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18 Cards in this Set
- Front
- Back
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Gametes made during sexual reproduction contain how much genetic material
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half the amount of genetic material
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They produce gametes that are morphologically similar
ex:simple eukaryotic species |
isogamous
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These produce gametes that are morphologically different.
ex. sperm cell, egg cell, or ovum |
heterogamous
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Ensures genetic continuity and variability
meiosis occurs in this. |
germline cells
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Unlike mitosis, meiosis involves two successive division called
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Meiosis I and meiosis II
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Reductional division, in which the chromosomes number is halved
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Meiosis I
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Equational division, in which the chromosome number remains that same
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Meiosis II
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Phases of Meiosis I
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Porphase I
Metaphase I Anaphase I Telophase I |
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Prophase I sub divided into 5 stages....
Longest and most complex stage of meiosis |
Leptonema
Zygonema Pachynema Diplonema Diakinesis |
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Leptonema (the leptotene stage)
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Chromatin begins to condense, but sister chromatids are not evident yet
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Zygonema (the zygotene stage)
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Chromosomes continue to shorten and thicken
Homologous chromosomes pair up side-by-side in intimate association In human males, the X and Y chromosomes line up end-to-end Each pair of synapsed homologous chromosomes is termed a Bivalent Synapsis is mediated by the nucleoprotein “synaptonemal complex” |
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Pachynema (the pachytene stage)
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Chromosomes continue to shorten and thicken, so that each is now seen to consist of two chromatids
Each bivalent is now a Tetrad of four strands Sister vs. Nonsister chromatid Crossing-over occurs |
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Diplonema (the diplotene stage)
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Chromosomes continue to shorten and thicken
Synaptonemal complex breaks down Homologues start to separate They remain held together at chiasmata |
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Diakinesis
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Chromosomes reach maximum contraction
Chiasmata move to ends of chromosomes as homologues continue to pull apart Nucleolus(i) disappears; Nuclear envelope degenerates; Spindle fibers form |
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Metaphase I
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Tetrads are organized along the metaphase plate
Pairs of sister chromatids are aligned in a double row, rather than a single row (as in mitosis) -The arrangement is random with regards to the homologues Furthermore A pair of sister chromatids is linked to one of the poles The homologous pair is linked to opposite pole |
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A short interphase between the two meiotic divisions
No DNA synthesis occurs |
INTERKINESIS
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MEIOSIS II Phases
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Prophase II
Metaphase II Anaphase II Telophase II Similar to mitosis! |
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Significance of Meiosis
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It allows the conservation of the chromosome number in sexually-reproducing species
2. It generates genetic variability through the various ways in which maternal and paternal chromosomes are combined into gametes In humans an individual can produce 2^23 = 8,388,608 different gametes So humans couples can produce 8,388,608 X 8,388,608 different offsprings == 70,368,744,177,664 3. It enhances the potential genetic variation in gametes through the phenomenon of crossing over between maternal and paternal chromatid pairs |
