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REPRODUCTION IN PLANTS AND ANIMALS Introduction The process by which mature individuals produce offspring is called reproduction. Reproduction is a characteristic of all living organisms and prevents extinction of a species. There are two types of reproduction: sexual and asexual reproduction. Sexual reproduction involves the fusion of male and female gametes to form a zygote. Asexual reproduction does not involve gametes. Cell Division Cell division starts with division of nucleus. In the nucleus are a number of thread-like structures called chromosomes, which occur in pairs known as homologous chromosomes. Each chromosome contains-genes that determine the characteristics of an organism. The cells in each organism contain a specific number of chromosomes. There are two types of cell division: Mitosis This takes place in all body cells of an organism to bring about increase in number of cells, resulting in growth and repair. The number of chromosomes in daughter cells remains the same as that in the mother cell. Meiosis This type of cell division takes place in reproductive organs (gonads) to produce gametes. The number of chromosomes in the gamete is half that in the mother cell. Mitosis Mitosis is divided into four main stages. Prophase, Metaphase, Anaphase and Telophase. These stages of cell division occur in a smooth and continuous pattern. Interphase The term interphase is used to describe the state of the nucleus when the cell is just about to divide. During this time the following take place: Replication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell. Division of cell organelles such as mitochondria, ribosomes and centrioles. Energy for cell division is synthesised and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process. interphase, the following observations can be made: Chromosomes are seen as long, thin, coiled thread-like structures. Nuclear membrane and nucleolus are intact. Prophase The chromosomes shorten and thicken. 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 Spindle fibres lengthen. In animal cells they attach to the centrioles at both poles. Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres. Chromatids begin to separate at the centromere. Anaphase Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres. Chromatids becomes a chromosome. In animal cell, the cell membrane starts to constrict. Telophase The cell divides into two. In animal cells it occurs through cleavage of cell membrane. In plants cells, it is due to deposition of cellulose along the equator of the cell.(Cell plate formation). A nuclear membrane forms around each set of chromosome. Chromosomes later become less distinct. Significance of Mitosis It brings about the growth of an organism: It brings about asexual reproduction. Ensures that the chromosome number is retained. Ensures that the chromosomal constitution of the offspring is the same as the parents. Meiosis Meiosis involves two divisions of the parental 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 and not chromatids separate. Each division has four stages Prophase, Metaphase, Anaphase and Telophase. Interphase As in mitosis the cell prepares for division. This involves replication of chromosomes, organelles and buildup of energy to be used during the meiotic division. First Meiotic division Prophase I Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents. Chromosomes shorten and thicken hence become more visible. Chromosomes may become coiled around each other and the chromatids may remain in contact at points called chiasmata (singular chiasma). 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. 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 Each chromosome is seen as a pair of chromatids. Metaphase II Spindle forms 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. 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 fertilization. It brings about new gene combinations that lead to genetic variation in the offspring. See also BIOLOGY PRACTICAL ACTIVITIES HUMAN DISEASES (PARASITIC DISEASES) HUMAN DISEASES – BACTERIAL DISEASES POLLUTION HYDROPHYTES (WATER PLANTS)