: Plasma membrane -- Cytoplasmic membrane/ cell membrane- coined by C. Nageli& C. Cramer (1855); Plasmalemma given by J.Q. Plowe (1931). Plasma membrane encloses every type of cell whether Prokaryotic or Eukaryotic cells. \(i) Separates the cytoplasm from the surrounding cellular environment. It is an ultrathin, elastic, living, dynamic and selective transport -- barrier A fluid-mosaic assembly of molecules of lipids (phospholipids and cholesterol), proteins and (CH~2~O)n. (ii). Controls the entry of nutrients and exit of waste products. (iii). Generates differences in ion concentration between the interior and exterior of the cell (permeability). (iv). Acts as a sensor of external signals (e.g. hormonal & immunological etc) and allows the cell to react/change in response to environmental signals. \- For bacteria and plants, p.m. is between the cell wall and cytoplasm. \- For cells having no cell walls (e.g. mycoplasma & animal cells), plasma membrane forms the cell surface. Examples of cell used in studying plasma membrane are mammalian red blood cell (erythrocytes), medullated nerve fibres, liver cells, striated muscle, *Amoebaproteus*, Sea urchin eggs and bacteria. **Types of plasma membrane according to permeability**: **Impermeable plasma membrane** e.g. unfertilized eggs of certain fishes allow nothing except gases to pass through it. **Semi permeable plasma membrane** -- allows only H~2~O but no solute to pass through it. **Dialysing plasma membrane** -- having extraneous coats around them which serves as a dialyzer. i.e. H~2~O molecules & crystalloids are forced through them by the hydrostatic pressure forces e.g. basement membrane of endothelial cells. **Mode of transport through plasma membrane:** **Passive transport**: a type of diffusion in which an ion/molecule crossing a membrane moves down its electrochemical or concentration gradient. No metabolic energy is consumed on passive transport and is of 3 types namely: 1. **Osmosis** (2). **Simple diffusion** (3). **Facilitated diffusion**, a special type of passive transport in which ions and molecules cross the membrane rapidly because specific permeases in the membrane facilitate their crossing. e. g. ionic transport through charged pores. **Structure**: sheets of unit membrane with ribosomes on the outside; forms a tubular network throughout the cell. **Function**: transports chemicals between cells and within cells. provides a large surface area for the organization of chemical reactions and synthesis. The cytoplasmic matrix is traversed by a complex network of interconnecting membrane bound vacuoles/cavities which are often concentrated in the endoplasmic portion therefore called [ER] b/c it resembles a ["net"] in the cytoplasm under light microscope \- Occurrence of ER varies from cell to cell. \- The erythrocytes (RBC), egg and embryonic cells have no ER. \- In reticulocytes (immature RBC) which produce only proteins retained in the cytoplasmic matrix (cytosol) e.g. haemoglobin), the ER is poorly developed or non-existent but may contain many ribosomes. \- The ER acts as a secretory, storage, circulatory & nervous system to the cell. (3). **[GOLGI APPARATUS/COMPLEX]** Tagged/referred to as the **"traffic police"** of the cell (Darnell et al., 1986) **Structure:** These are stacks of flattened sacs of unit membrane (cisternae) Vesicles pinch off the edges. **Occurrence**: Found in plant and animal cells' cytoplasm; - occurs in all cells except the prokaryotic cells (e. g. mycoplasmas, bacteria &b.g.a), eukaryotic cells of certain fungi, sperm cells of Bryophytes and Pteridophytes, cells of mature sieve tubes of plants, mature sperm & red blood cells of animals. **Function**: Modifies chemicals to make them functional. Secretes chemicals in tiny vesicles. Stores chemicals. May produce endoplasmic reticulum. Golgi apparatus is responsible for the performance of certain important cellular functions e.g. biosysnthesis of polysaccharides, packaging (compartmentalizing) of cellular synthetic products (proteins), production of exocytotic (secretory) vesicles and differentiation of cellular membranes. Golgi apparatus is a centre of reception, finishing, packaging and dispatch for a variety of materials in animals'& plants' cells. (4). **[LYSOSOMES]**: (Gr. Lyso = digestive + soma = body) **Structure**: membrane bound bag containing hydrolytic enzymes. Hydrolytic enzyme is water split biological catalyst i.e. using water to split chemical bonds. Lysosome -- lytic body having digestive enzymes capable of lysisi.e dissolution of a cell/tissue (De Roberts & De Robertos Jr. 1987). \- These are tiny membrane-bound vesicles involved in intracellular digestion. \- Contain a lot of (variety of) hydrolytic enzymes that remain active under acidic conditionslysosomal lumen (PH =5) by an ATP --driven proton pump in the membrane. **Functions**: - break large molecules into small molecules by inserting a molecule of \- Primarily meant for the digestion of a variety of biological materials. \- Secondarily causing aging and death of animal cells, human diseases e.g. cancer, gout, silicosis. (5). **[MITOCHONDRIA]** (Gr, mito = thread, chondrion = granule) **Structure:** composed of modified double unit membrane (protein, lipid) \- Inner membrane infolded to form cristae. \- They are filamentous or granular cytoplasmic organelles of all aerobic cells of higher animals and plants, certain micro organisms e.g. Algae, Protozoa and Fungi. Absent in bacterial cells. \- Contain a specific DNA for cytoplasmic inheritance and ribosomes for protein synthesis. \- Have uniform distribution in the cytoplasm (move autonomously in the cytoplasm) \- Distribution and number of mitochondria (& also of mitochondria cristae) is correlated with the type of function the cell performs. \- Mitochondria having many cristae are associated with mechanical & osmotic work situations, where there are **sustained demands of ATP** and where space is at a premium e.g. between muscle fibres, basal infolding of kidney tubule cells, and in a portion of inner segment of rod and cone cells of retina. \- Usually, mitochondria occur in greater concentrations at work sites. \- In animal cells, 95% ATP is produced by the mitochondria; the remaining 5% is produced during **anaerobic respiration** outside the mitochondria. \- ATP can be produced by chloroplasts in plant cells. **Function:** - Site of cellular respiration i.e. the release of chemical energy from food. - Actual respiratory organs of the cells, helps in oxidizing carbohydrates (CH~2~O)n and fats (food stuffs) into CO~2~ andH~2~O and large amount of energy is released which the mitochondria utilize for the synthesis of energy rich compound, Adenosine Triphosphate (ATP); because of production of ATP, mitochondria are referred to as the "**Power houses**" of the cell. **Glucose+ Oxygen Carbon Dioxide + Water + Energy (ATP)** (6). **PLASTIDS** Two membrane-bounded compartments -- vacuoles & plastids readily distinguish plant cells from animal cells; are related to the immobile life -- style of plant cells. **Structure of chloroplast:** composed of a double layer of modified membrane (protein, chlorophyll, lipid); inner membrane invaginates to form layers called "grana" (sing., granum) where chlorophyll is concentrated. - Present in all living plant cells & in Euglena (a protozoan) - Small bodies found free in the cytoplasm, more/less spherical or disc shaped (1µm - 1mm in diameter) \- May be elongated or lobed or show amoeboid characteristics. \- Double-bounding membranes, possession of pastoglobuli (spherical lipid droplet) is another identifying factor. \- A self-replicating organelles whose protein-synthesizing capacity is comparable to that of mitochondria. **Functions of chloroplasts**: site of photosynthesis (light & dark reactions) (7).**NUCLEUS** (L., nux = nut) -- the "heart" of the cell. **Structure**: The nucleus consists of the nuclear material envelope, nucleolus, chromatin and nucleoplasm. **Functions**: \- Almost all the cells' DNA is confined, replicated & transcribed. \- Controls diff metabolic as well as hereditary activities of the cell, synonymous to Greek word "karyon". - Serves as main distinguishing feature of eukaryotic cells. (8). **CHROMOSOMES** These are the nuclear components of special organization, individuality and function. **Functions**: They are capable of self reproduction and play a vital role in heredity, mutation, variation & evolutionary development of the species. (9). **RIBOSOMES** \- Small, dense, rounded & granular particles of the ribonucleo protein. \- Occur freely in the matrix of mitochondria, chloroplast and cytoplasm (matrix) or remains attached with the membranes of the ER and nucleus. \- Occur in most prokaryotic and eukaryotic cells. **Functions**:Known to provide a scaffold for the ordered interaction of all the molecules involved in protein synthesis i.e. in the biosynthesis of proteins. (10). **CENTRIOLES AND BASAL BODIES** Cytoplasm of some eukaryotic cells contains 2 cylindrical, rod-shaped, microtubular structures called centrioles, near the nucleus. They lack limiting membrane and DNA or RNA and form a spindle of microtubules, the mitotic apparatus during mitosis/meiosis and sometimes get arranged just beneath the plasma membrane to form and bear flagella and cilia in flagellated/ciliated cells (Futton 1971). **Stucture**: Nine triplets of microtubules form one centriole; two centrioles form one centrosome. **Function**: forms spindle fibres to separate chromosomes during cell division. When the centriole bears a flagellum/cilium is referred to as a basal body. synonymous tokinetosome, blepharoplast, basal granule/corpuscle & proximal centriole. (11). **CILIA & FLAGELLA** **Structure**: The cilia (L. cili= eyelash) and flagella (L. Little whip) are microscopic, contractile &filamentous processes of the cytoplasm which create: \(1) Food currents in lower aquaticanimals. \(2) Act as sensory organs. \(3) Perform many mechanical functions of the cell. \(4) The ciliary or flagella movement provides the locomotion to the cell/organism. The two (2) are identical morphologically & physiologically but can be distinguished by their number, size and functions. Flagella are less (1 or 2) in no while cilia are numerous (3000 -- 14000 or more) in number. Flagella occur at one end of the cell, while the cilia may occur throughout the surface of the cell. Flagella are longer (up to 150µm) processes while the