This involves 2 divisions of the nucleus The first division is...
Transcript of This involves 2 divisions of the nucleus The first division is...
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•This involves 2 divisions of the nucleus –
•The first division is called MEIOSIS 1 and separates the homologous chromosomes, halving the number of chromosomes
•The second is called MEIOSIS 2 and separates the sister chromatids
•This results in the formation of four cells each with half the original chromosome number - haploid
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• Meiosis is a slower and more complex process than mitosis.
• It is preceded by an interphase stage during which the chromosomes are replicated.
• This doubles the amount of DNA
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DNA condenses and chromosomes appearAs they shorten and open out they reveal
their chromatidsChromatids from one chromosome are
referred to as sister chromatidsHomologous chromosomes come together in
pairsEach chromosome is called a homologueTogether each pair is called a bivalent
(or tetrad)
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Bivalents:
each
chromosome
composed of
2 chromatids
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• In some places the non-sister chromatids may touch and join.
• The DNA may break at these points and rejoin, swapping portions with non-sister chromatids.
• This is called crossing over and generates genetic variation as the process is random.
• The points of exchange are called chiasmata.
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• Non-sister chromatids touch forming chiasma
sister
chromatids
chiasma
• Non-sister chromatids break at chiasma
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• Portions of chromosomes are swapped –crossing over
• This gives rise to new combinations of alleles
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• In late prophase 1 the homologues begin to repel each other
• but the chiasmata still hold the chromosomes together and a cross-shaped structure is formed.
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• The disappearance of the nucleoli and the nuclear membrane marks the end of prophase 1
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The spindle forms
The bivalents attach to the spindle
The bivalents move towards the equator
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Spindle fibres contract and pull the homologous chromosomes apart to opposite poles
two chromatids
homologous
chromosomes
separate
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Chromosomes form 2 groups at either end of the cell and the spindle disappearsThe nuclear membrane reforms.Sometimes this phase is indistinct and blends into prophase 2At this stage the groups of chromosomes are haploid, but each chromosome is still made up of 2 chromatids
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new nuclear
membrane may
form
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•New spindle fibres form at right angles to the old spindle
•Nuclear membranes break down
•The chromosomes are no longer in pairs and there is NO crossing over
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The chromosomes arrange themselves on the equator of the spindle.
The chromosomes (composed of two chromatids) attach to the spindle by their centromeres.
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chromosomes align
on spindle together
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Centromeres divide and separate
Sister chromatids get pulled to opposite poles
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chromatids separate
and move to opposite
poles of the spindle
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Nuclear membranes form around the 4 groups of chromatids (now chromosomes)
Chromosomes uncoil and become dispersed chromatin
Nucleoli reappear
Cytokinesis follows
The 4 new cells are haploid.
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4 haploid cells each with a different genetic make up
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meiosis whf
Cells alive meiosis
Interactive quiz to id stages of meiosis
http://bcs.whfreeman.com/thelifewire/content/chp09/0903004.html
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Differences between Mitosis & Meiosis
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