An animated film explaining the complex cell division in which the chromosome number is halved, preceding the formation of egg cells and sperm in animals and the higher plants. The chromosomes are shown as simple coloured rods.
Terms like `chromatids’, `centromere’ and `chiasma’ are explained.
[Music plays and screen appears with title, The Commonwealth Scientific and Industrial Research Organization Presents: Meiosis]
[Words appear on screen: Reproductive Cells]
Narrator: Adult individuals form reproductive cells, eggs and sperm or gametes.
[Image shows illustration charting the process of Meiosis from reproductive cells]
These reproductive cells fuse at fertilisation and by the processes of growth the fertilised cell develops to a new adult.
[Image charts the fertilisation process]
The reproductive cells have a basic number of chromosomes. This basic number may be different in different species and we call it n. With the fusion at fertilisation this basic number is therefore doubled and during growth to the new adult this 2n chromosome number is maintained.
In animals the halving of this 2n adult chromosome number to n before the formation of reproductive cells is brought about by a special type of cell division called Meiosis. Plant reproduction is essentially the same as animal but in some plants the lifecycle is more complex and Meiosis may not result in the direct formation of gametes.
Spores are formed instead of gametes. These spores with n chromosomes develop to an intermediate generation called the gametophyite which then produces the gametes by ordinary Mitosis. Two gametes fuse at fertilisation and so the cycle continues.
Meiosis and fertilisation are complementary to each other. Meiosis halves the adult chromosome number, fertilisation doubles the gamete chromosome number. This halving and doubling makes certain that the chromosome number of the adult cell remains constant from generation to generation.
[Image shows a group of cells]
Anyone looking at a group of cells dividing by Meiosis sees a complex pattern. The cells are at various stages of the division, the chromosomes in different positions and in different forms, thread like or thicker and contracted or even indistinguishable.
[Image shows close up of a single cell]
It is from this indistinguishable state that Meiosis begins, from the resting cell nucleus in which the individual chromosomes cannot be detected.
[Image shows close up of single cell showing long thin threads intertwining in centre]
They first become visible as long, thin threads but by representing them as simple rods the division is easier to follow.
[Image shows single cell with thin threads changing to rod shapes]
Most organisms have a large number of chromosomes in each body cell and each chromosome has an identical partner.
These pairs of identical chromosomes arise by fertilisation, when one member of each pair comes from the maternal gamete and its identical partner is donated by the paternal gamete.
[Image shows single cell with pairs of identical chromosomes highlighted]
Four such pairs are shown here but to explain the main features of Meiosis we need follow only one pair.
[Image changes to show single cell with one pair of chromosomes]
Remember that throughout Meiosis the chromosomes are changing continually. They twine around each other and are rarely parallel. For this complexity we have substituted simple rods and simple movements.
[Image shows chromosomes moving towards each other in a single cell]
The chromosomes first move toward their partners and pair closely.
After pairing each of the chromosomes splits along its length into two equal and identical parts.
[Image shows close up of chromosomes splitting in half]
These are known as Chromatids. Each chromosome has now become two sister Chromatids. One part of each chromosome doesn’t divide at this stage, it’s called the Centromere.
[Image shows close up of chromosomes with Centromeres highlighted]
The function of the Centromere is to attach the pair of Chromatids to a spindle. This is formed later.
Following the pairing and splitting the chromosomes cross over. A cross over results from the breakage of adjacent non-sister Chromatids and their re-joining with one another at a point called a Chiasma.
[Image shows close up of chromosomes splitting and crossing over]
Crossing over may occur simultaneously at several points along the paired chromosomes.
[Image pans out to show view of cell]
After crossing over the chromosomes repel each other
[Image shows chromosomes repelling each other]
and would separate if they were not held together by the mechanical intertwining of the Chromatids in the region of the Chiasma. The nuclear membrane disappears
[Image shows inner circle of cell disappearing]
and a spindle of cytoplasmic fibres is formed across the cell.
[Image shows connecting lines appearing across centre of cell]
The chromosomes become attached to this spindle at their Centromeres. They then move to the poles of the spindle drawing apart the Chiasma.
[Image shows chromosomes attaching to spindle and drawing apart]
Notice that crossing over results in an exchanging of segments of two Chromatids.
The spindle disappears and a cell wall divides the original cell into two daughter cells.
[Image shows single cell dividing into two cells]
This completes the first division of Meiosis and you can now see that the original chromosome number has been halved. The second division of Meiosis usually follows soon after the first. A new spindle is formed in each of the daughter cells
[Image shows spindle forming in each separate cell]
and the chromosomes become attached to them at their Centromeres. After the chromosomes move to the middle of the spindle
[Image shows chromosomes moving to middle of spindle]
the Centromeres which did not divide in the first division, divide.
[Image shows close up of one cell with Centromeres dividing]
The pairs of Chromatids have become new individual chromosomes
[Image pans out to show both cells]
and these now separate to opposite poles.
