Lecture 17 Gametogenesis at the Chromosomal level: Mitosis and Meiosis

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Lecture 17Gametogenesis at the Chromosomal
level Mitosis and Meiosis

• Animal Science 434
• John J. Parrish

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Definitions
Chromosome - linear DNA molecule
Centromere - special region of spindle attachment
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Definitions
A chromatid pair has an identical sequence of DNA
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Cell Cycle
M
Go
MPF
MPF
MPF
G1
G2
MPF
MPF
MPF
MPF
Interphase
S
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M - Phase

• Prophase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis

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Grand Scheme
Mitosis
Meiosis
2N
2N
Replicate Division
Replicate Division
2N
2N
2N
2N
Division
N
N
N
N
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Mitosis
Go
G1
M - phase
Prophase
S
G2
Metaphase
Homologous chromosomes pair along side of each
other
Chromatids
Centromere
MPF high
Spindles (microtubules)
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Mitosis
Anaphase
MPF decreases
Chromatids pull apart
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Mitosis
Anaphase
MPF decreases
Chromatids pull apart
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Mitosis
Telophase
Chromosomes pull apart further
Microtubules Disappears
Nuclear envelopes reform
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Mitosis
Telophase
Chromosomes pull apart further
Microtubules Disappears
Nuclear envelopes reform
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Mitosis
Cytokinesis
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Mitosis
Cytokinesis
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Meiosis
2N
4N
Go
G1
G2
S
Prophase
Primary Spermatocyte Primary Oocyte
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The Five Phases of Meiotic Prophase

• Leptotene
• Chromosomes condense

4N

• Zygotene
• Homologous chromosomesbegin to pair
• Bivalents

4N
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The Five Phases of Meiotic Prophase

• Pachytene
• Pairing is completed
• Crossing over of homologues occur

4N

• Diplotene/Dictyatene
• Oocytes stop here before puberty
• Homologous chromosomes pull apart but remain
  attached at crossover points (chiasma)
• RNA synthesis is possible

4N
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The Five Phases of Meiotic Prophase

• Diakinesis
• RNA synthesis stops
• The chromatids become visible

4N
Homologous chromosomes linked at
chiasma Chromatids linked at centromere
Crossover (chiasma)
Centromere
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Metaphase I of Meiosis
4N
Homologous chromosomes move toward opposite poles
and pair across from one another
Homologous chromosomes held together via chiasma
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Anaphase I of Meiosis
4N
Recombination as a result of crossing over
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Telophase I
4N
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Telophase I
Cytokinesis
4N
Secondary Spermatocyte orSecondary Oocyte
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Meiosis
Metaphase II
2N
2N
No Replication
Rest Same As Mitosis
2N
2N
Oocytes stop here until fertilized
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Meiosis
This does notexist for oocyte
Telophase II Cytokinesis
Anaphase II
N
N
2N
Spermatid
N
2N
N
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Abnormalities

• Anueploidy
• 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
Non-disjunction
XXYY
XY
XXYY
X
Kleinfelters Syndrome
Turners Syndrome
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Downs Syndrome

• Non-disjunction
• Extra copy of chromosome 21
• Probability increases with age
• Age 35 2 per 1000 births
• Age 40 6 per 1000 births
• Age 45 16 per 1000 births