Mitosis, Meiosis

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Mitosis, Meiosis Recombination

• Two Types of cellular reproduction in eukaryotes
• asexual ? Mitosis (somatic cells in multicellular
  eukaryotes)
• sexual ? Meiosis (gametes)
• Mitosis produces two genetically identical
  daughter cells
• Meiosis produces new genetic combinations
• This results in
• - greater genetic diversity
• potential adaptive advantages

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Mitosis chromosomes replicate then distribute
one copy to each daughter cell
Same strategy is used whether cells are haploid
or diploid Haploids One chromosome set Diploids
Two chromosome sets (ie. 2 copies of each
chromosome) (ignoring sex chromosomes, X and
Y) Eukaryotes generally spend part of their lives
in each state Higher Eukaryotes Mostly diploid
short haploid stages (sperm egg) Lower
Eukaryotes Much of life cycle could be haploid
diploid stages might be transient.
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View mitosis movie here
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Meiosis The process by which diploid cells are
made haploid (so that when egg and sperm are
fertilized it reconstitutes the diploid number of
chromosomes)
- One round of chromosome replication (as in
mitosis) followed by TWO rounds of division.
Homologous pair
1st Division Reductional Meiosis I
n
A
B
B
A
A
B
2n
diploid
haploid
2nd Division Meiosis II
n
A
B
n
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View meiosis movie here
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Comparison of Meiosis Mitosis
This is a diploid cell with 1 pair of homologous
chromosomes (red and blue)
Figure 20.6 (Mol. Biol. Cell)
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Human Meiosis Generates Huge Genetic Diversity
Amongst Gametes
e.g. an individual gamete receives only a
maternal or a paternal copy of each chromosome
pair and there are 23 pairs this results in a
minimum of 223 possible gametes produced by
meoisis in humans i.e. even with no
recombination -there is a much greater number of
possible gametes if recombination occurs
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During the pairing of Homologous chromosomes,
Homologous Recombination or exchange of DNA
between non-sister chromatids almost always
occurs. This typically occurs at several
locations over the length of a chromosome
Homologous Recombination
-occurs by precise breakage and rejoining of DNA
molecules of homologous chromosomes
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Meselson-Weigle experiment established that
homologous recombination occurs by breakage and
rejoining of DNA strands
of Mathews
Figure 25.18
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Progeny phage plaques sometimes contained phages
of two different genotypes- can be explained if
recombination involves the formation of
heteroduplex DNA
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Figure 25.20 (Mathews)
Holliday Model for Homologous Recombination
Involves formation of heteroduplex DNA, followed
branch migration or expansion of heteroduplex
region and generation of two unbroken duplexes
through a process of strand breakage and
rejoining
parental
recombinant
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Mobility of the Holliday Junction
An interesting property of holliday junctions is
their mobility. The branch point can migrate up
and down the double helixes.
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Holliday Structure (Junction)
The DNA structure at the Chiasma
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Resolvase
Recombinant Products
Parental Products
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Meselson-Radding Model for Homologous
Recombination
-triggered by a single nick in one duplex,
followed by strand displacement synthesis and
invasion of the unbroken duplex by the displaced
single-strand end
Figure 25.22 Mathews