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Lecture 17
Gametogenesis at the Chromosomal level: Mitosis and Meiosis
Animal Science 434John J. Parrish
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DefinitionsChromosome - linear DNA molecule
Centromere - special region of spindle attachment
Homologous Chromosomes
• Have the same kind of genes in the same order
• 1 from father, 1 from mother
Sister chromosomes (X, Y)
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DNA Replication
Definitions
Chromatids Chromatids
Homologous Chromosomes
A chromatid pair has an identical sequence of DNA
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Cell Cycle
Go
G1G2
S
M
MPF
MPF
MPF
MPF
MPF
MPF
MPF
Interphase
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M - PhaseProphaseMetaphaseAnaphaseTelophaseCytokinesis
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Grand Scheme
Mitosis
2N
2N 2N
2N
2N 2N
N N N N
Meiosis
ReplicateDivision
ReplicateDivision
Division
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MitosisGo
G1
S
G2
Homologous chromosomes pair along side of each other
M - phase
Spindles(microtubules)
Centromere
Chromatids
MPF high
Metaphase
Prophase
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Mitosis
Anaphase
MPF decreases
Chromatids pullapart
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Mitosis
Anaphase
MPF decreases
Chromatids pullapart
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Mitosis
Telophase
Chromosomes pull apart further
Nuclear envelopesreform
MicrotubulesDisappears
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Mitosis
Telophase
Chromosomes pull apart further
Nuclear envelopesreform
MicrotubulesDisappears
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Mitosis
Cytokinesis
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Mitosis
Cytokinesis
2N
2N
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Meiosis
Go G1 G2S
Primary SpermatocytePrimary Oocyte
2N 4NProphase
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The Five Phases of Meiotic Prophase
Leptotene• Chromosomes condense
Zygotene• Homologous chromosomes
begin to pair• Bivalents
4N
4N
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The Five Phases of Meiotic Prophase
Pachytene• Pairing is completed• Crossing over of homologues
occur
Diplotene/Dictyatene• Oocytes stop here before puberty
• Homologous chromosomes pull apart but remain attached at crossover points (chiasma)
• RNA synthesis is possible
4N
4N
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The Five Phases of Meiotic Prophase
Crossover (chiasma)
Centromere
Diakinesis
• RNA synthesis stops
• The chromatids become visible
Homologous chromosomes
linked at chiasma
Chromatids linked at centromere
4N
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Metaphase I of Meiosis
Homologous chromosomes move
toward opposite poles and pair across from one
another
4N
Homologous chromosomes held
together via chiasma
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Anaphase I of Meiosis
Recombination as a result of crossing
over
4N
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Telophase I
4N
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Telophase I
4N2N
2N
Secondary Spermatocyte or
Secondary Oocyte
Cytokinesis
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Meiosis
2N
2N
No Replication Rest Same As Mitosis
Metaphase II
2N
2N
Oocytes stop here until fertilized
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Meiosis
Anaphase IITelophase IICytokinesis
2N
2N
N
N
N
N
This does notexist for oocyte
Spermatid
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AbnormalitiesAnueploidy– Not an exact multiple– Hyper or hypo– Example: 2N - 1 or 2N + 1
Euploidy– Exact multiple of haploid– Example: 3N, 4N, etc.
Non-disjunction– Causes aneuploidy and occurs in meiosis– Example: N - 1 or N + 1
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Abnormalities
XXYY
XXYY O
XY XY O O
XXYY
XX YY
XX O Y Y
X XXY XXY XO XO XXX XO XY XY
Kleinfelter’sSyndrome
Turner’sSyndrome
Non-disjunctionXY
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Down’s SyndromeNon-disjunctionExtra copy of chromosome 21Probability increases with age– Age 35, 2 per 1000 births– Age 40, 6 per 1000 births– Age 45, 16 per 1000 births