REPRODUCTION IN PLANTS AND ANIMALS
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 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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 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.
- 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
- 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.
- Spindle fibres are fully formed and attached to the centromeres.
- The bivalents move to the equator of the spindles.
- 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.
- 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.
- Each chromosome is seen as a pair of chromatids.
- Spindle forms and are attached to the chromatids at the centromeres.
- Chromatids move to the equator.
- Sister chromatids separate from each other
- Then move to opposite poles, pulled by the shortening of the spindle fibres.
- 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.
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