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Mitosis: produces two daughter cells with the same
number of chromosomes as the parent cell and as each
other
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in the first meiotic division (meiosis 1) homologous chromosomes pair up and their chromatids wrap around each other
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By the end of
the division the homologous pairs have separated with one chromosomes from each pair
going into one of the two daughter cells
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In the second meiotic division (meiosis 2) the
chromatids move apart
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In humans each of these cells contains 23 chromosomes
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Importance of Meiosis:
In sexual reproduction, two gametes fuse to give rise to
new offspring
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Humans diploid number is
46, which means this cell would have 92 chromosomes
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This halving occurs
through meiosis (often in the formation of gametes)
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During
meiosis, homologous pairs of chromosomes separate so
that only one chromosome from each pair enters a
daughter cell
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When two haploid gametes fuse at fertilisation, the diploid number of chromosomes is restored
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It does this by:
1
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New combinations of maternal and paternal
alleles by crossing over

Independent segregation of homologous chromosomes:
During meiosis 1 each chromosome lines up alongside its
homologous partner
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One of each
pair will pass to each daughter cell
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Since the pairs line up at random the combination of chromosomes of maternal and
paternal origin that go into the daughter cell at meiosis
1 is also a matter of chance = independent segregation
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However, the alleles of
these genes may differ (may code for blood group
A or B)
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Where the cells produced in meiosis are gametes these will
be genetically different as a result of the different combinations of the maternal and paternal chromosomes/alleles
they contain
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Each gamete has a different makeup and their random fusion therefore produces
variety in the offspring
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Example:

Genetic Recombination by crossing over:

Stage 1: one of the pair of chromosomes includes
the gene for tongue rolling and caries one allele for
roller and one for non-roller
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there are two possible arrangements (1st
chromosome: non-roller and A/ 2nd chromosome:
roller and B OR– 1st chromosome: roller and A/ 2nd
chromosome has non-roller and B)
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During meiosis 1, each chromosome lines up alongside
its homologous partner
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1
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2
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3
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4
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5
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The chromatids cross over one another many times and
Stage 3: at the end of meiosis 2, the chromosomes so this process is known as crossing over
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The actual gametes = recombination
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stage 1
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either one of a pair can
pass into each daughter cell (independent segregation)
and so there are a large number of possible combinations of chromosomes in any daughter cell
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So an organism with 4 homologous pairs of chromosomes can produce 2(4) or 16 possible combinations of
chromosomes of maternal and paternal origin in its
daughter cells as a result of meiosis
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Where the gametes come from different parents, two
different genetic complements with different alleles are
combined = more variety:
(2n)2 where n = number of pairs of homologous chromosomes
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As recombination occurs each time gametes
are made, it will greatly increase the number of
possible chromosome combinations in the
gametes

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