Cell division is a process which leads to cell multiplication. It occurs in both plants and animals. Original cells which undergo division are known as parent

1

CELL DIVISION

Cell division is a process which leads to cell multiplication.

It occurs in both plants and animals. Original cells which undergo division are known as parent

cells and the new on ones resulting from division are known as daughter cells.

Ther

e are two types of cell division i.e

.

Mitosis (mitotic cell division) which occurs in somatic

cells

leading to growth and developments

and Meiosis (meiotic cell division) which occurs in

reproductive cells

leading to production of gametes (sex cells).

MITO

SIS

Each of us began as a single cell, so one important question is:

How did that single cell develop into a body with more than a trillion cells?

The production of such a large number of body cells is accomplished by many, many repeats of a cycle of

cell

division

in which one cell divides to form two cells, then each of these cells divides resulting in a

total of four cells, etc. Thus, repeated cell division is needed for growth and development.

The type of cell division that produces almost all the cell

s in our bodies is called

mitosis

.

Mitosis

is the cell division in which two identical daughter cells are produced by the division of one parent cell.

Each of the daughter cells needs to have a complete set of chromosomes

containing an exact

copy of all

the DNA in the original cell.

Stages of mitosis

1)

Interphase (resting stage of the parent cell) During this stage the following happens to

prepare a cell for nuclear division)

In this stage the cell builds up energy reserve in form of ATP

It also builds up f

ood/nutrient reserve

Replication of DNA also takes place in the chromosomes. i.e the amount of DNA is doubled

There is synthesis/replication of new cell organelles/structures eg mitochondria, endoplasmic

reticulum, centrioles, chloroplasts etc

2

Early pr

ophase



Chromosomes become visible as long thin entangled threads in the nucleus.



The nucleolus begins to shrink and centrioles move to the opposite ends of the cell

Late prophase



Chromosomes shorten and they can be seen to comprise of 2 chromatid joined

at the

centromere



Nucleolus disappears



Nuclear membrane breaks up and disappears

Early metaphase

3



Spindle fibres are formed. They radiate from the centrioles at the opposite poles



Chromosomes arrange them selves at the centre of the spindle or equatoria

l plane

Late metaphase

Spindle fibres contract and shorten, separating the

sister

chroma

t

ids

of each chromosome

slightly

at the centromere.

A

naphase

Sister c

hromatids

of each chromosome

begin to move to the opposite poles/ends of the cell

and

reach their

respective poles.

This movement is facilitated by the contraction/shortening of the

spindle fibres.

4

Early telophase



Chromatids become chromosomes after reaching their poles



The cell membrane starts to constrict at the centre so that the cell is divide

d into two

daughter cells.

Late telophase.



Constriction of the cell membrane is completed



Nuclear membrane and nucleolus reform in each cell



Spindle fibres disappear



Chromosomes become invisible thread like structures and each cell enters the inter phase.

Introduction to meiosis

Meiosis is a type of cell division which involves reduction in the amount of genetic material and

production of reproductive cells (eggs and sperm).

Meiosis is very similar to mitosis. The major difference is that meiosis consis

ts of two divisions,

meiosis I and meiosis II. In meiosis I, homologous chromosomes forms pair at the metaphase

plate, and then the homologues chromosomes migrate to opposite poles. In meiosis II, the