cilia are short (5 -- 10µm) appendages of the cytoplasm. Flagella usually beat independently, while the cilia tend to beat in a coordinated rythym. Flagella exhibit undulatory motion, while the cilia move in a sweeping/pendular stroke. (12). **Vacuole**: This is a single layer of unit membrane enclosing fluid in a sack. **Functions:** - produces turgor pressure against cell wall for support. - stores water and various chemicals. - may store insoluble wastes. (13). **[MICROBODIES]**: **Structure**: There is the presence of membrane bound, spherical bodies of 0.2 -- 1.5µm diameter in close association of ER and mitochondria or chloroplast or both in cells of protozoa, fungi, plants, liver and kidney of vertebrates. - The organelles have a **central granular/crystalloid core** containing some enzymes & called **microbodies**. - These organelles are surrounded only by a single membrane which have no DNA (genome) or ribosomes. - They use molecular O~2~ like mitochondria but have flavin-linked oxides and catalases for the H~2~O~2~ metabolism and enzyme for fatty acid metabolism instead of cytochromes and capacity for ATP synthesis as in mitochondria. - Peroxisomes and Glycoxysomes both differ in cell compartment and in the type of tissue in which they are found. - Peroxisomes are found in animal cells and leaves of higher plants. - Glycoxysomes present in plant cells only and abundant in germinating seeds that store fat as a reserve food material, cells of yeast, Neurospora (fungus) and oil rich seeds of many higher plants. **FUNCTIONS OF PEROXISOMES** 1\. H~2~O~2~metabolism 2. Glycolate cycle 3. β-oxidatio[n] **FUNCTIONS OF GLYCOXYSOMES** 1\. Fatty acid metabolism 2. β-oxidatio[n] 3. Glycoxylate cycle **CHARACTERISTICS AND CLASSIFICATION OF LIVING THINGS** **CHARACTERISTICS**: MR.NIGER D M - Movement R- Reproduction N - Nutrition I- Irritability G- Growth E- Excretion R- Respiration D - Death **CLASSIFICATION OF LIVING THINGS** The classification of living organisms has been controversial throughout time, and these schemes are among those in use today. Aristotle's system distinguished only between plants and animals on the basis of movement, feeding mechanism, and growth patterns. This system groups prokaryotes, algae and fungi with plants, and moving, feeding protozoa with the animals. The increasing sophistication of laboratory methods and equipment, however, revealed the differences between prokaryotic and eukaryotic cells promoting a classification system that reflects them. Most recently, five kingdoms have emerged to take both cellular organization and mode of nutrition into account. Greek philosopher Aristotle (384-322BC) grouped life forms as either plant or animal. Microscopic organisms were unknown. Plants Animals Plants Animals Fungi In 1735, Swedish naturalist Carolus Linnaeus formalized the use of 2 Latin names to identify each organism; a system called binomial nomenclature. He grouped closely related organisms and introduced the modern classification groups: Phylum Class Order Family Genus Species Single-celled organisms were observed but not classified **Kingdom**: Plantae Animalia **Organisms**: Plants Animals Fungi In 1866, German biologist Ernst Haeckel proposed a third kingdom, Protista, to include all single-celled organisms. Some taxonomists also placed simple, multicellular organisms, such as seaweeds, in kingdom Protista. Bacteria, which lack nuclei, were placed in a separate group within Protista called Monera Kingdom: Protista Plantae Animalia Organisms: All single-celled Plants Animals Organisms such as Amoebas and diatoms, Sometimes simple Multicellular organisms Such as seaweeds In 1938, American biologist Herbert Copeland proposed a fourth kingdom, Monera, to include only Bacteria. This was the first classification proposal to separate organisms without nuclei, called prokaryotes, from organisms with nuclei, called eukaryotes, at the kingdom level. PROKARYOTES EUKARYOTES Monera **Kingdom**: (Prokaryote) Protista Plantae Animalia **Organisms**:Bacteria Amoebas, diatoms, and other single- celled Eukaryotes, and sometimes Simple, multicellular In 1957, American biologist Robert H. Whittaker proposed a fifth kingdom; Kingdom, Fungi, based on Fungi's unique structure and method of obtaining food. Fungi do not ingest food as animals do, nor do they make their own food, as plants do; rather, they secrete digestive enzymes around their food and then absorb it into their cells. **Kingdom**: MoneraProtistaFungiPlantae Animalia (Prokaryote) Amoeba, diatoms Multicellular MulticellularMulticellular and other single celled filamentous organisms organisms eukaryotes and sometimes organisms that obtain that ingest **Organisms**: Bacteria simple,multicellular that absorb food through food. organisms such as food. Photosynthesis sea weeds In 1990, American molecular biologist Carl Woese proposed a new category, called a Domain, to reflect evidence from nucleic acid studies that more precisely reveal evolutionary, or family relationships. He suggested three domains, Archaea, Bacteria, and Eucarya, based largely on the type of ribonucleic acid(RNA) in cells. **PROKARYOTESEUKARYOTES** **Domain**: Archaea Bacteria Eukarya **Kingdom**: CrenarchaeotaEuryachaeootaProtista Fungi Plant Animal **Organisms**: Ancient bacteria Ancient bacteria That produce that grow in high Methane temperatures **BIOLOGICAL INTERACTIONS** Organisms do not exist alone in nature but they do in the midst of sea of other organisms of many species. The presence or absence of other organisms may have adverse effects or beneficiary effects on one another. Hence, there are various interspecific interactions which can be classified either as Positive interactions or Negative interactions. \(A) **POSITIVE INTERACTIONS** \(1) **Mutualism(Symbiosis)**:The type of symbiosis resulting in mutual benefit to the interdependent organisms, or symbionts, is commonly known as mutualism.It is obligatory positive interspecific interaction that is strongly beneficial to both species. **Pollination by animals** - Bees, moths, birds and flowering plants. **Dispersal of fruits and seeds** - Birds and mammals and fruiting plants. **Mutual defence in ants and acacias** - The ants attack herbivores (insects) and sting and bite any foreign vegetation that touches the acacia. **SymbioticNitrogenfixation** - Rhizobium and leguminous plants. **Microorganismsand cellulosedigestion** - *Trichonympha*and termites. **Lichens** - Fungi and algae. t012712a Lichen Symbiosis Lichens typify one of the three types of symbiotic relationships: mutualism. In this type of association, both the organisms that make up the lichen, an alga and a fungus, depend on each other and cannot live independently. Through photosynthesis, the alga produces the food the lichen requires, while the fungus absorbs vital nutrients and water for it. G.I. Bernard/Oxford Scientific Films **Microsoft ® Encarta ® 2008.** © 1993-2007 Microsoft Corporation. All rights reserved. \(2) **Commensalism**:This defines the coaction in which two or more species are associated and one species at least, derives benefit from the association, while the other associates are neither benefited nor harmed. This is a food-sharing association between two different kinds of non-parasitic animals that is harmless to both and in many cases is mutually advantageous. Many commensals are free to separate. Other commensals grow together so completely that they cannot separate; they are not considered parasitic, however, because they do not harm each other. Examples of this association are the following. **(a) Lianas**- The woody stem of lianas is closely attached to the supporting tree but it is not involved with it in any nutritional relationship. Lianas are classified as Leaners, thorn lianas, twiners or tendrils on the basis of the type of device used for climbing their supports. **(b) Epiphytes**- These are the plants that grow on other plants and use other plants only as support and not for water or food supply. They differ from the lianas in that they are not rooted into the soil. **(c)Epizoans**- Some plants grow on the surfaces of animals. Examples include: Some green algae grow on the long, grooved hair of sloth. Basicladia grows on the backs of fresh water turtle. **(d)Epizoites**- Here animals are associated with another animal for the purpose of anchorage and protection. Examples include the following: Branchiobdellid annelids on crayfish. The barnacles that attach themselves on whales. The remora fishes on the bellies of sharks, seaturtle, barracuda etc. It must however be noted that commensal association can be internal as found in the [Escherichiacoli] in human colon and the hermit crab inside the gastropod shell. \(3) **Protocooperation**:This is a type of mutualism but it is not obligatory to the parties that are involved. The water moccasins and the large birds like herons and ibises. Sea anemone on the empty molluscan shell harbouring hermit crab. **NEGATIVE INTERACTIONS** The negative interactions are the following: This is when one species harm the other by using it directly or indirectly for support, shelter or food. This is also called **PARASITISM or EXPLOITATION**. There are two types of parasitism-Social parasitism and parasitism. Social parasitism is the exploitation of one species by another for various advantages. E.g. egg parasitism in old world cuckoos and brood parasitism in Indian Koel and crow. One species of ant snatches food from forage workers of another or rob their nest of their food. Parasitism is used for the harmful coactions between two species; one, the host and the other the parasite. It is mainly a food coactions but the parasite derives shelter and protection from the host also. A parasite is usually smaller than the host. **Classification of Parasites**: Parasites are classifiedthus-viral parasites, microbial parasites, phytoparasites, zooparasites, endoparasites and ectoparasites, permanent and temporary parasites. **Parasitic Adaptations**: Reduction of organs of special sense of nervous system and locomotory organs. Development of clinging organs such as hooks, suckers to attach to the body of the hosts. Most endoparasites exhibit anaerobic respiration, high rate of reproduction, parthenogenesis (growth of an organism from an unfertilized gamete/sex cell; in case of plants (lower), biological reproduction of fruit without fertilization is called parthenocarpy), hermaphrodism, polyembryony (multiple embryo production; i.e. production of more than one embryo from a single egg), intermediate and a complicated life cycle. Many parasites pass their entire existence in a single host; others need one, two, three intermediate hosts and all the hosts occur in the same community. Parasites are transferred from one host to another by active locomotion of the parasite itself; by ingestion, as one animal sucks the blood of or eats another; by ingestion as an animal takes in eggs, spores, or encysted stages of the parasite along with its food or drinking water; as a result of bodily contact between hosts; or transportation from host by way of vectors. **EFFECTS OF PARASITE ON THE HOST**: Parasites may not cause immediate mortality but they cause damage to body structures, should it become excessive, may cause death. There are a number of causes of diseases due to parasitic association viz, the parasite, physiological stress, nutritional deficiency and poisoning. Common parasitic agents of disease and the resulting mortality of animals are the following: Viruses are the potent agents of several disastrous diseases of plants and animals including man. Bacteria may produce localized inflammatory changes in tissue, enter the blood stream, or produce powerful poison known as TOXINS. Fungus spores of *Aspergillus*may be drawn into the lungs of ground-feedings, where they germinate and grow, causing aspergillosis (disease of the skin, nasal sinuses, lungs and other internal organs caused by inhalation of spores of the molds of genus *Aspergillus*). Protozoan parasites are especially important in the alimentary tract and in the blood. Worm parasites e.g. tapeworms, nematodes may wander through the host's body doing mechanical injury as well as destroying and consuming tissues. External parasites e.g. ticks, fleas, licedo not commonly produce serious mortality by themselves, but they are often vectors, transmitting protozoa, bacteria and viruses from one animal to another. Nutritional deficiency in vitamins or minerals, or improper balance among carbohydrates, proteins, fats may produce malformations, lack of vigour, or even death. Food poisoning occurs when certain food become contaminated with the toxins released by the bacterium *Clostridiumbotulinum*. (2)**Predation**: This occurs when members of one species eat those of another species. Four types of predation are identified viz, i. **Herbivores** ii. **Carnivores** iii. **Insect parasitism** **Cannibalism** **Characteristics of Predation** The hunting ability of a predator is well developed. Predators are specialized or generalized by their hunting activities. Age, size, and strength of prey influence the direction that predation takes. Predators hunt only when it is necessary for them to procure food. Habitat preferences or overlapping territories can bring predators and prey into close contact, increasing prey risks. Predators may have strong preference for a particular prey; it can turn in the time of relative scarcity to an alternative. **Defensive Mechanisms of Prey** \(i) **AposematicColouration**: Distastefulness because of toxins. Many of the preys have bright colours with which they advertise their noxiousness. \(ii) **Groupliving**: Early detection of predators - Deterrence to predators attack. - Distraction of the predators by confusing them. iii. **Camouflaging**: The animal takes the colour of its environment by blending with it e.g. Chameleon (3)**Amensalism** It is a comprehensive term in biotic interactions that means the adverse effect of one of the competing population or species while the other remains stable. Here, one population definitely inhibits the other while remaining unaffected itself but by modifying the environment, the organism improves its chance of survival. It is chemical interaction. The inhibitor substances may be inorganic chemicals, organic toxins or antibiotic. The suppression of the growth through the release of chemicals by a higher plant is known as **ALLELOPATHY** e.g. *Salvialeucophylla* emits some volatile oils which reach the surface of the soil and inhibit the germination of seeds of other species. **ANTIBIOSIS** refers to the complete or partial inhibition or death of one organism by another through the production of some substance or environmental conditions as a result of metabolic pathway. It is more commonly referred to secretions by microorganisms that check the growth of others. It is common among microbial world. e.g. *Chlorellavulgaris* produces substance which inhibits the growth of diatoms. In marine waters, populations of some microbes popularly known as **REDTIDE**, cause catastrophic destruction of fish and other animals. \(4) **Competition**: Occurs when individuals attempt to obtain a resource that is inadequate to support all the individuals seeking it, or even if the resource is adequate individuals harm one another in trying to obtain it. There are two types viz: **Intraspecific competition**-It is an important density-dependent factor regulating populations. This is also called Scrabble or Resource competition. **Interspecific competition**- It is also called Contest competition. Here, \- One species only survives, it being the one with the greater negative effect on its competitor. Growth of the surviving population to its carrying capacity is slower than if the second population had been absent. \- Both species coexist indefinitely. This occurs when interspecific competition is less intense than intraspecific one in both species. Neither population reaches the carrying capacity it would have in the absence of other species. \- The species beginning at higher density persists, and the other is eliminated. This is a special case when the populations have equally negative effects on the growth of each other, but interspecific competition is stronger than intraspecific one. **Gause's Principle** states that "Given a region of physical space in which two species do persist indefinitely, there exists one or more properties of the environment or species, or both that ensures ecological distinction between the two species. That is, complete competitors cannot coexist. **ECOSYSTEMS** (1)**DEFINITION**: Is any unit that includes all the organisms (the communities in a given area) which interacts among themselves and with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycles within the systems. It is the system resulting from integration of all the living and non-living factors of the environment. It is a relatively self-contained, dynamic system composed of a natural community along with its physical environment. The term ecosystem may also be used to describe geographical areas which contain a wide range of habitat types which are linked by ecological processes. (2)**KINDS OF ECOSYSTEMS**: Various constituents of ecosystems of biosphere (earth-giant ecosystem) fall into two categories: Natural and Artificial. **(A)NATURAL ECOSYSTEMS:** These operate by themselves under natural conditions without any major interference by man. These are divided on the basis of their habitats as follows: \(i) **Terrestrial Ecosystems**-Forest, Grasslands, Deserts, Savannah, a single log e.t.c. (ii)**Aquatic Ecosystems:** **(a)Freshwater Ecosystems-**These can be *LOTIC* (running --water as spring, streams or rivers) or *LENTIC* (standing-water as lake, ponds, pools, puddles, ditches, swamp etc.). **(b)Marine Ecosystems -**These include deep bodies of water such as oceans, sea or shallow ones such as estuary. **(B) Artificial Ecosystems:** Man-made or man- enginered ecosystem. These are maintained artificially by man whereby addition of energy and planned manipulation, natural balance is disturbed regularly. Examples include cropland like maize, wheat, rice-field, villages, cities, dams, aquarium etc. **(3) STRUCTURE OF AN ECOSYSTEM** By structure,we mean(i) the composition of the biological community including species, numbers, biomass, life history, population and distribution in space.(ii)the quantity and distribution of non-living materials such as nutrients, water etc. (iii) the range or gradient of conditions of existence such as temperature, light etc. **(A) Abiotic or Non-living Components:** \(i) Moisture(ii) Inorganic substances - Phosphorus, Sulphur, Carbon, Nitrogen, Hydrogen which are involved in material cycle in the ecosystem(biogeochemical cycles) (iii) Organic substances such as protein, carbohydrates, lipids etc. present either in biomass or in the environment. The amount of inorganic substances present at any given time in an ecosystem is designated as the standing state or standing quality. **(B) Biotic or Living components:** This is indeed the trophic structure of an ecosystem, where living organisms are distinguished on the basis of their nutritional relationships. **(i) Autotrophic Component:** These are the producers or energy transducers which convert solar energy into chemical energy in form of carbohydrates, lipids, proteins. **(a) Photoautotrophs-**Contain green photosynthetic pigment -- chlorophyll e.g. trees, grasses, algae, phytoplanktons and photosynthetic bacteria (blue green algae). **(b) Chemoautotrophs -** Use energy generated in oxidation-reduction process. Their significance in the ecosystem as producers is minimal. e.g. [Beggiatoa.] **(ii) Heterotrophic Component:** Organisms here predominate the activities of utilization, rearrangement and decomposition of complex organic materials. They are consumers. Macroconsumers(phagotrophs)-Depending on their food habits, consumers can be herbivores orcarnivores or omnivores. Herbivores are known as the primary consumers while secondary consumers and tertiary consumers are a. carnivores and omnivores. b. Microconsumers-They are called decomposers, reducers, saprotrophs, osmotrophs and scanvengers. Microconsumers include microorganisms such as bacteria and fungi. **(4) FUNCTION ECOSYSTEM** By function, we mean (i) the rate of biological energy flow, that is, the production and respiration rates of the community, (ii)the rate of materials or nutrient cycles and(iii) biological or ecological regulation including both regulation of organisms by environment and vice versa. **(5) FOOD CHAIN IN ECOSYSTEMS** The transfer of food energy from the producers, through a series of organisms (herbivores, carnivores to decomposers) with repeated eating and being eaten, is known as a FOOD CHAIN. Producers utilize the radiant energy of