Meiosis

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Meiosis
Meiosis
IB Biology
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• Meiosis cell division for the production of
  gametes (sperm or egg or spores)
• Called reduction division because it divides
  the number of chromosomes in half
• Diploid (2n) 4 Haploid (n) 2

B B D D b b d d
B d B d b D B
d B D D b b d
D b
cells NOT identical
OR
Cell of ovary or testes
In humans... 2n 46 n 23
Gametes or spores
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Karyotype for pea plants

• The diploid number for this organism would be 14
• (The haploid number is 7)
• Locus a genes specific position on a
  chromosome
• The locus for flower
• color might be at this point
• on the 5th chromosome

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2 stages (meiosis I and meiosis II)

• Meiosis I
• Prophase I
• Synapsis homologous chromosomes pairing up
  closely on the same spindle fiber

A A a
a B b B b r r R
R d D d D
CHIASMATA point where chromatids touch, to allow
for crossing over
Homologous pairs are called bivalents when
together
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Tetrad 4 chromatids (paired)Crossing over
homologous chromosomes can exchange pieces at the
chiasmata this is a type of recombination
A A a
a B b B b r r
R R d D d D
The lining up here of homologous pairs (ex
which homologue will be on the right and which
will be on the left?) is completely random this
will eventually add to the variation of the
gametes ? Mendels Law of Independent Assortment
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Metaphase I
A A a
a B b B b r r
R R d D d D

• Chromosomes are at the equator
• (Homologous chromosomes still on the same spindle
  fiber)
• Note the different arrangement than in
  mitosis

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Anaphase I chromosomes move to poles
A A
a a B b
B b r r
R R
D d D
d
Centro-meres do NOT split!... The bivalents
separate.

• Spindle fibers pull chromosomes to poles (spindle
  fibers are shortening.) Homologous pairs are
  separated.
• Creates (nearly) infinite variety
• Mendels Law of Segregation Two alleles of each
  gene separate into different gametes (As to one
  side, as to the other)

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Telophase I
R R a a d D B b
r r A A d D B b

• Chromosomes at the poles
• Spindle fibers disintegrate
• Nuclear membrane begins to reform each new
  nucleus is haploid!

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Cytokinesis I
Down to 2 chromosomes n (DNA is not
replicated again after meiosis I)
Down to 2 chromosomes n (DNA is not
replicated again after meiosis I)

• After cytokinesis I, maybe a brief interphase.
  Otherwise, Meiosis II !!.

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Meiosis II -Prophase II
R R a a d D B b
A A r r B b
d D

• Chromosomes evident
• Nuclear envelopes break down

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Metaphase II
A A B b r r d
D
a a B b
R R d D

• Spindle fibers attach
• Chromosomes line up at equator
• Note similar arrangement to mitosis

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Anaphase II
A A B b r
r d D
a a B
b R R d D
centriole (pole)

• Spindle fibers pull sister chromatids apart
• Chromosomes move to poles
• Centromeres have split

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Telophase II
A A B b r
r d D
a a B
b R R d D

• Spindle fibers disintegrate
• Chromosomes at poles!
• Nuclear envelopes reform!
• Still Haploid!!

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Result after cytokinesis...4 gametes 4 spores,
4 sperms, or 1 egg and 2 polar bodies

• Potential for lots of variety!!
• Recombination the re-assortment of genes into
  different combinations from those of the parents

A A a
a B b
B b r r
R R d
D d D