Secondary Biology, Reproduction in Plants and

AnimalsCell division; Mitosis

By Mr. Kanyoro Peter

Introduction Reproduction is the process by which mature

living organisms produce young ones of their kind/offspring.

It’s of two types;

Sexual reproduction – involves fusion of sex cells (male and female gametes) in a process called fertilization.

Asexual reproduction – where parts of mature organisms develop into new individuals in several mechanisms.

Sexual (a) and Asexual (b) reproduction

Importance of reproduction Leads to procreation hence

perpetuation/survival/sustenance of species so that it dose not become extinct

Helps in improvement of quality of species through exchange of genetic material during crossing over hence variation and adaptability.

Helps to maintain all the characteristics of the parent as in asexual reproduction.

Cell division

Organisms start as a single cell; spore or zygote

Cells of multicellular organisms divides into two and the division continues in multiples of two.

Cells of unicellular organisms divide into separate daughter cells which develop into adult individuals.

Cell division starts with division of nucleus (karyogamy) – chromosomes and then cytoplasm (cytogamy)

There are two types of cell division; mitosis and meiosis

Chromosomes Are long thread-like structures found in the nucleus

which become visible with microscope only during cell

division.

Each somatic cell (body cell) has a fixed diploid (double

number) of chromosomes (also referred to as “2n”

constitution) e.g. 23 pairs for human, 4 pairs for

Drosophila, 20 pairs for mouse and 7 pairs for garden

pea.

Each chromosome is made of two parallel strands called

chromatids. Each chromatid pair is connected at a

point by centromere.

Structure of Chromosomes

Homologous chromosomes

This is a pair of chromosomes with a

characteristic length i.e. have a similar

appearance though their genetic

composition may be different.

Each parent gives one pair of such

chromosomes during sexual reproduction.

Homologous chromosomes

Genes

Located along the length of chromosome.

Are basic units of heredity that are transferred from a

parent to offspring.

Are made of protein chemical substance called

Deoxyribonucleic acid (DNA), which contain coded

information determine the characteristics of the

offspring e.g. whether male or female.

Mitosis The type of reproduction in which a cell divides into

two daughter cells each with the same number of chromosomes as the parent cell.

Importance of Mitosis

Functions of mitosis

For simple organisms, it is a means of asexual reproduction. (For example cells of fungus)

In multicellular organisms, it allows for entire cell to grow by forming new cells and replacing older cells.

In certain species, mitosis is used to heal wounds or regenerate body parts.

Phases of mitosis It is a continuous process, divided into four

phases;

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

In order to remember the phases easily, we can use the acronym P-MAT

However, prior to cell division (P-MAT), the cell spends a lot of time in INTERPHASE where it undergoes intense activities in preparation to division.

Interphase

During interphase;

The genetic material multiplies so that

daughter cells have the same number of

chromosomes as the parent cell.

New cell organelles are synthesized e.g. golgi

apparatus, centrioles, mitochondria and

ribosomes.

Enough energy stores in form of ATP

(adenosine triphosphate) is build up

Phases of the Cell Cycle

Interphase- phase in which cells spends most time performing functions that makes it unique.

Mitosis- phase of cell during which cell divides into two daughter cells.

Stages

Prophase

In animal cells, duplicated

microscopic bodies called

centrioles separate and move

to opposite sides of the cell.

These are not present in most

plant or fungal cells.

Spindle fibres begin to form.

Nuclear membrane begins to

break down and nucleolus

disappear.

Chromosomes thicken and

shorten and become visible

through staining.

Metaphase

Nuclear membrane disappears hence chromosomes are found free in cytoplasm

Spindle fibres lengthen (they attach to centrioles by at both poles in animal)

Chromosomes align themselves at equatorial plate (equator of spindle).

Each pair is connected at the centromere where the spindle fiber is attached.

Anaphase

Spindle fibres shorten, chromatids move apart (separate) from each other and move to the opposite poles.

The spindle apparatus begin to disappear.

The cell membrane in animal cells begins to constrict towards the end.

Telophase Chromosomes

finally arrive/collect together at the opposite poles of the cell.

Nuclear membrane forms around set of chromosomes

Cytoplasm divides into two leading to the formation of two daughter cells.

Chromosomes become less distinct.

Cytokinesis

Process in which cytoplasm divides into two separate cell forms.

In animal cells, it begins with the formation of a furrow in the center of the cell which constricts the cell membrane.

The membrane begins to pinch into cytoplasm and formation of two cells begins. (Process referred to as cell cleavage).

Plant cell Final Stage Cleavage doesn’t

take place.

New cell wall is assembled at center of cell by formation of middle lamellae and separation of two daughter cells occurs.

The End;thanks for

walking with me.