[Image shows chromosomes in each cell separating]
Walls divide the initial cells into four daughter cells,
[Image shows two cells dividing into four cells]
each of which contains half the original number of chromosomes. Nuclear membranes then form around these chromosomes.
[Image shows membranes forming around each chromosome in each cell]
You will notice that two of the resultant chromosomes contain segments of both parental chromosomes while the other two remain unaltered. This is the result of a single cross over. Usually more than one cross over occurs
[Image shows added chromosomes in each cell]
so that all the resultant chromosomes contain segments of both parent chromosomes. If we had considered two pairs we would have seen exactly the same process in each pair
[Image shows added pair of chromosomes appearing in each cell]
and similarly with all the pairs resulting in an exchanging of Chromatid segments and the halving of the adult chromosome number, the essential feature of Meiosis.
[Image changes to show one single cell]
We have followed the main features of Meiosis using simplified chromosomes. Actually there are many chromosomes in each cell and many changes in their shape and position so we will attempt to show Meiosis more realistically. At the beginning of Meiosis individual chromosomes cannot be detected in the nucleus but as Meiosis proceeds the chromosomes appear as long thin threads.
[Image shows close up of chromosomes as long thin threads in cell]
These threads pair and begin to contract and thicken.
[Image shows threads pairing off]
As pairing proceeds the chromosomes twist around each other so that each pair may actually appear to be one thick chromosome.
[Image shows chromosomes moving and twisting together]
The chromosomes then divide into Chromatids and crossing over occurs.
[Image shows chromosomes dividing and crossing over]
The chromosomes, each of which is a pair of Chromatids repel each other but are held together by the intertwining of the Chromatids near the Chiasma.
[Image shows chromosomes moving and intertwining]
If there has been only one cross over repulsion results in the paired chromosomes taking up the appearance of a cross.
[Image shows chromosomes moving into cross shapes]
If there are two crossovers they form a circular structure as the Chiasmata pull towards the ends of the chromosomes. The thickening and contraction become more pronounced and the internal structure is increasingly difficult to see. At this stage the nuclear membrane disappears and the spindle forms.
[Image shows membrane in cell disappearing and spindle forming]
The chromosomes which are attached to the spindle by their Centromeres become orientated on its equator
[Image shows chromosomes moving together]
and now move to the opposite poles.
[Image shows chromosomes stretching and moving out to either side of cell]
A cell wall divides the original cell into two and the spindle disappears.
[Image shows single cell dividing into two cells]
The second cell division commences with the formation of new spindles in each daughter cell and the chromosomes become attached to them by their Centromeres. Again the chromosomes become orientated at the equators of the spindles.
[Image shows chromosomes dividing and moving to opposite sides of the cells]
The Centromeres divide and the Chromatids separate to opposite poles. The spindles disappear and the second cell wall completes the meiotic division.
[Image shows spindle disappearing and two cells dividing into four cells]
Nuclear membranes then form around each group of chromosomes which elongate and return to the original thin thread stage.
[Image shows membranes forming around each group of chromosomes in each of the four cells, then chromosomes returning to thread form]
The original chromosome number has been halved. Later the cells enter a so-called resting stage in which the individual chromosomes cannot be detected.
[Image shows chromosomes disappearing, then image fades out. Image of single cell appears with chromosomes]
So summarising Meiosis, again we use a schematic cell and schematic chromosome movements.
Firstly the chromosomes pair.
[Image shows chromosomes moving together]
Each chromosome then splits into two except at the Centromere.
[Image shows each chromosome splitting in two]
Crossing over occurs by breakage and reunion of adjacent Chromatids.
[Image shows chromosomes intertwining]
The pairs of Chromatids then repel each other but are held together near the Chiasma.
[Image shows chromosomes moving apart]
A spindle forms and the chromosomes separate to opposite poles.
[Image shows spindle appearing and chromosomes splitting apart and moving to opposite sides]
The disappearance of the spindle and the formation of a cell wall
[Image shows spindle disappearing and single cell dividing into two cells]
complete the first division of Meiosis. The chromosome number has been halved. New spindles are formed upon which the pairs of Chromatids orientate at their equators.
[Image shows spindles forming in each cell and chromosomes moving into place]
The Centromeres which did not divide during the first cell division now divide.
[Image shows Centromeres dividing]
Each Chromatid is now an individual chromosome. The chromosomes now separate.
[Image shows chromosomes separating]
The spindles disappear and a second cell wall is formed.
[Image shows spindles disappearing and two cells dividing into four cells]
With the formation of new nuclear membranes the meiotic division is complete.
[Image shows membranes forming inside each cell]
Again notice the halving of the chromosome number and the exchanging of Chromatid segments due to the cross over.
[Music plays – End and credits roll: Produced by C.S.I.R.O Animal Genetics Section and C.S.I.R.O film unit. Scientific Direction and Animation A.S Fraser. Direction Photography J.J Walker. Supervision S.T Evans. Backgrounds T.Nay, K.A Mason]