BIO 101 Course Outline PDF

COURSE OUTLINE  What is biology all about  Characteristics of living organism  Level of organization of life  Cell structure and organization  function of cellular organelles  diversity, characteristics and classifications of living things  General reproduction  interrelationship of organism...

COURSE OUTLINE  What is biology all about  Characteristics of living organism  Level of organization of life  Cell structure and organization  function of cellular organelles  diversity, characteristics and classifications of living things  General reproduction  interrelationship of organism  heredity and evolution  elements of ecology and types of habitat.  Definition of biology  The term biology derived from the Greek word βίος, bios, "life" and the suffix - λογία, -logia, "study is a science concerned with the study of life and living organisms, including their structure, function, growth, evolution, distribution, identification and taxonomy.  Despite the broad scope of modern biology, there are certain general and unifying concepts within it that govern its study and research, consolidating it into single, coherent field.  In general, biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the synthesis and creation of new species.  Branches of biology  Molecular biology studies the complex interactions among biological molecules;  Botany studies the biology of plants  cellular biology examines the basic building-block of all life, the cell;  Physiology examines the physical and chemical functions of tissues, organs, and organ systems of an organism;  Evolutionary biology examines the processes that produced the diversity of life;  Ecology examines how organisms interact  Biochemistry examines the rudimentary chemistry of life; etc.  Characteristics of Organism Made of CELLS Require ENERGY (food) REPRODUCE (species) Maintain HOMEOSTASIS ORGANIZED RESPOND to environment GROW and DEVELOP EXCHANGE materials with surroundings (water, wastes, gases)  Level of organization of life Nonliving Levels: ATOM (element) MOLECULE (compounds like carbohydrates & proteins) ORGANELLES (nucleus, ER, Golgi …) Living Levels: CELL (makes up ALL organisms) TISSUE (cells working together ORGAN (heart, brain, stomach …)  Level of organization of life Living Level cont. ORGAN SYSTEMS (respiratory, circulatory …) ORGANISM POPULATION (one species in an area) COMMUNITY (several populations in an area ECOSYSTEM (forest, prairie …) BIOME (Tundra, Tropical Rain forest…) BIOSPHERE (all living and nonliving things on Earth)  Cell Introduction All living organism are made up of small unit of protoplasm which is surrounded by a surface membrane in animals and a non-living wall in plants and bacteria.  This unit of protoplasm is termed ‘Cell’ The cell is very tiny, and can only be viewed with the aid of a microscope.  Even though it is tiny and not visible to the naked eyes, it is capable of carrying out the processes that make the organism a living thing. Definition of a cell A cell is defined as the basic structural, functional, and biological unit of all known living organisms.  It is a unit of structure and function because it serves as the building blocks of life and is also capable of performing all the characteristics of living organism.  Its discovery by Robert Hooke in 1665 was possible due to advancement in the magnification technology i.e. Development of the microscope, by Anton van Leeuwenhoek. Diagram of a typical animal cells Diagram of a typical plant cells  History of discovery and study of cell Robert Hooke( first to observe dead plant cell) In 1665, Robert Hooke used a microscope to examine a thin slice of cork (dead plant cell walls) What he saw looked like small boxes and named that structure as a cell Hooke called them “CELLS” because they looked like the small rooms that monks lived in called Cells  History cont. Leeuwenhoek ( first to observe living organisms) In 1673, Leeuwenhoek (a Dutch microscope maker), was first to view organism (living things) Leeuwenhoek used a simple, handheld microscope to view pond water & scrapings from his teeth Matthias Schleiden In 1838, a German botanist named Matthias Schleiden concluded that all plants were made of cells Schleiden is a cofounder of the cell theory  History cont. Theodore Schwann  In 1839, a German zoologist named Theodore Schwann concluded that all animals were made of cells  Schwann also cofounded the cell theory Rudolph Virchow  In 1855, a German medical doctor named Rudolph Virchow observed, under the microscope, cells dividing  He reasoned that all cells come from other pre-existing cells by cell division  Working together, Matthias Jacob Schleiden and Theodor Schwann recognized the fundamental similarities between plant and animal cells in 1839, they proposed the revolutionary theory (cell theory) that gave rise to modern biology  Cell theory The cell theory states that: All living organisms are composed of one or more cells. The cell is the most basic unit of life. All cells come from pre-existing, living cell, by biogenesis. The modern version of the cell theory includes the idea that: Energy flow (metabolism and biochemistry) occurs within cell Heredity information (DNA) is passed on from cell to cell. All cells have the same basic chemical composition in organisms of similar species.  