Mitosis, Meiosis

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Mitosis, Meiosis Recombination
Two Types of cellular reproduction in
eukaryotes asexual ? Mitosis sexual ?
Meiosis Mitosis produces two genetically
identical daughter cells Sexual reproduction
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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During the pairing of Homologous chromosomes,
Recombination or exchange of DNA between
chromatids almost always occurs. This typically
occurs at several locations over the length of a
chromosome
Homologous Recombination
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Meselson-Weigle experiment Established that
recombination occurs by breakage and rejoining of
DNA strands
Figure 25.18
Figure 25.22
Mathews
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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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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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Figure 25.20 (Mathews)
parental
recombinant