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:
1. Sexual
2. 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 contains a specific number of chromosomes.
There are two types of cell division:
1. 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.
2. 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:
a. Prophase
b. Metaphase
c. Anaphase
d. 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.
a. 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.
b. 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.
c. 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.
d. 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
1. It brings about the growth of an organism
2. It brings about asexual reproduction.
3. Ensures that the chromosome number is retained.
4. 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
a. Prophase
b. Metaphase
c. Anaphase
d. 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
a. 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.
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:
1. Sexual
2. 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 contains a specific number of chromosomes.
There are two types of cell division:
1. 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.
2. 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:
a. Prophase
b. Metaphase
c. Anaphase
d. 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.
a. 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.
b. 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.
c. 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.
d. 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
1. It brings about the growth of an organism
2. It brings about asexual reproduction.
3. Ensures that the chromosome number is retained.
4. 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
a. Prophase
b. Metaphase
c. Anaphase
d. 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
a. 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.