The cell structure: Cells are of different kinds, different sizes, shapes and form. But there is one thing common among them, they all possess 1. Cell membrane that keeps the inside and outside separate. 2. DNA-containing region that holds the instructions to run the processes of life. 3. Cytoplasm: a semi-fluid region containing the rest of the cell’s machinery.  Cell structure cont. The most conspicuous structure of a cell is the nucleus found only in eukaryotic organisms. This contains the genetic material of the cell (DNA) within a chromosomes. Between the cell nucleus and the cell surface membrane is a living material known as cytoplasm which contains cells’ organelles. The organelles which perform specific functions are surrounded by one or more membrane layers.  Cell size  Cells are visible only with a microscope. However, A few types of cells such as egg cells, nerve cells in a giraffe can be seen without the aid of a microscope.  Cells are limited in size by the ratio between their outer surface area and their volume.  This means that if a cell keeps the same shape as it grows, its volume will increase more rapidly than its surface area  At some point, its surface area becomes too small to allow nutrients, oxygen, and other materials to enter the cell quickly enough to meet the cell’s needs.  Shape Cells come in a variety of shapes. This diversity reflects diversity of functions. For examample Skin cells are flat covering the body’s surface also White blood cells can change shape (leave the blood, enter the areas surrounding blood vessels, so they can do their job---attack invaders like bacteria). End of lecture one  Classification of cells Living cells are classified in to two basic classes: prokaryote, from the Greek words pro (before) and karyon (nucleus),and Eukaryote, from eu (true) and karyon (nucleus).  Prokaryotic cell  These cells were the first form of life on Earth, they are characterized by having vital biological processes and being self-sustaining.  They are simpler and smaller than eukaryotic cells, Ribosomes and lack nucleus and other membrane-enclosed structures. Inclusion capsule  Prokaryotes include two of the domains of life, Chromosome Cell wall bacteria and archaea. Cytoplasm Cell membrane  The DNA of a prokaryotic cell consists of a single Plasmid chromosome that is in direct contact with the pilus cytoplasm.  Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter even though not all of them are spherical in shape. flagellum  Prokaryotic cell organization A prokaryotic cell has three architectural regions:  Enclosing the cell is the cell envelope – generally Ribosomes consisting of a plasma membrane covered by a cell wall Inclusion which, for some bacteria, may be further covered by a capsule third layer called a capsule. Chromosome Cell wall Cytoplasm Cell membrane  Though most prokaryotes have both a cell membrane Plasmid pilus and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. flagellum  Prokaryotic cell organization cont.  Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. The genetic material is freely found in the Ribosomes cytoplasm. Prokaryotes can carry extrachromosomal Inclusion capsule DNA elements called plasmids, which are usually Chromosome Cell wall circular and encodes additional genes, such as antibiotic resistance genes. Cytoplasm Cell membrane  Though not forming a nucleus, the DNA is condensed in a nuclear region called nucleoid. pilus Plasmid  On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement flagellum and communication between cells.  The structure and function of prokaryotic cell organelles Capsules These are slimy or gummy secretions of some bacteria (gram negative) outside the cell membrane and cell wall. The capsule may be polysaccharide or polypeptide bilayer that is covalently bonded to the peptidoglycan of the cell wall. Function Unite bacteria into colonies,  enable bacteria to stick to surfaces such as teeth, mud rocks etc.  provides additional protection to the organism.  The structure and function of prokaryotic cell organelles cont. The Cell Wall This is a semi rigid structure that lies outside the cell membrane. The cell wall of bacteria is in many cases is porous, but it does not play role in regulating the entry of material in to the cells. Components  its main chemical component is called peptidoglycan (murein from murus).  Functions  Maintain shape of the organism  It prevents the cell from bursting when fluids flow into the cell by osmosis.  The outer membrane acts as a coarse sieve and exerts little control over the movement of sub- stances into and out of the cell.  The structure and function of prokaryotic cell organelles cont. Periplasmic space This is a gap between the cell wall and the cell membrane. Components The components of the periplasmic space are peptidoglycan, digestive enzymes, and transport proteins. Function Destruction of potentially harmful substances  Transportation of metabolites (byproduct of metabolism) into the bacterial cytoplasm. The structure and function of prokaryotic cell organelles cont. The Cell Membrane The cell membrane, or plasma membrane, is a living membrane that forms the boundary between a cell and its environment. Components  The main components are phospholipids and proteins. The phospholipid molecule consists of polar head that contains phosphate group and two non-polar hydrocarbons tails. These Membrane phospholipids forms a bilayer with each of the phosphate ends of the lipid molecules extending toward the membrane surface, and the fatty acid ends extending inward. The charged phosphate ends of the molecules are hydrophilic.  