sun which is transformed to chemical form, ATP during photosynthesis. Thus green plants occupy in any food chain, the first trophic (nutritional) level. They are called the **PRIMARYPRODUCERS**. Plant eaters constitute the second trophic level and they are called **PRIMARYCONSUMERS**. Carnivores which feed on herbivores form the third trophic level and they are called the **SECONDARYCONSUMERS**. These in turn may be eaten still by other carnivores at **TERTIARYCONSUMERS** level by the tertiary consumers. The number of steps in a food chain is always restricted to four or five, since the energy available decreases with each step e.g. *Cynodondactylon* *Melanoplusdifferentialis* *Bufomelanostictus* grass species a grasshopper a toad *Zamensismucosus* At each trophic level in a food chain, a large portion of energy is used for its own maintenance and ultimately lost as heat. Consequetly, organisms in each trophic level pass on less and less energy than they received. This leads to limit the number of steps or trophic levels to four or five. In nature, there are basically two types of food chains viz: **(i) GRAZING FOOD CHAIN:** This type of food chain starts from the living green plants, goes to grazing herbivores and on to the carnivores. Ecosystems with such type of food chain are directly dependent on an influx of solar radiation. This type of chain thus depends on autotrophs energy captured and the movement of this captured energy to herbivores e.g. phytoplanktons zooplanktons fish sequence **(ii) DETRITUS FOOOD CHAIN:** This type of food chain goes from organic matter into microorganisms and then to organisms feeding on detritus (detritivores) and their predators. The ecosystems are thus less dependent on direct solar energy. Detritus food chain ends up in a manner similar to the grazing food chain but the way in which the two chains begin is quite different. In detritus food chain, the detritus consumers, in contrast to grazing herbivores, are a mixed group in terms of trophic levels. These include herbivores, omnivores and carnivores. In addition, the energy storage for the detritus food chain may be largely to understand biological magnification. **IMPORTANCE OF FOOD CHAIN** i. Helps to understand the relationship and the interaction between organisms. ii. Helps to appreciate the energy flow mechanism and matter circulation in the ecosystem. iii. To understand the movement of toxic substance in the ecosystem iv. To understand biological magnification. **FOOD WEB** In a given ecosystem, various food chains are linked together and intersect each other to form a complex network called the **FOODWEB**. The complexity of any food web depends on the length of the food chain and the alternative at different points of consumers in the chain. The length of the food chain is determined by the diversity in the organisms based upon their food. **ECOLOGICAL PYRAMIDS** This is the graphic representation of the trophic structure and function at successive trophic levels, i.e. producers herbivores carnivores, where the first or producer level constitutes the base of the pyramids and the successive levels, the tiers making up the apex. Ecological pyramids are of three types viz: **(a) Pyramids of Numbers:** These pyramids show the relationship between producers, herbivores and carnivores at successive trophic levels in terms of their number. They do not give a true picture of the food chain as they are not functional. **(b) Pyramids of Biomass:** The biomasses of members of the food chain present at any one time form the pyramids of the biomass. **(c) Pyramids of Energy:** This is when production is considered in terms of the energy. It shows the amount of energy flow at each level and the actual role the various organisms play in transfer of energy. The shape of the pyramid is always upright as there is always a gradual decrease in the energy content at successive trophic levels from producers to various consumers. **ENERGY FLOW IN ECOSYSTEM** Ecosystems function with energy flowing in one direction from sun, and through nutrients which are continually recycled. Light energy is used by plants, which by process of photosynthesis; convert it to chemical energy in the form of carbohydrate and other carbon compounds. When primary consumers (herbivores) eat the producers, the energy changes into a form that can be stored in animal cells. Secondary consumers (carnivores) transform the energy once again. Decomposers may occupy several positions in the pyramid, both receiving energy from decaying plants and animals and supplying it to detrivores and fungus-eaters. Since energy is lost in each step, populations are necessarily smaller at each higher level of the pyramid. To determine the energy flow in ecosystem, it is essential to understand the efficiency of production in the absorption and conversion of solar energy; the use of this converted chemical form of energy by consumers; the total input of energy in form of food and its efficiency of assimilation; the loss through respiration, heat, excretion etc. and the gross net production. **ECOLOGICAL NICHE** This is the ultimate unit within which each species is held by its structural and instinctive limitations.The place of the species in the formal community structure. The specific physical space occupied by an organism; the role of an organism in the ecosystem. Three Aspects of Ecological Niche: 1. Spatial or habit niche- the physical space occupied. 2. Trophic Niche-the functional role, the trophic position. 3. Multifactor or Hyper volume Niche-the position in the environment gradient. Niche that is completely overlaps lead to competition while the one that is partial overlaps leads to co-existence. **CELL DIVISION** Cell division is the process by which a cell divides to form two new cells, either to produce identical cells of mitosis or to produce cells with half the number of chromosomes of meiosis. Why Divide? ----------- All cells are produced by division of pre-existing cell. Continuity of life depends on cell division. A cell born after a division, proceeds to grow by macromolecular synthesis, reaches a species-determined division size and divides. Continued growth of cell threatens to disturb the balance and the cell, in order to avert this imbalance and ageing, divides into two daughter cells, which in turn grow and finally divide in the same number. This cycle acts as a unit of biological time and defines life history of a cell. Cell cycle is the entire sequence of events happening from the end of one nuclear division to the beginning of the next. Growth of the body is thus affected, not by the growth of the individual cells, but by the increase in the number of cells in the body. **To produce sex cells**: Cell division in multi-cellular organisms enables the organisms to grow larger while the cells remain small. A large surface: volume ratio is due to small cell size. Introductory Concepts ===================== ### Chromatin, Chromosomes ***Chromatin*** is a mass of uncoiled DNA and associated proteins called ***histones***. When cell division begins, DNA coils around the proteins forming visible structures called ***chromosomes***. **Haploid, Diploid** Diploid cells (2n) have two complete sets of chromosomes. The body cells of animals are diploid, having two haploid sets of homologous chromosomes Haploid cells have one complete set of chromosomes. In animals, gametes (sperm and eggs) are haploid - having a single set of unpaired chromosomes. **Homologous Chromosomes** Diploid organisms have two copies of each chromosome (except the sex chromosomes). Each pair of chromosomes is ***homologous***. **Genes** A small segment of DNA that contains the information necessary to construct a protein or part of a protein (polypeptide) is called a gene. Genes are the unit of inheritance. **Centromere:** Double-stranded chromosomes have two chromatids; normally, each one is identical to the other. The point where the two **chromatids** are attached is called the ***centromere***. **STEPS IN CELL DIVISION** Cell division has three important aspects: I. Replication of DNA. II. Division of nucleus/karyokinesis. III. Division of cytoplasm/cytokinesis. **TYPES OF CELL DIVISION** Three types of cell division occur: binary fission, mitosis, and meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multi-cellular organisms use mitosis for growth, cell repair, and cell replacement. It is the **EQUATIONAL DIVISION**, as it results in two daughter cells having the same number of chromosomes as the parent cell. The type of cell division required for sexual reproduction is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and animals -- including, of course, human beings. Meiosis differs from mitosis in that cell division begins with a cell that has a full complement of chromosomes and ends with gamete cells, such as sperm and eggs that have only half the complement of chromosomes. When a sperm and egg unite during fertilization, the cell resulting from the union, called a zygote, contains the full number of chromosomes. It is the **REDUCTIONAL DIVISION**, as it results in daughter cells having half the number of chromosomes present in the parent cell. **MITOSIS** Mitosis occurs in the somatic cells. It is also known as SOMATIC CELL DIVISION. Mitotic division results in an increase in the number of cells without change in the genetic composition as the number and structure of chromosome remain unchanged. Repair of tissues and replacement of cells occur by mitosis. A somatic cell mitotic cycle includes two parts viz, **INTERPHASE** and **MITOTICPHASE**. **INTERPHASE:** Inter-phase is a time of intense preparation for cell division. It is a time when the DNA and organelles are duplicated. This is the longest phase of cell cycle. It consists of three parts- G~1~ (first gap/growth); S (DNA synthesis) and G~2~ (second gap/growth). All three parts of inter-phase are times of cell growth, characterized by the production of organelles and the synthesis of proteins and other macromolecules. **There are, however, some events specific to certain parts of inter-phase**: **G~1~**-A time of major growth before DNA synthesis begins. High rate of synthesis of cell components like RNA, ribosomes, mitochondria, lysosomes, ER, chloroplasts and proteins. Metabolic rate of cell is high. This is the longest phase of inter-phase in which cell grows in size. **S**-The time during which DNA is synthesized. Synthesis