General Biology 1 - Past Notes PDF

**COURSE TITLE: GENERAL BIOLOGY 1** **COURSE CODE: BIO 101** **TOPIC: General Reproduction** **OBJECTIVE:** At the end of the lesson, the student is expected to understand - The principle of General reproduction. - Differentiate between the two types of reproduction - List the advantages...

**COURSE TITLE: GENERAL BIOLOGY 1** **COURSE CODE: BIO 101** **TOPIC: General Reproduction** **OBJECTIVE:** At the end of the lesson, the student is expected to understand - The principle of General reproduction. - Differentiate between the two types of reproduction - List the advantages and disadvantages of the two types of reproduction - Discuss cell division and stages involved 1. Discuss reproduction in animals and plants and unicellular organisms together to investigate shared mechanisms 2. Describe the reproductive system of both man and woman 3. Discuss the processes involved in the development of the embryo after fertilization has taken place. We cannot begin to discuss reproduction without introducing the very science of biology. Biology is the study of living things and by living things we mean man, animals and plants. According to *Carolus linneaus* (1707-1778) who introduced the Binomial system of classification, living things are classified into 7 distinct groups (Kingdom, Phylum, class, order, family, genus, specie). They can also be classified based on their cellular structures. Every living thing is made of small box like structures which we call cells and they are divided into two: Prokaryotes and Eukaryotes. The cell is the origin of life and has greater significance in the characteristics of every living thing. Reproduction, one of the vital characteristics is a process by which an organism gives rise to an offspring that is biologically similar to the parent organism. This process is seen in all living organisms- both [plants](https://www.toppr.com/guides/biology/life-processes/nutrition-in-plants/) and [animals](https://www.toppr.com/guides/biology/diversity-in-living-organisms/animal-kingdom/). It ensures the continuity of species, generation after generation and is the main characteristic of life on earth. There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and reproduce an offspring. In lieu of this, the organism's habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. 1. Asexual Reproduction 2. Sexual Reproduction *Asexual reproduction refers to the type of reproduction in which only a single organism gives rise to a diploid new individual by a process of cell division called mitosis. It involves one individual which divides into two or more portions or cut off bits of its body to produce new individuals. It* does not involve the fusion of gametes. Therefore, offspring produced are genetically identical to the parent (clones). The organisms produced by asexual reproduction are less diverse in nature. It is practiced mostly by unicellular organisms. There is rapid population growth and no mate is required for the process. However, a lack of genetic diversity makes organisms more susceptible to diseases and nutritional deficiencies. Asexual reproduction is further divided into: 1. **Binary Fission: ** The cell splits into two. Each cell carrying a copy of the DNA from the parent cell. This kind of asexual reproduction is seen in prokaryotes and single celled organism. The circular DNA begins to replicate and the original and replicating DNA are attached to different parts of the cell membrane, at same time it begins to grow in size and divide to form an identical daughter cell eg. Amoeba, Bacteria 2. **Budding: ** A small bud-like outgrowth gives rise to a new individual. The outgrowth remains attached to the organism until it is fully grown. It detaches itself and lives as an individual organism. eg. hydra 3. **Fragmentation: ** The parent organism splits into several parts and each part grows into a new individual. eg. Planaria 4. **Sporogenesis: **In this type of reproduction, a new organism grows from the spores. These can be created without fertilization and can spread through wind and animals e.g Fungi 5. **Parthenogenesis --** In this type of asexual reproduction, the female organism produces eggs without fertilization, and offspring are created. Lizards, some fishes, and insects are the common examples of Parthenogenesis asexual reproduction. 6. **Gemmules --** In this type, the parents release a specialized mass of cells that develop into offspring. These gemmules can be formed at a time when parents feel harsh environmental conditions around them. 7. **Regeneration --** It is the formation of the whole body of an organism from the small segment (morphallaxis) or the replacement of the lost part (epimorphosis). It is mainly found in amoeba, hydra, sponge, planaria, and many more organisms. 8. **Vegetative reproduction**: The formation of new plants from the vegetative parts of the plant called vegetative propagules is called vegetative propagation or reproduction. Only one parent is involved, it doesn't involve meiosis and fusion of gametes, the offspring are genetically similar to the parents. Asexual reproduction - Spore Formation  Budding-REPRODUCTION-IN-HYDRA-BY-BUDDING.png **NB. Please find out other diagrams representing the other types of asexual reproduction** +-----------------------------------------------------------------------+ | **Advantages Of Asexual Reproduction** | +=======================================================================+ | - The process requires less energy. | +-----------------------------------------------------------------------+ | - It takes place in various environments. | +-----------------------------------------------------------------------+ | - It allows for the survival of species since a higher number of | | offspring is produced. | +-----------------------------------------------------------------------+ | - A single organism can develop a colony. | +-----------------------------------------------------------------------+ | - All