The fatty acid ends, consisting largely of nonpolar hydrocarbon chains, are hydrophobic (water-fearing) and form a barrier between the cell and its environment. The protein molecules that are interspersed between the phospholipids extends through the entire membrane and act as carriers or form pores or channels through which material enter and leaves the cell. Cell Membrane  Functions of cell membrane Its regulates the movement of materials into and out of a cell. In bacteria, this membrane also performs some functions carried out by other structures in eukaryotic cells. It synthesizes cell wall components, assists with DNA replication, secretes proteins, carries on respiration, and captures energy as ATP.  It also contains bases of appendages called flagella.  Finally, some proteins in the bacterial cell membrane respond to chemical substances in the environment. Cytoplasm This is a semifluid area that is bounded by the cytoplasmic or cell membrane. its constitute about four-fifth water and one fifth dissolved substances such as enzymes, proteins, carbohydrates, lipids, inorganic ions, and other suspended substances such as chromosomes and some ribosomes. Function Many chemical reactions, both anabolic and catabolic, occur in the cytoplasm. Ribosome Ribosomes consist of RNA and protein.  They are nearly spherical comprising of large and small subunits. Whole bacterial ribosomes, which are smaller than eukaryotic ribosomes, have a rate of 70S, and their subunits have rates of 30S and 50S. Functions of ribosome Ribosomes serve as sites for protein synthesis Nuclear region One of the key features differentiating prokaryotic cells from eukaryotic cells is the absence of a nucleus bounded by a nuclear membrane.  Instead of a nucleus, bacteria have a nuclear region, called nucleoid. The centrally located nuclear region (nucleoid) consists mainly of DNA, but has some RNA and protein associated with it.  Inclusions This are small bodies in Bacteria cytoplasm. Two main types are granules and vesicles. Granules(none membranous) contain substances such as such as glycogen(as energy source) or polyphosphate(for Metabolic processes) Vesicle (membrane bound) contains lipid deposit that serves as storehouses of energy and as sources of carbon for building new molecules.  Endosporesspore coat Core cortex  Vegetative cells of some bacteria(Bacillus and Clostridium) produce within cells special resistant, dormant structure that help the organism to survive.  This endospore consists of a core, surrounded by a cortex, and a spore coat, and in some species, a delicately thin layer called the exosporium. The core has an outer core wall, a cell membrane, nuclear region, and other cell components. Functions The main function is for survival  This if because they contain very little water and are highly resistant to adverse conditions such as heat, drought, acids, bases, disinfectants, radiation etc. Endospores are capable of surviving adverse environmental conditions for over 10,000 years  FLAGELLUM In addition to cell walls, many bacteria have Flagella and pili that extend from the cell membrane through the cell wall and beyond it. The Flagellum is a long, thin, and helical appendage that is made up of protein. There may be one or more flagella attached to Bactria cell wall. Function It is used for movement in most bacteria.  Pili Pili (singular: pilus) are tiny, hollow projections. Types i. long conjugation pili, or F pili whose main functions are to attach two cells in order to provide a pathway for the transfer of the genetic materials. ii. Short attachment pili, called fimbriae (fim`-bre-e; singular: fimbria) whose main function are for attachment of bacteria to surfaces.  End of class  The structure of and function of eukaryotic cell organelles Eukaryotic cell are about fifteen times wider than a typical prokaryotic cell and can be as much as thousand times greater in volume.  They have diameter of more than 10 µm, and are much larger.  Eukaryotic cell includes only the Domain Eukarya (plants, animals, fungi and protoctista)  The structure and function of eukaryotic cell organelles The main distinguishing feature is compartmentalization (i.e. the presence of membrane-bound organelles) Like prokaryote, eukaryote have three architectural regions, membrane, cytoplasm and a nucleus.  Within the cytoplasm of Eukaryotes cells are variety of internal structures that are surrounded by one or more membranes. Cell walls may or may not be present. Some possess cilia and flagella attached to their surfaces.  Cell Wall A number of unicellular eukaryotic organisms have cell wall that lack peptidoglycan that characterize bacteria. Chemical composition  Algal cell wall for example consist mainly of cellulose, chitin or both and in some polysaccharides. Arthropod such as insects and crustacean contain Chitin(polysaccharide) Protozoans have flexible external coverings called pellicles. Function Regardless of composition, cell walls give cells rigidity and protection.  