of histone proteins. Replication of DNA. **G~2~**- A time of growth after DNA is synthesized and before mitosis begins. Division of mitochondria and chloroplasts. Amount of DNA in cell doubled than in the original cell. Doubling of centrioles. Volume of cell increases. **MITOTIC PHASE** (M PHASE): This includes KARYOKINESIS/NUCLEAR DIVISION during which duplicated chromosomes are separated into two nuclei and CYTOKINESIS during which the entire cell (cytoplasm) is divided into two daughter cells. The division of body cells (after inter-phase) consists of two processes that overlap somewhat in time. The first process, division of the nucleus, is called **karyokinesis** (MITOSIS). The second process is **cytokinesis**, which is the division of the cytoplasm that occurs toward the end of mitosis. Mitosis is usually divided into four stages-prophase, metaphase, anaphase and telophase. **PROPHASE**: Mitosis begins with prophase, a time when changes occur both in the nucleus and in the cytoplasm. **In the nucleus**: - Chromatin threads or chromosomal material, coil and condense to form compact chromosomes. - Chromosomes composed of two chromatids attached together at centromere. - Nuclear membrane disappears. - Nucleolus disintegrates. - Golgi complex and E.R. fragment. **In the cytoplasm**: - The mitotic spindle forms. - The centrioles, duplicated during inter-phase, move away from each other toward opposite ends of the cell. **METAPHASE** The first part of metaphase after the end of prophase is called pro-metaphase. Here, condensed chromosomes are scattered in the nuclear region; chromosomal microtubules attach to kinetochores of chromosomes; chromosomes move to spindle equatorial plate. Later, chromosomes arrange on equatorial plate, attached by chromosomal microtubules to both poles. This line of chromosomes is called **METAPHASE PLATE**. Metaphase ends when chromosomes split, thus doubling the number of chromosomes **ANAPHASE** In this phase: - Centromeres split and the chromatids separate (chromosomes). - Chromosomes move to opposite spindle poles. - Spindle poles move farther apart. **TELOPHASE**: **In telophase**: - Chromosomes are clustered at opposite spindle poles. - Nuclear membrane is formed around chromosome clusters. - The mitotic spindle disassembles. - Nucleolus reappears. - Golgi complex and E.R. reform. **CYTOKINESIS** Division of the cytoplasm begins during the telophase. **At this time**: - Cell organelles and cytoplasm are equally distributed in the two daughter cells. - Constriction or cell plate formation occurs. **[Overview of the Cell Cycle]** +-----------------------------------+-----------------------------------+ | ![mitosi3](media/image6.png) | **Interphase (G~1~ and G~2~)** | | | | | | Chromosomes are not visible | | | because they are uncoiled | +===================================+===================================+ | mitosi11 | **Prophase** | | | | | | The chromosomes coil.\ | | | The nuclear membrane | | | disintegrates.\ | | | The spindle apparatus forms. | +-----------------------------------+-----------------------------------+ | ![mitosi5](media/image8.png) | **Metaphase** | | | | | | The chromosomes become aligned. | +-----------------------------------+-----------------------------------+ | mitosi6 | **Anaphase** | | | | | | The chromatids separate (The | | | number of chromosomes doubles). | +-----------------------------------+-----------------------------------+ | ![mitosi9](media/image10.png) | **Telophase** | | | | | | The nuclear membrane reappears.\ | | | The chromosomes uncoil.\ | | | The spindle apparatus breaks | | | down.\ | | | The cell divides into two. | +-----------------------------------+-----------------------------------+ | mitosi8 | **G~1~ Interphase** | | | | | | The chromosomes have one | | | chromatid. | +-----------------------------------+-----------------------------------+ | ![mitosi10](media/image12.png) | **G~2~ Interphase** | | | | | | The chromosomes have two | | | chromatids. | +-----------------------------------+-----------------------------------+ **SIGNIFICANCE OF MITOSIS** 1. Mitosis helps the cell in maintaining proper size. 2. It helps in the maintenance of equilibrium in the amount of DNA and RNA in the cell. 3. It provides the opportunity for growth and development of organs and the body of the organisms. 4. The old decaying and dead cells of the body are replaced by the help of mitosis. 5. In certain organisms, the mitosis is involved in asexual reproduction. 6. The gonads and the sex cells depend on the mitosis for the increase in their number. 7. The cleavage of egg during embryogenesis and division of blastema during blastogenesis, both involve mitosis. **DIFFERENCES BETWEEN MITOSIS IN ANIMAL CELL AND PLANT CELL** **ANIMALCELL** **PLANTCELL** --------------------------------------------------------------------- ------------------------------------------------------------ 1\. It occurs in almost all tissues of the body. 1\. Occurs generally in meristematic tissues. 2\. Centrioles are generally present. 2\. Centrioles are generally absent. 3\. Astral or amphiastral mitosis occurs because asters are formed. 3\. Anastral mitosis occurs because asters are not formed. **MEIOSIS** This is cell division forming four daughter cells each with half the number of chromosomes as compared to the parent cell. It occurs in the cells of gonads resulting in the formation of gametes (sperms and ova). It is known as GAMETOGENIC DIVISION. **PROCESS OF MEIOSIS** Meiosis superficially resembles two mitotic divisions without an intervening period of DNA replication. The first meiotic division, the reduction division, includes a long prophase in which homologous chromosomes become closely associated to each other and interchange of hereditary material takes place between them and resulting in two haploid daughter cells. In the second meiotic division, the equational division, the haploid cell divides mitotically and results into four haploid cells. **FIRST MEIOTIC DIVISION/HETEROTYPIC DIVISION**: It can be distinguished into inter-phase I, prophase I, metaphase I, anaphase I, telophase I. **Inter-phase 1**-This includes the G~1~, S, and G~2~ stages as in mitosis. **Prophase I**-This is the longest phase of meiosis and it is more complex than prophase in mitosis. It has five sub-stages namely, leptotene, zygotene, pachytene, diplotene and diakinesis. Leptotene: - The centrioles duplicate and start moving towards opposite poles. - The chromatin material condenses to form thin, long and thread-like chromosomes. - Each chromosome appears longitudinally double due to sister chromatids. **Zygotene**: - The pairing of homologous chromosomes takes place. - The pairing of the homologous chromosomes is known as SYNAPSIS. - The synapsis begins at one or more points along the length of the homologous chromosomes-pro-terminal, pro-centric and random synapsis. **Pachytene**: - The pair of homologous chromosomes become twisted spirally around each other and cannot be distinguished separately. - Crossing over occurs during this stage. - The points of contacts are called chiasmata. **Diplotene**: - The chromosomes of each homologous pair repel each other. - The chromatids sometimes break at points where chiasmata occur. - The breakage is accompanied by reunion in such a way that corresponding fragments are interchanged between the two homologous chromosomes. - The process called crossing over produces recombinant chromosomes, which combine genes inherited from both parents. - The nuclear membrane and nucleolus start disappearing at the end of this stage. **Diakinesis**: - The homologous chromosomes move further away from each other. - The nuclear membrane disappears. - The nucleolus also disappears. - The two set of centrioles reach the opposite poles with each one surrounded by aster rays. **METAPHASE I**-A spindle of fibres appears between two sets of centrioles with the chromosomes arrange at the equatorial region in such a way that the two members of a homologous pair lie on opposite sides of the equatorial plane. The centromere of each chromosome gets attached to the spindle fibres. Independent assortment that is random arrangement of chromosomes which promotes variation occurs here. The number of combinations possible when chromosomes assort independently into gametes during meiosis is 2^n^, where n is the haploid number of the organism. **ANAPHASE I**-The partners of homologous chromosomes go to opposite poles. Each centromere is still undivided and has the chromatids attached to it. Due to crossing over each set of chromosome consists of a mixture of maternal and paternal chromosome segment. **TELOPHASE I**-At each pole a nucleus is reconstructed but daughter nuclei are haploid. Nuclear division is usually followed by cytokinesis, resulting in two haploid daughter cells. Both cells pass through a short resting phase of inter-phase during which, no DNA replication. **SECOND MEIOTIC DIVISION/HOMOTYPIC DIVISION**: This is the mitotic division which divides each haploid meiotic cell into two haploid cells and it has four stages-prophase II, metaphase II, anaphase II, telophase II. **PROPHASE II** - The centrioles duplicate and migrate towards opposite poles. - Each set of centrioles is surrounded by aster rays. - Nuclear membrane and nucleolus disappear. **METEPHASE II** - The spindle of fibres appears. - The chromatids of the chromosomes, held together by a centromere are arranged at the equatorial plane. - The centromere of each chromosome gets attached to the spindle fibres. - The equatorial plane in this meiotic division is perpendicular to that of first meiotic division. **ANAPHASE II** - The centromere divides to separate the two chromatids. - The chromatids move to the opposite poles as a chromosome. **TELOPHASE II** - Nucleus is recognized at the poles. - Nucleolus reappears. Cytokinesis, cytoplasm division, follows karyokinesis and results in two daughter cells. Cytokinesis can be successive cell plate formation or simultaneous cell plate formation. Summary of the Phases of Meiosis -------------------------------- A cell undergoing meiosis will divide two times; the first division is meiosis 1 and the second is meiosis 2. The phases have the same names as those of mitosis. A number indicates the division number (1st or 2nd): meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1 meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2 In the