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NadiaA

Mitosis and Meiosis
LO:
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Recall the processes of mitosis and meiosis

Mitosis
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production of somatic cells
It is a form of asexual reproduction

1
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3
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5
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7
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Interphase
 Also known as the resting phase
 Consists of G1, S and G2 phase- stands for;
Gap/growth phase 1, Synthesis phase, Gap/growth phase 2
 G1
- Protein synthesis occurs and reaches a ‘restriction point’ which means it is then
confirmed to reach the mitosis (M) phase
- Organelle synthesis
- DNA checking and repair
- Cell growth
- This is about 11 hours in humans
 S
- Chromosome replication
...
So there are 2x the chromosomes as
there were in G1
...

 G2
- accumulates materials for M phase
- second round of proof reading enzymes check for mutations/errors
- RNA and protein synthesis resumes
- microtubule synthesis
- cell confirms the number of chromosomes and organelles present
 Respiration occurs to harness ATP for the cell’s activities in mitosis e
...
spindle contract and
shorten, movement of centrioles
...

 This stage takes the longest in comparison to the other stages i
...
M phase
2
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Spindle begins to form
...
Prometaphase
 Centrosomes move to opposite poles of cell (centrosomes consist of centrioles at 90 degrees
to each other)
 By end of this stage, microtubule (spindle) is completely formed
 Microtubules shoot out of centriole and tries to make contact with kinetochore, those that
do not retract to the centrosome / degrade( (WTR: Spider Man))
 Nuclear membrane completely fragments into vesicles
 Chromosomes undergo active movement
4
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Anaphase
 Centrosome divide
 Spindle contracts and shorten pulling sister chromatids apart by centromere first (thus the
‘v’ shape)
 Centromeres divide by enzyme ‘separase’ (the separase cuts the cohesin)
 Poles move apart due to elongation of kinetochore microtubules to allow the completion of
segregation of sister chromatids
 Sister chromatids are thus separated
6
...

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Telophase
Chromatids uncoil and unthicken (decondensation)
Spindle breaks down and disappears
Cannot be seen under light microscope
Nuclear envelope reforms
2 genetically identical nuclei formed
Cytokinesis -this is already in progress during late anaphase and carries on to the end of
telophase
Cytoplasm cleaves/divides
2 genetically identical nuclei are formed – 2 daughter cells genetically identical to mother
In animals- a cleavage furrow forms as the cell pinches into two; there is a contractile ring
(see image)
Contractile ring is made of actin and myosin filaments
Plants cannot form contractile rings
In plants- cytokinesis starts from inwards and a cell plate forms
In some organisms the cell division is complete without a cytokinesis stage

Note in plant cells, the cells
produced are not physically
separated unlike in animal cells

Contractile ring starts to contract

The cell plate is formed as
plasma membrane forms a cell
wall which then forms the cell
plate
...
REMEMBER, that the M phase occupies a short amount of
time in comparison to interphase in the cell division
...

Meiosis

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Production of germ cells- gametes
Occurs in gonads- sex organs i
...
Testes and ovaries
There are two meiosis stages of division; Meiosis I and Meiosis II
You need to be specific in what stage you refer to in the exam
In meiosis II, haploid cells are produced that are non-genetically identical

Meiosis I
 Interphase
1
...
Metaphase I
3
...
Telophase I
 Interkinesis ( no DNA replication here—this is what distinguishes it from interphase)
Meiosis II
1
...
Metaphase II
3
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Telophase II
 Cytokinesis (remember this starts in late anaphase and goes on right till the end of
telophase)
A little more info;
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Asexual reproduction should be more common; in sexual reproduction each organism
contributes half its genes to the new organism, whereas in asexual, the whole genome is
passed on, so asexual should be taking over
...
But sexual reproduction is
more popular
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g
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g
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g
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g
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This is because spindle fibres are just one type of microtubule, as
the composition of protein in it make it a spindle fibre
...

The cell division duration depends on organisms
...
g
...
During anaphase of both M1 and M2, the DNA
content (number of chromosomes in a cell) is halved, although the ploidy level changes only
when the number of chromosome sets in the cell change
...


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Variation and heredity is characterised by;

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(Random assortment/ independent alignment at Metaphase and anaphase)
Fertilisation increases genetic variation by randomly combining two sets of chromosomes,
one of each from 2 genetically unrelated organisms
Also mutations can also cause change and are another cause of variation e
...
cause
disabilities ((remember mutations can cause cancer in somatic cells))
Genetic variation increases the chance of evolution as natural selection can favour the
organisms that are better adapted to the ever-changing environment
Recombination causes chiasma formation- the genetic material from one parent changes to
the other parent’s genetic background
Aneuploidy- cells with more or less DNA than normal e
...
Downs Syndrome
Polyploidy- cells with many genomes e
...
human agriculture- plants produce larger fruits and
flowers and seedless fruits which is great commercially
The possible daughter cells; you can predict this from metaphase I diagrams- try this and see
if you get the right answers!

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