Cell surface membrane The membrane is the same with prokaryotic cell membrane but contain sterols in addition. It is also less versatile than that of the prokaryotes because they do not have respiratory enzymes. Function Serve as partially permeable barriers that control the exchange between the cell and its environments.  Cytoplasm In eukaryotes, the cytoplasm makes up a relatively smaller portion of the cells. It is a semifluid substance consisting mainly of water with the same substances with prokaryote dissolved in it. In addition, this cytoplasm contains elements of a fibrous network known as cytoskeleton that gives larger cells shape and support.  Nucleus This is a large cell organelle that is enclosed by an envelope of two membranes The membrane is is perforated by nuclear pores. Within it is the genetic information(DNA)and (nucleolus) Functions The nucleus is vitally important in controlling the activities of the cell. This is because it contain the DNA (deoxyribonucleic acid) Replication of DNA also occurs in the nucleus prior to nuclear division. Nucleolus serves as site for the assemblage of protein  Nuclear envelope / nuclear pore This is a two membrane structure around the nucleus. It may be covered with ribosomes that carry out protein synthesis. Function Control the movement of substances The nuclear pore is responsible for exchange of substances such as mRNA between the nucleus and the cytoplasm.  Cytoskeleton This is scaffolding like structure that is found within the cytoplasm. It is made up of protein and compose of microfilament and microtubules.  Microfilaments are threadlike & made of ACTIN Microtubules are tubelike & made of TUBULIN Functions helps the cell maintain or change its shape Also help move organelles around  Centrioles These are situated in the cytoplasm close to the nuclear envelop in animals and simpler plants. They occur in pairs and lie at right angle to each other. Each is composed of nine groups of microtubules arranged in triplets, with each of the triplet group connected to the neighboring triplet by fibrils. Functions The main function is to assist the orientation of spindle fiber during nuclear division. Also serve as basal bodies of flagella and cilia.  Endoplasmic reticulum(ER) This is a system of tubes and flattened, membrane-bound sacs called cisternae. They may or may not have ribosomes attached to them. Functions The rough ER is concern with the transport of proteins. The smooth ER is concern with lipid synthesis, and production of steroids.  Mitochondria  These are structures found in all aerobic eukaryotic cells. Outer membrane Cristae  They are surrounded by envelop of two membranes and contain fluid filled matrix. Matrix Inner The inner membrane is folded to form membrane cristae (crista) which extend in to the Respiratory matrix. a Mitochondrion enzymes the matrix contains ribosomes, circular DNA, and phosphate granules. Functions The main functions is aerobic respiration Cristae is the site of oxidative phosphorylation and electron transport The matrix is the site of creb cycle.  Chloroplast  is surrounded by an envelope of two membranes with a jelly-like stroma through which runs a system of membranes stacked together to form grana (s. granum) That are connected through lamellae. Unlike mitochondria, chloroplasts have separate internal membranes, called thylakoids that contain chlorophyll, other pigments, enzymes and electron carriers.  It also contains within the stroma DNA that can replicate independent of the cell in which they function.  Outer Stroma membrane Inner Lamella Granum Thylakoid membrane Function It is an organelle in which photosynthesis take place, producing sugar from carbon dioxide and water using light energy trapped by chlorophyll. Light energy is also converted to chemical energy in the chloroplast.(light reaction during photosynthesis)  Ribosomes Ribosomes of eukaryotic cells, are larger than those of prokaryotic cells.  They are about 60% RNA and 40% protein. They have sedimentation rate of 80S, and their sub- units have sedimentation rates of 60S and 40S. Ribosomes are assembled in the nucleoli of the nucleus.  Golgi apparatus This consists of a stack of flattened membranous sacs. Functions  it receives substances transported from the ER, stores the substances, and alters their chemical structure.  formation of vesicles The Golgi apparatus also helps to form the plasma membrane and membranes of the lysosomes.  Lysosomes These are extremely small membrane- covered organelles made by the Golgi apparatus in animal cells.  They contain multiple kinds of digestive enzymes such as proteases, nucleases, and lipase. Functions Endocytosis (digestion of materials taken in) Autophagy(the process by which unwanted structures within the cells are engulf and digested within the lysosomes) Exocytosis (this is the release of enzyme outside the cell) Autolysis (simply self-digestion)  Peroxisomes these are small, membrane-enclosed organelles filled with enzymes. They are found in both plant and animal cells but appear to have different functions in the two kinds of cells.  