the positive traits of the species are transferred to future | | generations. | +-----------------------------------------------------------------------+ | - The organisms mature rapidly. | +-----------------------------------------------------------------------+ **Disadvantages of asexual reproduction** - Lack of genetic variation leading to lack of diversity - Lowered resistance to disease - Loss of hybrid vigor - Competition for resources due to overcrowding - They have short lifespan +-----------------------------------------------------------------------+ | - Since the offspring is an exact copy of the parent, any negative | | mutation will also pass on to the offspring. | +-----------------------------------------------------------------------+ - It is difficult to control the increasing population - The organisms produced cannot adapt to the changing environments **SEXUAL REPRODUCTION** Sexual reproduction involves the formation of male and female gametes, either by the same individual or by different individuals of opposite sex. These formed gametes then fuse to form the zygote, which develops into a new organism. It is a complex and slow process compared to asexual reproduction. The offspring are not identical to the parents or between them. The process of sexual reproduction is fundamentally similar in all organisms. However, the structures associated with sexual reproduction are quite different. Sexual reproduction is considered a better mode of reproduction as there is a variation in the [population](https://www.toppr.com/guides/geography/population/), along with the evolution of species. Genetically speaking, sexual reproduction involves both the process of [mitosis](https://www.toppr.com/guides/biology/cell-cycle-and-cell-division/mitosis/) and meiosis. All organisms go through general growth before reproductive growth. General growth is called the juvenile phase and in the case of plants, the vegetative phase.  **In Flowering Plants:** In sexual reproduction in plants, the reproductive organ is the flower. The flower produces both the male and female reproductive parts. Stamens (Androecium) which produce pollen are the male part and contain male sex cells. Each stamen (Androecium) has two parts. The upper part is known as anther which bears pollen and is held on the lower part known as the filament. The pistil (Gynoecium) is the female part and its basal part is the ovary carrying eggs or ovules or female sex cells. Its parts include the stigma, style and ovary. In most plants, each flower bears both male and female parts and are called bisexual while in some plants there are male flowers with only androecium and female flowers bearing only gynoecium and are termed as unisexual. Fertilization is said to occur when the pollen and ovule are fused together by agents like the wind, water, and insects which brings about the transfer of pollen grain from anther to the stigma of a flower (pollination). Self pollination is when pollen of a typical flower falls on its own stigma and fertilizes the ovule. In Cross pollination pollen from one flower falls on the stigma of another flower of a different plant of the same species and then fertilizes the ovule of that flower. The fertilized ovules develop into seeds which are capable of germinating into fruits and new plants. **IN ANIMALS** The juvenile phase is the phase of growth and development till the animal reaches its maturity, after which it can reproduce sexually. In different animals, there are some cyclical changes that occur during reproduction. In animals such as cows, dogs, deer, sheep etc., it is called the Oestrus cycle and in mammals like humans, apes, and monkeys, it is called the Menstrual cycle. Old age often signifies the end of the reproductive phase. For the sake of simplicity, these sequential events can be examined as three different stages, namely pre-fertilization, fertilization, and post-fertilization.  - ### Pre-Fertilization Events 1. **Gametogenesis** **2. Gamete transfer** - ### Fertilization - ### Post-Fertilization Events 1. **The formation of the zygote** 2. **Embryogenesis** ### Advantages of Sexual Reproduction - The offsprings produced through sexual reproduction has greater adaptability and survival changes due to its variations.  - Since there is a fusion of male and female gametes in sexual reproduction, the offspring produced are genetically diverse.  - It leads to the evolution of new species.  - It increases the variation in the population due to the genetic diversity, which is not possible in asexual reproduction as there is no fertilization **CELL DIVISION** 044c145168c30498964000f01b297e818ccb6785.png The sequence of events leading to cell division in plants and animals is basically the same. There are two types of cell division namely *mitosis* and *meiosis.* [Mitosis](https://peda.net/kenya/css/subjects/biology/form-three/ripaa/mitosis2#top) ==================================================================================== This is a type of cell division whereby a cell divides into two daughter cells each having the same number of chromosomes as the parent cell. It occurs in body (somatic) cells of an organism in order to increase number of cells involved in growth and repair. Mitosis is usually described as a series of continuous stages which do not interfere with the activities of the next stage. The stages include **Interphase, Prophase, Metaphase, Anaphase and Telophase.