first meiotic division, the number of cells is doubled but the number of chromosomes is not. This results in ½ as many chromosomes per cell. The second meiotic division is like mitosis; the number of chromosomes does not get reduced. +-----------------------------------+-----------------------------------+ | img011 | **Prophase I** | | | | | | Homologous chromosomes become | | | paired. | | | | | | Crossing-over occurs between | | | homologous chromosomes. | | | | | | | | | | | | | +===================================+===================================+ | ![img020](media/image14.png) | Crossing over | | | | | | | +-----------------------------------+-----------------------------------+ | img012 | **Metaphase I** | | | | | | Homologous pairs become aligned | | | in the center of the cell. | | | | | | | | | | | | | +-----------------------------------+-----------------------------------+ | ![meiosi6](media/image16.png) | The random alignment pattern is | | | called independent assortment. | | | For example, a cell with 2N = 6 | | | chromosomes could have any of the | | | alignment patterns shown at the | | | left. | +-----------------------------------+-----------------------------------+ | img013 | **Anaphase I** | | | | | | [Homologous | | | chromosomes](http://faculty.clint | | | oncc.suny.edu/faculty/michael.gre | | | gory/files/Bio%20101/Bio%20101%20 | | | Lectures/Mitosis/mitosis.htm#Homo | | | logous%20Chromosomes) | | | separate. | | | | | | | +-----------------------------------+-----------------------------------+ | ![img014](media/image18.png) | **Telophase I** | | | | | | This stage is absent in some | | | species | | | | | | | +-----------------------------------+-----------------------------------+ | | **Interkinesis** | | | | | | Interkinesis is similar to | | | [interphase](http://faculty.clint | | | oncc.suny.edu/faculty/michael.gre | | | gory/files/Bio%20101/Bio%20101%20 | | | Lectures/Mitosis/mitosis.htm#Inte | | | rphase) | | | except DNA synthesis does not | | | occur. | | | | | | | +-----------------------------------+-----------------------------------+ | | ** ** | +-----------------------------------+-----------------------------------+ | img015 | **Prophase II** | +-----------------------------------+-----------------------------------+ | ![img016](media/image20.png) | **Metaphase II** | +-----------------------------------+-----------------------------------+ | img017 | **Anaphase II** | +-----------------------------------+-----------------------------------+ | ![img018](media/image22.png) | **Telophase II** | +-----------------------------------+-----------------------------------+ | img019 | **Daughter Cells** | +-----------------------------------+-----------------------------------+ **SIGNIFICANCE OF MEIOSIS** 1. Crossing over provides new combinations of chromosomes and hence new combinations of characters in offspring. 2. It results in the formation of haploid sex cells, which after fertilization restore the original diploid number in the zygote. 3. The four chromatids of a homologous pair of chromosomes are passed on to four different daughter cells. This is called the segregation of chromosomes. It results in its different combinations and hence brings about genetic variations in the daughter cells. **DIFFERENCES BETWEEN MITOSIS AND MEIOSIS** **MITOSIS** **MEIOSIS** ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ It occurs in the somatic cells. It occurs in the sex cells (gonads). One parent cell gives rise to two daughter cells. One parent cell gives rise to four daughter cells. The daughter cells resemble the parent cell in all the essential characters. The daughter cells differ from the parent cell in containing half the number of chromosomes and the chromosomes bear different combinations of genes as a result of crossing over. The whole process completes in one sequence or phase. The whole process completes in two successive divisions which occur one after the other. Daughter cells replace worn-out cells or add to the bulk of the body, thus affecting growth. Daughter cells do not affect growth, but act as haploid sex cells, capable of bringing about reproduction by fertilization. Mitotic division is completed in a shorter period. Meiosis requires a longer period for its completion. The prophase is of short duration and includes no sub-stage. It is a longer phase, consisting of five stages. Duplication of chromosomes takes place in early prophase. Duplication or splitting of chromosomes takes place in late prophase The homologous chromosomes duplicate into two chromatids. The two chromatids separate and form new chromosomes. Each daughter cell receives the daughter chromosome or chromatids of each homologous chromosome. Out of two homologous chromosomes only one type of chromosome either maternal or paternal moves to the daughter cells. **HEREDITY** - **SOME TERMINOLOGIES IN GENETICS** - **Alleles ** - Alternate forms or varieties of a [gene](javascript:JumpTo('#gene')). The alleles for a trait occupy the same locus or position on [homologous chromosomes](javascript:JumpTo('#homologous_chromosomes')) and thus govern the same trait. However, because they are different, their action may result in different expressions of that trait. - **Carrier** - An individual who is [heterozygous](javascript:JumpTo('#heterozygous')) for a trait that only shows up in the [phenotype](javascript:JumpTo('#phenotype')) of those who are [homozygous](javascript:JumpTo('#homozygous')) recessive. Carriers often do not show any signs of the trait but can pass it on to their offspring. This is the case with [hemophilia](javascript:JumpTo('#hemophilia')). - **Chromosomes ** - Thread-like, gene-carrying bodies in the nucleus of a cell. Chromosomes are composed primarily of [DNA](javascript:JumpTo('#DNA')) and [protein](javascript:JumpTo('#protein')). They are visible only under magnification during certain stages of cell division. Humans have 46 chromosomes in each [somatic cell](javascript:JumpTo('#somatic_cell')) and 23 in each [sex cell](javascript:JumpTo('#sex_cell')). - **Codominance ** - The situation in which two different [alleles](javascript:JumpTo('#allele')) for a trait are expressed unblended in the [phenotype](javascript:JumpTo('#phenotype')) of [heterozygous](javascript:JumpTo('#heterozygous')) individuals. Neither allele is dominant or recessive, so that both appear in the phenotype or influence it. Type AB blood is an example. Such traits are said to be codominant. - **Cross-pollination** - The mating of two genetically different plants of the same species. Usually, the term is used in reference to the crossing of two pure breeding ([homozygous](javascript:JumpTo('#homozygous'))) plants. - **Dominant allele** - An[allele](javascript:JumpTo('#allele')) that masks the presence of a [recessive allele](javascript:JumpTo('#recessive_allele')) in the [phenotype](javascript:JumpTo('#phenotype')). Dominant alleles for a trait are usually expressed if an individual is [homozygous dominant](javascript:JumpTo('#homozygous')) or [heterozygous](javascript:JumpTo('#heterozygous')). - **DNA (Deoxyribonucleic acid)** - A large organic molecule that stores the genetic code for the synthesis of [proteins](javascript:JumpTo('#protein')). DNA is composed of sugars, phosphates and bases arranged in a double helix shaped molecular structure. Segments of DNA in [chromosomes](javascript:JumpTo('#chromosomes')) correspond to specific [genes](javascript:JumpTo('#gene')). - **Evolution ** - Genetic change in a population of organisms that occurs over time. The term is also frequently used to refer to the appearance of a new species. - **F1 generation ** - The first offspring (or filial) generation. The next and subsequent generations are referred to as f2, f3, etc. - **Gene pool** - All of the [genes](javascript:JumpTo('#gene')) in all of the individuals in a breeding population. More precisely, it is the collective [genotype](javascript:JumpTo('#genotype')) of a population. - **Genes ** - Units of inheritance usually occurring at specific locations, or loci, on a [chromosome](javascript:JumpTo('#chromosomes')). Physically, a gene is a sequence of DNA bases that specify the order of amino acids in an entire protein or, in some cases, a portion of a protein. A gene may be made up of hundreds of thousands of DNA bases. Genes are responsible for the hereditary traits in plants and animals. - **Genetic drift** - Evolution, or change in [gene pool](javascript:JumpTo('#gene_pool')) frequencies, resulting from random chance. Genetic drift occurs most rapidly in small populations. In large populations, random deviations in allele frequencies in one direction are more likely to be cancelled out by random changes in the opposite direction. - **Genetics ** - The study of gene structure and action and the patterns of inheritance of traits from parent to offspring. Genetic mechanisms are the underlying foundation for evolutionary change. Genetics is the branch of science that deals with the inheritance of biological characteristics. - **Genome ** - The full genetic complement of an individual (or of a species). In humans, it is estimated that each individual possesses approximately 2.9 billion base units in his or her DNA. See [Human Genome Project](javascript:JumpTo('#Human_Genome_Project')). - **Genome imprinting ** - An inheritance pattern in which a [gene](javascript:JumpTo('#gene')) will have a different effect depending on the gender of the parent from whom it is inherited. Genome imprinting is also known as genetic imprinting. - **Genotype ** - The genetic makeup of an individual. Genotype can refer to an organism\'s entire genetic makeup or the [alleles](javascript:JumpTo('#allele')) at a particular locus. See [phenotype](javascript:JumpTo('#phenotype')). - **Heterozygous ** - A [genotype](javascript:JumpTo('#genotype')) consisting of two different [alleles](javascript:JumpTo('#allele')) of a gene for a particular trait (Aa). Individuals who are heterozygous for a trait are referred to as heterozygotes. See [homozygous](javascript:JumpTo('#homozygous')). - **Homologous chromosomes ** - [Chromosomes](javascript:JumpTo('#chromosomes')) that are paired during the production of of sex cells in [meiosis](javascript:JumpTo('#meiosis')). Such chromosomes are alike with regard to size and also position of the centromere. They also have the same genes, but not necessarily the same [alleles](javascript:JumpTo('#allele')), at