In animal cells, their enzymes oxidize amino acids, whereas in plant cells they typically oxidize fats. Peroxisomes are so named because their enzymes convert hydrogen peroxide to water in both plant and animal cells.  Vacuoles In eukaryotic cells vacuoles are membrane- enclosed structures that store materials such as starch, glycogen, or fat to be used for energy.  Flagella Flagella in eukaryotes are larger and more complex.  They consist of two central microtubules and nine pairs of peripheral microtubules (a 9 2 arrangement). Associated with each pair of peripheral microtubules are small molecules of the protein dynein. two centrally located Microtubules Peripheral microtubules  Cilia  found mainly among ciliates are shorter and more numerous than flagella, but they have the same chemical composition and basic arrangement. Functions Cilia allow ciliated organisms, such as paramecia, to move much more rapidly than those with flagella. Cilia on some cells can also propel fluids, dissolved particles, bacteria, mucus, and so on past the cell. They also play role in host defenses against diseases. e.g. respiratory tract in man  Pseudopodia These are temporary projections of cytoplasm that occurs only in cells without walls, such as amoebas and some white blood cells Function Movement  The structure AND ROLE of a chromosomes IN CELL DURING DIVISION Chromosomes are the most important structures in every cell during division as they are responsible for the transmission of hereditary information from one generation the next.  It is made up of two parts known as chromatids, which are held together at a point called centromere.  chromosomes within cells exist in pairs with each of the paired chromosomes termed homologous. The number of chromosome in a cell varies with species of organisms. Fruit fly for example have eight chromosome, cats have thirty eight, dog have seventy eight. Human beings have forty six or 23 pairs with only one pair(sex or X and Y) differing in their composition and structure.  CELL DIVISION This is the process where a cell’s cytoplasm divides giving rise to another cell. Cell division involves two type of nuclear division I. Mitosis: II. meiosis.  The cell cycle This is the sequence of activities occurring between one cell division and the next. three stages of cell cycle are: i. Interphase:  synthesis of all the materials required for the functioning and growth  DNA replication takes place at this stage. ii. Mitosis: where the nucleus of the cell divides. iii.Cell division/ cytokinesis: division of the cytoplasm followed by the division of the entire cell.  Mitosis Mitosis is the process by which a cell’s nucleus divides to produce two daughter nuclei that contain identical sets of chromosomes to the parent’s cell. Mitosis although a continuous process with no distinction between the phases, is divided into interphase, Prophase, metaphase, anaphase and telophase. The stages of Mitosis Interphase i. During interphase chromosomes become loosely coiled thread-like materials(chromatin) that are difficult to see. ii. all the materials required by the cell are synthesized. I. The DNA of each chromosome also replicates. At this stage the cell is 4n (4 copies of each DNA molecule, 2 in each homologous chromosome).  prophase This is the longest phase in mitosis. I. The chromosomes become thicker and shorter as a result of tight packaging of their components. II. Centriole replicates and moves to the opposite poles of the cell and begins to produce short microtubules called asters. And III.Finally the nuclear envelop breaks away and spindle fibers are formed.  Metaphase At metaphase:  the chromosome line up around the equator of the spindle and become attached through the centromere on the spindle fiber.  ANAPHASE The spindle fiber split in two  the spindle fiber pull the daughter centromere to opposite poles.  The separated chromatids are pulled along behind the centromeres.  telophase The chromatid on reaching the poles of the cell uncoils and lengthens to form a chromatin again. Spindle fibers disintegrate, nuclear envelop reforms around the each of the polar chromosomes.  Telophase may leads directly to cytokinesis.  Cell division Cytokines This is the division of the cytoplasm or the entire cell in to two or more daughter cells. In preparation for division, the cell organelles become evenly distributed towards the two poles of the telophase cell along with the chromosomes.  In animal cells, the cell surface membrane begins to invaginate during telophase toward the region previously occupied by the spindle equator forming a furrow. The cell’s surface membranes in the furrow eventually join up and completely separate the cell.  Cell division cont. In plants, the spindle fiber disappears everywhere except the region of equatorial plane.  Here they move outward in diameter and increased in number to form a barrel shaped region called phragnoplast.  Golgi apparatus, ER, ribosomes, mitochondria, are attracted to this region and the Golgi apparatus produces a number of small fluid-filled vesicles which appear first in the center of the cells.  