Mitosis (1958)

[The Commonwealth Scientific and Industrial Research Organization Presents, appears on screen. Image changes to show one cell dividing into two with the words Mitosis, how cells divide and multiply overlayed. Produced by the CSIRO Film Unit in collaboration with the Animal Genetics Section. Individual credits roll. Image pans across animated illustration of flora and fauna, people and buildings.]

Narrator: Every living thing is dependent for its growth on the division of cells called Mitosis.

[Image shows close up of animated illustration of a laboratory]

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.

[Image shows close up of a group of cells. Images of various cell structures]

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,

[Image shows cells of a muscle]

bone cells,

[Image shows cells of a bone]\

blood cells

[Image shows blood cells]

and nerve cells,

[Image shows nerve cells]

to mention only a few.

[Image shows an amoeba]

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.

[Image shows various cells]

Although cells are found in a wide variety of shapes and types they’re essentially similar.

[Images continue to change to show different types of cells]

Each cell is separated from its neighbours by a wall or a membrane.

Living cells have a nucleus

[Image shows the nucleus of a cell]

enclosed by a nuclear membrane.

[Image shows the membrane of a cell]

The nucleus contains a number of chromosomes.

[Image shows chromosomes inside a cell]

In this section of a pine leaf

[Image shows cells inside a pine leaf]

only the cell walls remain. In some cells it is possible to stain the nucleus or even the chromosomes.

[Image shows chromosomes inside cells. Image changes to show chromosomes inside a single cell]

The number of chromosomes in each cell is important. Different species have their own particular chromosome number.

[Image changes to show a different cell]

The lowest is in the parasite ascaris

[Images changes to show long thin threads]

which has only two chromosomes, ranging to several hundred in some deep sea crustacea

[Image changes to animated picture of a lobster]

and in some plants.

Maize has a chromosome number of 20.

[Image shows animated picture of maize field]

Wheat has 42.

[Image changes to animated picture of wheat field]

The chromosome number is 46 for man, 66 for horses.

[Image changes to animated picture of man on horse]

Dogs, 78.

[Image pans down to picture of dog next to man on horse]

And sheep, 54.

[Image pans across to animated picture of flock of sheep]

Usually the chromosome number is the same for all cells of a body

[Image changes to close up of sheep]

and for all individuals of a species.

[Image shows animated picture of flock of sheep. Image changes to animated picture of man]

Man has a chromosome number of 46 whether he is black or white,

[Image pans out to show group of different nationalities]

Indian or Eskimo, giant or pygmy.

[Image returns to animated picture of man on horse with dog and flock of sheep]

How is such constancy in chromosome number maintained? Why is the chromosome number constant for a particular species?

[Image shows close up of animated horse, then changes to close up of man, then close up of sheep]

The answer lies in the mechanism by which cells divide, called Mitosis.

[Image shows animated picture of a single cell]

Old cells divide to form new cells and each new cell is an image of the parent cell.

[Image shows single cell dividing into two]

Growth of an individual or a population depends on this continuing process.

[Image shows a single cell dividing into two cells]

The division of cells is accompanied by division of the nucleus and the chromosomes it contains.

[Image returns to show a single cell]

It is in the division of the chromosomes

[Image shows close up of single cell with chromosomes inside]

that we find the essential feature of Mitosis.

Each chromosome divides to form two daughter chromosomes

[Image shows chromosomes splitting into two]

which are identical.

Each pair of daughter chromosomes separates into opposite daughter cells

[Image shows chromosomes moving away from each other]

and consequently each daughter cell contains a set of chromosomes

[Image shows two cells each with four chromosomes]

identical to those of the parent cell.

[Image changes to show a different cell with chromosomes]

Let us follow the course of a mitotic division using a schematic cell with simplified chromosomes.

[Image changes to show close up of chromosomes inside cell]

First the chromosomes thicken and contract, the nuclear membrane disappears and a spindle is formed.

[Image shows membrane disappearing and spindles forming around chromosomes]

The chromosomes become attached to the spindle at their centromeres.

[Image shows chromosomes moving towards centre of spindle]

The chromosomes are drawn towards the centre of the spindle and the centromeres now divide.

[Image shows centromeres dividing]

The two daughter chromosomes separate and move to opposite poles.

[Image shows chromosomes moving apart]

The spindle disappears and nuclear membranes form around each group of chromosomes.

[Image shows lines of spindle disappearing and membranes appearing around each group of chromosomes]

Finally a new cell wall divides the original cell into two daughter cells,

[Image pans out to show two cells inside two cell walls]

each an image of the parent cell. The parent cell

[Image changes to show one cell]

had two pair of chromosomes making four in all.

[Image changes back to show two separate cells with four chromosomes in each]

Likewise each daughter cell contains four chromosomes. The chromosomes are undiminished in both quantity and quality.

[Image changes to show different image of a single cell]

Mitosis is in fact very complex and we’ll now attempt to show it more realistically.

When cells are in the resting stage

[Image pans in to close up of the inside of the cell]

the individual chromosomes can’t be detected until Mitosis begins.

The first sign of Mitosis

[Image shows long thin threads]

is the appearance of the chromosomes as long thin threads. Their double structure is already obvious some time before the formation of the spindle.

[Image changes to show long thin threads inside a cell]

First the chromosomes thicken and contract.

[Image shows chromosomes moving and thickening]

The nuclear membrane disappears

[Image changes to show membrane disappearing and a spindle forming]

and a spindle is formed.

The chromosomes become attached to the spindle at their centromeres.

[Image changes to show various close ups of chromosomes moving]

The chromosomes move to the equator of the spindle and the centromeres divide.

[Image shows centromeres dividing]

After division each original parent chromosome consists of two identical daughter chromosomes

[Image shows pairs of chromosomes moving apart]

which now move to opposite poles.

[Image shows spindle disappearing]

The spindle disappears and nuclear membranes form

[Image shows membranes forming around each group of chromosomes]

around each group of chromosomes.

[Image shows cells changing colour and dividing]

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.

[Image shows close up of single cell with thin threads inside]

It is important to remember that in all mitotic divisions

[Image shows chromosomes moving, then changes to close up of pairs of chromosomes]

each chromosome divides to form two daughter chromosomes which are identical.

[Image pans out to show pairs of chromosomes separating and moving apart]

Each one of a pair of daughter chromosomes separates into opposite cells and consequently

[Image shows spindle disappearing and single cell dividing into two cells]

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.

[Image shows single cell]

Mitosis is the process by which cells divide.

[Image shows single cell dividing into two cells]

New cells originate only by division of existing cells.

[Image changes back to single cell, then divides again to two cells]

So Mitosis is the basis of life and growth.

[The End]