the same locus or location. - **Homozygous ** - Having the same [allele](javascript:JumpTo('#allele')) at the same locus on both members of a pair of [homologous chromosomes](javascript:JumpTo('#homologous_chromosomes')). Homozygous also refers to a [genotype](javascript:JumpTo('#genotype')) consisting of two identical alleles of a gene for a particular trait. An individual may be homozygous dominant (AA) or homozygous recessive (aa). Individuals who are homozygous for a trait are referred to as homozygotes. See [heterozygous](javascript:JumpTo('#heterozygous')). - **Hybrids ** - Offspring that are the result of mating between two genetically different kinds of parents\--the opposite of [purebred](javascript:JumpTo('#purebred')). - **Meiosis ** - Cell division in specialized tissues of ovaries and testes which results in the production of [sperm](javascript:JumpTo('#sperm')) or [ova](javascript:JumpTo('#ovum')). Meiosis involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes\--23 in the case of humans. - **Monozygotic twins ** - Identical twins. Twins that come from the same [zygote](javascript:JumpTo('#zygote')) are essentially the same genetically. Differences between monozygotic twins later in life are virtually always the result of environmental influences rather than genetic inheritance. Fraternal twins may look similar but are not genetically identical. - **Multiple-allele series ** - a situation in which a [gene](javascript:JumpTo('#gene')) has more than two [alleles](javascript:JumpTo('#allele')). The ABO blood type system is an example. Multiple-allele series only partly follow simple [Mendelian genetics](javascript:JumpTo('#Mendelian_genetics')). - **Mutation ** - An alteration of genetic material such that a new variation is produced. For instance, a trait that has only one allele (A) can mutate to a new form (a). This is the only mechanism of [evolution](javascript:JumpTo('#evolution')) that can produce new [alleles](javascript:JumpTo('#allele')) of a gene. - **Phenotype ** - The observable or detectable characteristics of an individual organism\--the detectable expression of a [genotype](javascript:JumpTo('#genotype')). - **Pleiotropy ** - The situation in which a single [gene](javascript:JumpTo('#gene')) is responsible for a variety of traits. The collective group of symptoms known as [sickle-cell trait](javascript:JumpTo('#sickle-cell_trait')) is an example. - **Purebred ** - Offspring that are the result of mating between genetically similar kinds of parents\--the opposite of [hybrid](javascript:JumpTo('#hybrid')). Purebred is the same as true breeding. - **Recessive allele ** - An[allele](javascript:JumpTo('#allele')) that is masked in the [phenotype](javascript:JumpTo('#phenotype')) by the presence of a [dominant allele](javascript:JumpTo('#dominant_allele')). Recessive alleles are expressed in the phenotype when the [genotype](javascript:JumpTo('#genotype')) is homozygous recessive (aa). - **Sex cell** - A gamete, either a sperm or an ovum. Sex cells are produced by the [meiosis](javascript:JumpTo('#meiosis')) process. See [somatic cell](javascript:JumpTo('#somatic_cell')). - **Sickle-cell trait** - A genetically inherited [recessive](javascript:JumpTo('#recessive%20allele')) condition in which red blood cells are distorted resulting in severe anemia and related symptoms that are often fatal in childhood. Sickle-cell trait is the result of a [pleiotropic](javascript:JumpTo('#pleiotropy')) gene. Sickle-cell trait is also known as sickle-cell anemia. - **Somatic cell ** - Any cell in the body except those directly involved with reproduction. Most cells in multicellular plants and animals are somatic cells. They reproduce by mitosis. See [sex cell](javascript:JumpTo('#sex_cell')). - **X-linked** - Referring to a [gene](javascript:JumpTo('#gene')) that is carried by an X sex chromosome. - **Zygote ** - A \"fertilized\" [ovum](javascript:JumpTo('#ovum')). More precisely, this is a cell that is formed when a [sperm](javascript:JumpTo('#sperm')) and an ovum combine their [chromosomes](javascript:JumpTo('#chromosomes')) at conception. A zygote contains the full complement of chromosomes (in humans 46) and has the potential of developing into an entire organism. **Mendel\'s Pea Plants** **What\'s so interesting about Pea plants?** These purple-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. The plants are common garden pea plants, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. With his careful experiments, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring. You may not care much about heredity in pea plants, but you probably care about your own heredity. Mendel's discoveries apply to you as well as to peas---and to all other living things that reproduce sexually. Johann Gregor Mendel (1822-1884) -------------------------------- Father of Genetics ------------------ Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel\'s Laws of Heredity are usually stated as: 1\) The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization. 2\) The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another. 3\) The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant. **Mendel and His Pea Plants** People have long known that the characteristics of living things are similar in parents and their offspring. Whether it's the flower color in pea plants or nose shape in people, it is obvious that offspring resemble their parents. However, it wasn't until the experiments of Gregor Mendel that scientists understood how characteristics are inherited. Mendel's discoveries formed the basis of **genetics**, the science of heredity. That's why Mendel is often called the \"father of genetics.\" It's not common for a single researcher to have such an important impact on science. The importance of Mendel's work was due to three things: a curious mind, sound scientific methods, and good luck. You'll see why when you read about Mendel's experiments. Gregor Mendel was born in 1822 and grew up on his parents' farm in Austria. He did well in school and became a monk. He also went to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through experimentation and to use math to make sense of his results. Mendel is best known for his experiments with the pea plant *Pisumsativum.* During Mendel's time, the blending theory of inheritance was popular. This is the theory that offspring have a blend, or mix, of the characteristics of their parents. Mendel noticed plants in his own garden that weren't a blend of the parents. For example, a tall plant and a short plant had offspring that were either tall or short but not medium in height. Observations such as these led Mendel to question the blending theory. He wondered if there was a different underlying principle that could explain how characteristics are inherited. He decided to experiment with pea plants to find out. In fact, Mendel experimented with almost 30,000 pea plants over the next several years! ### Why Study Pea Plants? Why did Mendel choose common, garden-variety pea plants for his experiments? Pea plants are a good choice because they are fast growing and easy to raise. They also have several visible characteristics that may vary. These characteristics, which are shown in **Figure**[below](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.1/#x-ck12-QmlvLTA2LTAyLU1lbmRlbC1TZXZlbi1DaGFyYWN0ZXJpc3RpY3M.), include seed form and color, flower color, pod form and color, placement of pods and flowers on stems, and stem length. Each characteristic has two common values. For example, seed form may be round or wrinkled, and flower color may be white or purple (violet). ![C:\\Users\\USER\\Desktop\\mendel.jpg](media/image24.jpeg) ### Controlling Pollination To research how characteristics are passed from parents to offspring, Mendel needed to control pollination. **Pollination** is the fertilization step in the sexual reproduction of plants.**Pollen**consists of tiny grains that are the male gametes of plants. They are produced by a male flower part called the **anther** (see **Figure**[below](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.1/#x-ck12-QmlvLTA2LTAzLVBhcnRzLW9mLWEtZmxvd2Vy)). Pollination occurs when pollen is transferred from the anther to the stigma of the same or another flower. The **stigma** is a female part of a flower. It passes the pollen grains to female gametes in the ovary. Flowers are the reproductive organs of plants. Each pea plant flower has both male and female parts. The anther is part of the stamen, the male structure that produces male gametes (pollen). The stigma is part of the pistil, the female structure that produces female gametes and guides the pollen grains to them. The stigma receives the pollen grains and passes them to the ovary, which contains female gametes. Pea plants are naturally self-pollinating. In **self-pollination**, pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand with pollen from other parent plants of his choice. When pollen from one plant fertilizes another plant of the same species, it is called **cross-pollination**. The offspring that result from such a cross are called **hybrids.** Review ------ 1. What is the blending theory of inheritance? Why did Mendel question this theory? 2. List the seven characteristics that Mendel investigated in pea plants. 3. How did Mendel control pollination in pea plants? 4. What are hybrids? 