** ============================================================================================================================================================================================================================================================================================================================================================================================================================================================================== **Interphase (S Stage or Preparatory Stage)** The term Interphase is used to describe the state of the nucleus when the cell is just about to divide. Chromosomes are seen as long, thin, coiled thread-like structures while the nuclear membrane and nucleolus remain intact (Chromatin). During this time the following take place: - Multiplication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell (Chromatin Duplication). - Manufacture of cell organelles such as mitochondria, golgi bodies, centrioles, ribosomes and centrioles. - Energy for cell division is synthesized and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process. **Prophase (M Phase or Dividing Phase)** - Nucleus dissolves - The chromatin coils, shorten and thicken into chromosomes or chromatids. - Each chromosome is seen to consist of a pair of chromatids joined at a point called centromere. - Centrioles (in animal cells) separate and move to opposite poles of the cell. - The centre of the nucleus is referred to as the equator. - Spindle fibres begin to form, and connect the centriole pairs to the opposite poles. - The nucleolus and nuclear membrane disintegrate and disappear. **Metaphase** - The nuclear membrane disappears hence chromosomes are free in the cytoplasm. - The spindle fibres lengthen. In animal cells they attach to the centrioles at both pole - Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres. **Anaphase** - Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres. - The spindle apparatus begins to disappear. - In animal cells, the cell membrane starts to constrict towards the end of the anaphase. **Telophase** This is the final phase of cell division - The chromatids collect together at the two opposite ends of the spindle. - A nuclear membrane forms around each set of chromatids and are now referred to as chromosomes. - The cytoplasm divides into two leading to the formation of two daughter cells. - Chromosomes later become less distinct. **Significance of Mitosis** - It brings about the growth of an organism: - It is a basis of asexual reproduction. - Ensures that the chromosome number is retained. - Ensures that the chromosomal constitution of the offspring is the same as the parents. [**Meiosis**](https://peda.net/kenya/css/subjects/biology/form-three/ripaa/meiosis#top) Meiosis involves two divisions of the parent cell resulting into four daughter cells. The mother cell has the diploid number of chromosomes. The four cells (gametes) have half the number of chromosomes (haploid) that the mother cell had. In the first meiotic division there is a reduction in the chromosome number because homologous chromosomes separate. Each division has four stages; *Prophase, Metaphase, Anaphase and Telophase.* This type of cell division takes place in reproductive organs (gonads) to produce gametes. **Interphase**\ As in mitosis, the cell prepares for division. This involves replication of chromatin, organelles and build up of energy to be used during the meiotic division. **First Meiotic division\ Prophase I** - Chromatins become coiled around each other and remain in contact at points called chiasmata. - Chromosomes shorten and thicken hence become more visible. - Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents or tetrads. - Chromatids cross-over at the chiasmata exchanging chromatid portions. Important genetic changes usually result. **Metaphase I** - Spindle fibres are fully formed and attached to the centromeres. - The bivalents move to the equator of the spindles. **Anaphase I** - Homologous chromosomes separate and migrate to opposite poles. - This is brought about by shortening of spindle fibres, hence pulling the chromosomes. - The number of chromosomes at each pole is half the number in the mother cell. **Telophase I** - Cytoplasm divides to separate the two daughter cells (Cytokinesis).  **Second Meiotic Division\ **Usually, The two daughter cells go into a short resting stage (interphase) But, sometimes the chromosomes remain condensed and the daughter cells go straight into metaphase of second meiotic division. The second meiotic division takes place just like mitosis.\ \ **Prophase II** - chromosomes condense and the nuclear envelope breaks down, if needed. - The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes. - Each chromosome is seen as a pair of chromatids. **Metaphase II\ ** Spindle fibre is formed and are attached to the chromatids at the centromeres.\ Chromatids move to the equator.\ \ **Anaphase II\ ** Sister chromatids separate from each other\ Then move to opposite poles, pulled by the shortening of the spindle fibres.\ \ **Telophase II**\ The spindle apparatus disappears.\ The nucleolus reappears and nuclear membrane is formed around each set of chromatids.\ The chromatids become chromosomes.\ Cytoplasm divides and four daughter cells are formed.\ Each has a haploid number of chromosomes. 9a74225bd24998f2c863366b8b57d238b9b9b653.png\ \ **Significance of Meiosis**\ Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells.\ It helps to restore the diploid chromosomal constitution in a species at fertilisation.\ It brings about new gene combinations that lead to genetic variation in the offsprings. [The differences between mitosis and meiosis](https://peda.net/kenya/css/subjects/biology/form-three/ripaa/tdbmam#top) ====================================================================================================================== **Mitosis** **Meiosis** ---------------------------------------------- -------------------------------------------------------- Maintenance of chromosome number (diploid) Reduction/halving of chromosomes (haploid) Takes place in somatic cells/growth Occurs in reproductive cells/gonads/produces gametes No crossing over/no variations Crossing over takes place/variation occurs Results into 2 daughter cells Results into 4 daughter cells No pairing/no synapsis/no bivalent formed There is paring/synapsis/bivalent A one division process of four stages A two division process of four stages each **Similarities between mitosis and meiosis**\ both occur in cells\ both involve division of cells (cell multiplication)