Introduction to 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 contains a specific number of chromosomes.

- There are two types of cell division: Mitosis and Meiosis

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 remain the same as that in the mother cell.
- 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.

- During 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

- 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.
- 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 build up 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 fertilisation.
• It brings about new gene combinations that lead to genetic variation in the offsprings.