5. Briefly state Mendel\'s three laws. Mendel's First Set of Experiments --------------------------------- Mendel first experimented with just one characteristic of a pea plant at a time. He began with flower color. As shown in **Figure**[below](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.2/#x-ck12-QmlvLTA2LTA0LUNoYXJ0LVNob3dpbmctb3V0Y29tZS1vZi1NZW5kZWwtZmlyc3QtZXhwZXJpbWVudA..), Mendel cross-pollinated purple- and white-flowered parent plants. The parent plants in the experiments are referred to as the **P** (for parent) **generation**. This diagram shows![](media/image26.png) Mendel\'s first experiment with pea plants. The F1 generation results from cross-pollination of two parent (P) plants, and contained all purple flowers. The F2 generation results from self-pollination of F1 plants, and contained 75% purple flowers and 25% white flowers. This type of experiment is known as a monohybrid cross. ### F1 and F2 Generations The offspring of the P generation are called the **F1** (for filial, or "offspring") **generation**. As you can see from **Figure**[above](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.2/#x-ck12-QmlvLTA2LTA0LUNoYXJ0LVNob3dpbmctb3V0Y29tZS1vZi1NZW5kZWwtZmlyc3QtZXhwZXJpbWVudA..), all of the plants in the F1 generation had purple flowers. None of them had white flowers. Mendel wondered what had happened to the white-flower characteristic. He assumed some type of inherited factor produces white flowers and some other inherited factor produces purple flowers. Did the white-flower factor just disappear in the F1 generation? If so, then the offspring of the F1 generation---called the **F2 generation**---should all have purple flowers like their parents. To test this prediction, Mendel allowed the F1 generation plants to self-pollinate. He was surprised by the results. Some of the F2 generation plants had white flowers. He studied hundreds of F2 generation plants, and for every three purple-flowered plants, there was an average of one white-flowered plant. ### Law of Segregation Mendel did the same experiment for all seven characteristics. In each case, one value of the characteristic disappeared in the F1 plants and then showed up again in the F2 plants. And in each case, 75 percent of F2 plants had one value of the characteristic and 25 percent had the other value. Based on these observations, Mendel formulated his first law of inheritance. This law is called the **law of segregation**. It states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces. Review ------ 1. Describe in general terms Mendel's first set of experiments. 2. State Mendel\'s first law. 3. Assume you are investigating the inheritance of stem length in pea plants. You cross-pollinate a short-stemmed plant with a long-stemmed plant. All of the offspring have long stems. Then, you let the offspring self-pollinate. Describe the stem lengths you would expect to find in the second generation of offspring. Mendel's Second Set of Experiments ---------------------------------- After observing the results of his first set of experiments, Mendel wondered whether different characteristics are inherited together. For example, are purple flowers and tall stems always inherited together? Or do these two characteristics show up in different combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The results of this cross, which is a **dihybrid cross**, are shown in **Figure**[below](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.3/#x-ck12-QmlvLTA2LTA1LURpaHlicmlkLUNyb3Nz). C:\\Users\\USER\\Desktop\\mendel2.png This chart represents Mendel\'s second set of experiments. It shows the outcome of a cross between plants that differ in seed color (yellow or green) and seed form (shown here with a smooth round appearance or wrinkled appearance). The letters R, r, Y, and y represent genes for the characteristics Mendel was studying. Mendel didn't know about genes, however. Genes would not be discovered until several decades later. This experiment demonstrates that in the F2 generation, 9/16 were round yellow seeds, 3/16 were wrinkled yellow seeds, 3/16 were round green seeds, and 1/16 were wrinkled green seeds. ### F1 and F2 Generations In this set of experiments, Mendel observed that plants in the F1 generation were all alike. All of them had yellow and round seeds like one of the two parents. When the F1 generation plants self-pollinated, however, their offspring---the F2 generation---showed all possible combinations of the two characteristics. Some had green round seeds, for example, and some had yellow wrinkled seeds. These combinations of characteristics were not present in the F1 or P generations. ### Law of Independent Assortment Mendel repeated this experiment with other combinations of characteristics, such as flower color and stem length. Each time, the results were the same as those in **Figure**[above](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.3/#x-ck12-QmlvLTA2LTA1LURpaHlicmlkLUNyb3Nz). The results of Mendel's second set of experiments led to his second law. This is the **law of independent assortment**. It states that factors controlling different characteristics are inherited independently of each other. Review ------ 1. What was Mendel investigating with his second set of experiments? What was the outcome? 2. State Mendel's second law. 3. If a purple-flowered, short-stemmed plant is crossed with a white-flowered, long-stemmed plant, would all of the purple-flowered offspring also have short stems? Why or why not? Mendel's Laws and Genetics -------------------------- You might think that Mendel\'s discoveries would have made a big impact on science as soon as he made them. But you would be wrong. Why? Because Mendel\'s work was largely ignored. Mendel was far ahead of his time and working from a remote monastery. He had no reputation among the scientific community and no previously published work. Mendel's work, titled *Experiments in Plant Hybridization*, was published in 1866, and sent to prominent libraries in several countries, as well as 133 natural science associations. Mendel himself even sent carefully marked experiment kits to Karl von Nageli, the leading botanist of the day. The result - it was almost completely ignored. Von Nageli instead sent hawkweed seeds to Mendel, which he thought was a better plant for studying heredity. Unfortunately hawkweed reproduces asexually, resulting in genetically identical clones of the parent. Charles Darwin published his landmark book on evolution in 1869, not long after Mendel had discovered his laws. Unfortunately, Darwin knew nothing of Mendel\'s discoveries and didn't understand heredity. This made his arguments about evolution less convincing to many people. This example demonstrates the importance for scientists to communicate the results of their investigations. ### Rediscovering Mendel's Work Mendel's work was virtually unknown until 1900. In that year, three different European scientists --- named Hugo De Vries, Carl Correns, and Erich Von Tschermak-Seysenegg --- independently arrived at Mendel's laws. All three had done experiments similar to Mendel's. They came to the same conclusions that he had drawn almost half a century earlier. Only then was Mendel's actual work rediscovered. As scientists learned more about **heredity** - the passing of traits from parents to offspring - over the next few decades, they were able to describe Mendel's ideas about inheritance in terms of genes. In this way, the field of genetics was born. At the link that follows, you can watch an animation of Mendel explaining his laws of inheritance in genetic terms. ### Genetics of Inheritance Today, we known that characteristics of organisms are controlled by genes on chromosomes(see **Figure**[below](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.4/#x-ck12-QmlvLTA2LTA2LVBhcnRzLW9mLWEtY2hyb21vc29tZQ..)). The position of a gene on a chromosome is called its **locus**. In sexually reproducing organisms, each individual has two copies of the same gene, as there are two versions of the same chromosome (**homologous chromosomes**). One copy comes from each parent. The gene for a characteristic may have different versions, but the different versions are always at the same locus. The different versions are called **alleles**. For example, in pea plants, there is a purple-flower allele (*B*) and a white-flower allele (*b*). Different alleles account for much of the variation in the characteristics of organisms. ![C:\\Users\\USER\\Desktop\\mendel3.png](media/image28.png) **Chromosome, Gene, Locus, and Allele.** This diagram shows how the concepts of chromosome, gene, locus, and allele are related. What is the difference between a gene and a locus? Between a gene and an allele? During meiosis, homologous chromosomes separate and go to different gametes. Thus, the two alleles for each gene also go to different gametes. At the same time, different chromosomes assort independently. As a result, alleles for different genes assort independently as well. In these ways, alleles are shuffled and recombined in each parent's gametes. ### Genotype and Phenotype When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent. The alleles an individual inherits make up the individual's **genotype**. The two alleles may be the same or different. As shown in **Table**[below](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.4/#x-ck12-dGFibGU6Zmxvd2VyLWNvbG9yLXBlYXM.), an organism with two alleles of the same type (*BB* or *bb*) is called a **homozygote**. An organism with two different alleles (*Bb*) is called a **heterozygote**. This results in three possible genotypes. **Alleles** **Genotypes** **Phenotypes** -------------- --------------------- ---------------- *BB* (homozygote) purple flowers *B* (purple) *Bb* (heterozygote) purple flowers *b* (white) *bb* (homozygote) white flowers The expression of an organism's genotype produces its **phenotype**. The phenotype refers to the organism's characteristics, such as purple or white flowers. As you can see from **Table**[above](http://www.ck12.org/book/CK-12-Biology-Concepts/section/3.4/#x-ck12-dGFibGU6Zmxvd2VyLWNvbG9yLXBlYXM.), different genotypes may produce the same phenotype. For example, *BB* and *Bb*genotypes both produce plants with purple flowers. Why does this happen? In a *Bb*heterozygote, only the *B* allele is expressed, so the *b* allele doesn't influence the phenotype. In general, when only one of two alleles is expressed in the phenotype, the expressed allele is called the **dominant** allele. The allele that isn't expressed is called the **recessive** allele. ### How Mendel Worked Backward to Get Ahead Mendel used hundreds or even thousands of pea plants in each experiment he did. Therefore, his results were very close to those you would expect based on the rules of probability (see \"Probability and Inheritance\" concept). For example, in one of his first experiments with flower color, there were 929 plants in the F2 generation. Of these, 705 (76 percent) had purple flowers and 224 (24 percent) had white flowers. Thus, Mendel's results were very close to the 75 percent purple and 25 percent white you would expect by the laws of probability for this type of cross. Of course, Mendel had only phenotypes to work with. He knew nothing about genes and genotypes. Instead, he had to work backward from phenotypes and their percents in offspring to understand inheritance. From the results of his first set of experiments,

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