(Narrator) Every living thing is dependent for its growth
on the division of cells called Mitosis.
In the laboratory we can learn what Mitosis really is
and how it works.
We see through the microscope that all living matter
is made up of tiny units called cells.
Cells come in a variety of shapes,
some regular, others irregular.
In the higher forms of life a number of cell types are found.
For example human beings have muscle cells,
bone cells,
blood cells
and nerve cells, to mention only a few.
By way of contrast the amoeba
is a single living cell which is able to feed,
grow, and even to reproduce when conditions are suitable.
Although cells are found in a wide variety of shapes and types
they’re essentially similar.
Each cell is separated from its neighbours
by a wall or a membrane.
Living cells have a nucleus
enclosed by a nuclear membrane.
The nucleus contains a number of chromosomes.
In this section of a pine leaf only the cell walls remain.
In some cells it is possible to stain the nucleus or even the chromosomes.
The number of chromosomes in each cell is important.
Different species have their own particular chromosome number.
The lowest is in the parasite ascaris
which has only two chromosomes,
ranging to several hundred in some deep sea crustacea and in some plants.
Maize has a chromosome number of 20.
Wheat has 42.
The chromosome number is 46 for man,
66 for horses.
Dogs, 78.
And sheep, 54.
Usually the chromosome number is the same for all cells of a body
and for all individuals of a species.
Man has a chromosome number of 46
whether he is black or white,
Indian or Eskimo,
giant or pygmy.
How is such constancy in chromosome number maintained?
Why is the chromosome number constant for a particular species?
The answer lies in the mechanism by which cells divide, called Mitosis.
Old cells divide to form new cells
and each new cell is an image of the parent cell.
Growth of an individual or a population
depends on this continuing process.
The division of cells is accompanied by division
of the nucleus and the chromosomes it contains.
It is in the division of the chromosomes
that we find the essential feature of Mitosis.
Each chromosome divides to form two daughter chromosomes
which are identical. Each pair of daughter chromosomes
separates into opposite daughter cells
and consequently each daughter cell contains a set of chromosomes
identical to those of the parent cell.
Let us follow the course of a mitotic division
using a schematic cell with simplified chromosomes.
First the chromosomes thicken and contract,
the nuclear membrane disappears and a spindle is formed.
The chromosomes become attached to the spindle at their centromeres.
The chromosomes are drawn towards the centre of the spindle
and the centromeres now divide.
The two daughter chromosomes separate and move to opposite poles.
The spindle disappears and nuclear membranes
form around each group of chromosomes.
Finally a new cell wall divides the original cell into two daughter cells,
each an image of the parent cell.
The parent cell had two pair of chromosomes
making four in all.
Likewise each daughter cell contains four chromosomes.
The chromosomes are undiminished in both quantity and quality.
Mitosis is in fact very complex
and we’ll now attempt to show it more realistically.
When cells are in the resting stage
the individual chromosomes can’t be detected until Mitosis begins.
The first sign of Mitosis is the appearance of the chromosomes
as long thin threads.
Their double structure is already obvious
some time before the formation of the spindle.
First the chromosomes thicken and contract.
The nuclear membrane disappears and a spindle is formed.
The chromosomes become attached to the spindle at their centromeres.
The chromosomes move to the equator of the spindle
and the centromeres divide.
After division each original parent chromosome
consists of two identical daughter chromosomes
which now move to opposite poles.
The spindle disappears and nuclear membranes
form around each group of chromosomes.
The original cell divides into two daughter cells
and the individual chromosomes become difficult to distinguish
as the cell enters the resting stage.
The cells absorb nutrient, grow, divide again,
and so the process of growth continues.
It is important to remember that in all mitotic divisions
each chromosome divides to form two daughter chromosomes
which are identical.
Each one of a pair of daughter chromosomes
separates into opposite cells
and consequently each daughter cell contains a set of chromosomes
identical to those of the parent cell.
Apart from oddities the chromosome number
is the same for all cells of a body
and usually the same for all individuals of a species.
Mitosis is the process by which cells divide.
New cells originate only by division of existing cells.
So Mitosis is the basis of life and growth.