[Music playing]
(Narrator) Adult individuals form reproductive cells,
eggs and sperm or gametes.
These reproductive cells fuse at fertilisation
and by the processes of growth
the fertilised cell develops to a new adult.
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.
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.
It is from this indistinguishable state that Meiosis begins,
from the resting cell nucleus in which the individual chromosomes
cannot be detected.
They first become visible as long, thin threads
but by representing them as simple rods the division is easier to follow.
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.
Four such pairs are shown here but to explain the main features of Meiosis
we need follow only one pair.
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.
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.
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.
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.
Crossing over may occur simultaneously
at several points along the paired chromosomes.
After crossing over the chromosomes repel 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
and a spindle of cytoplasmic fibres is formed across the cell.
The chromosomes become attached to this spindle at their Centromeres.
They then move to the poles of the spindle drawing apart the Chiasma.
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.
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
and the chromosomes become attached to them at their Centromeres.
After the chromosomes move to the middle of the spindle
the Centromeres which did not divide in the first division, divide.
The pairs of Chromatids have become new individual chromosomes
and these now separate to opposite poles.
Walls divide the initial cells into four daughter cells,
each of which contains half the original number of chromosomes.
Nuclear membranes then form around these chromosomes.
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
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
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.
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.
These threads pair and begin to contract and thicken.
As pairing proceeds the chromosomes twist around each other
so that each pair may actually appear to be one thick chromosome.
The chromosomes then divide into Chromatids and crossing over occurs.
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.
If there has been only one cross over
repulsion results in the paired chromosomes
taking up the appearance of a cross.
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.
The chromosomes which are attached to the spindle by their Centromeres
become orientated on its equator
and now move to the opposite poles.
A cell wall divides the original cell into two and the spindle disappears.
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.
The Centromeres divide and the Chromatids separate to opposite poles.
The spindles disappear
and the second cell wall completes the meiotic division.
Nuclear membranes then form around each group of chromosomes
which elongate and return to the original thin thread stage.
The original chromosome number has been halved.
Later the cells enter a so-called resting stage
in which the individual chromosomes cannot be detected.
So summarising Meiosis,
again we use a schematic cell and schematic chromosome movements.
Firstly the chromosomes pair.
Each chromosome then splits into two except at the Centromere.
Crossing over occurs by breakage and reunion of adjacent Chromatids.
The pairs of Chromatids then repel each other
but are held together near the Chiasma.
A spindle forms and the chromosomes separate to opposite poles.
The disappearance of the spindle and the formation of a cell wall
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.
The Centromeres which did not divide during the first cell division
now divide.
Each Chromatid is now an individual chromosome.
The chromosomes now separate.
The spindles disappear and a second cell wall is formed.
With the formation of new nuclear membranes
the meiotic division is complete.
Again notice the halving of the chromosome number
and the exchanging of Chromatid segments due to the cross over.
[Music playing]