Cell division cont. The vesicles then fused together to form a cell plate which grows across the equatorial plane. The contents of the vesicles at this stage contribute to the new middle lamella and cell wall of the daughter cell while their membrane forms the new cell surface membrane. The spreading plate eventually fuses with the parent’s cell wall and separate the two daughter cell.  The significance of Mitosis Genetic Stability In Mitosis, the daughter cells have the same amount and type of genetic constitution as their parent.  The number of chromosomes remains the same in all the cells produced making the daughter cells retain the same characters as those of the parent cell. Growth  It is responsible for growth and development of multi-cellular organisms from a single-celled zygote. It helps the cell in maintaining proper size. It is also a method of multiplication in unicellular organisms. Regeneration Mitosis helps in:  restoring wear and tear in body tissues  replacement of damaged or lost part  healing of wounds and regeneration of detached parts (as in tail of a lizards). Asexual Reproduction new individual species can be produced by one parent organism.  Meiosis It occurs mainly during the formation of gametes (sperm and egg) in animals and spore formation in plant. In Meiosis the chromosomes number is halved from the diploid (2n) number to haploid number (n). In meiosis two cycles of nuclear divisions occur. These are: I. meiosis I where the homologous chromosomes separates and II. meiosis II where the sister chromatid separates. Interphase I During this stage the cell gets ready for meiosis by duplicating its chromosomes and centriole.  Meiosis cont. Prophases I This is the longest phase. At the beginning of this stage chromatin strand of DNA coils up in to individual chromosomes. During meiotic prophase there are three major events that occur and do not occur during mitosis. These are: These are Synapsis: Homologous chromosomes which are usually laying randomly in the nucleus pair up together point to point to form a bivalent.  This process of pairing is called synapsis. Crossing over: Once the pairing is completed, the homologous chromosomes begin to exchange the genes through a process called crossing over. Chiasma: The paired chromosomes after crossing over begin to separate back. When these chromosomes are separating with each other, they make a unique pattern with each of the cross over points still attached.  This situation is called chiasma.  At this point the chromosomes are not purely paternal or maternal but will have their gene being shuffled thus, causing genetic variation. prophase continues after these events as follows  The spindle fiber starts to form from the centriole and the centriole start to move away from the each other towards the poles of the cell. The nucleus disappears and the nuclear Envelop breaks apart.  Meiosis Cont. Metaphase I The tetrads (bivalent) line up around the equator of the spindle, and attach themselves by their centromeres. Anaphase I The fifth stage begins with the spindles fibre contracting in to the centriole. The sister chromatids remain attached but the tetrad spits up. The cell begins to elongate ready to split. Telophase I At telophase the chromosomes are now at the poles of the cell. The cells continue to elongate and is almost ready to be divided.  Nuclei and the nuclear envelop begins to appear around the chromosomes. At this stage the chromosome number is half but still having double chromatid.  Cytokines I The cell is splits into two daughter cells. The daughter cells are haploid cell because the homologous chromosome pairs Split-up during interphase I. the sister chromatids are still intact.  Meiosis II Prophase II This is similar to prophase I except that synapsis, crossing over, and chiasma do not occur. This stage is absent if interphase II is absents.  The nucleoli and nuclear envelop disappears and the chromatids shortened and thicken. The centrioles begin to move apart from each other and begin to form spindle fibre. The chromosomes begin to move towards the center of the cell. Metaphase II During metaphase two the sister chromatids are lined up separately at the metaphase plane as in metaphase I. the spindle fibre is complete and the sister chromatids are attached to it through their centromere. Anaphase II Anaphase II is a little different from the anaphase I. in anaphase II the sister chromatids detached and move away from each other.  The split chromosome begins to move towards the centrioles as the spindle fibre contracts. Cell also begins to elongate.  Meiosis II Telophase II  The chromosomes arrived at the poles of the cell. Nuclei and nuclear envelop begins to form around the chromosomes. The spindle fibre disappears, the cell are almost ready to be divided. Cytokines II The cell spits and there are four different cell created from the original. Each cell is the haploid cell with the different single set of the chromosomes.  Significance of meiosis Sexual reproduction This restores the normal diploid state of the cell otherwise it will lead to doubling for the chromosome for each successive sexual reproduction generation.. Genetic variation Meiosis also provides opportunity for new combination of genes to occur in the gametes. This leads to genetic variation in the offspring produces by the fusion of the gametes.