Meiosis Overview
The Importance of Meiosis
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Three ways individuals are assured a
different genetic combination than either
parent.
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Crossing-over recombines genes on sister
chromosomes of homologous pairs.
Following meiosis, gametes have all possible
chromosome combinations.
At fertilization, sperm and egg carry varied
chromosome combinations.
Spermatogenesis and
Oogenesis
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Spermatogenesis occurs in the testes
of males and produces haploid sperm.
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Once started, continues to completion.
Oogenesis occurs in the ovaries of
females, and produces haploid eggs.
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Does not necessarily go to completion.
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Meiosis (Reduction Division)
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Cell division occurring in ovaries and
testes to produce gametes (ova and
sperm cells).
Has 2 divisional sequences:
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First division:
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Homologous chromosomes line up side by side
along equator of cell.
Spindle fibers pull 1 member of the
homologous pair to each pole.
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Each of the daughter cells contains 23 different
chromosomes, consisting of 2 chromatids.
Stages of Meiosis
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First Division.
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Prophase I - Spindle appears, and nuclear
envelope fragments.
Metaphase I - Tetrads line up at equator.
Anaphase I - Homologous chromosomes of
each pair separate and move to opposite
poles of the spindle.
Telophase I - Spindle disappears and
nuclear envelope reforms.
Cytokinesis - Plasma membrane furrows.
Meiosis (Reduction Division)
„ Second division:
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(conti nued)
Each daughter cell divides, with
duplicate chromatids going to each
new daughter cell.
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Testes: produce 4 sperm cells.
Ovaries: produce one mature egg, polar
bodies die.
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Stages of Meiosis
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Second Division.
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Prophase II - Spindle appears and nuclear
envelope disassembles.
Metaphase II - Dyads line up at equator.
Anaphase II - Sister chromatids separate and
move towards poles.
Telophase II - Spindle disappears and nuclear
envelope reforms.
Cytokinesis - Plasma membrane furrows.
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Four haploid daughter cells produced.
Chromosomal Inheritance
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Humans have 22 pairs of autosomes,
and one pair of sex chromosomes.
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Abnormal chromosome number or
structure often leads to a syndrome.
Amniocentesis and Chorionic Villi
Sampling can be used to obtain a
genetic sample to produce a karyotype.
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Visual display of chromosomes arranged by
size, shape, and banding pattern.
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Telomeres and Cell Division
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Decreased ability of cells to divide is an indicator
of senescence (aging).
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May be related to the loss of DNA sequences at the ends
of chromosomes (regions called telomeres).
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Telomeres serve as caps on the ends of DNA.
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DNA polymerase does not fully copy the DNA at end-regions.
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Prevent enzymes from mistaking the norma l ends for broken DNA.
Each time a chromosome replicates it loses 50-100 base pairs in its
telomeres.
Germinal cells can div ide indefinitely due to an enzyme
telomerase.
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Duplic ates telomere DNA.
Cell Death
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Pathologically:
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Apoptosis:
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Cells shrink, membranes become bubbled, nuclei
condense.
Capsases (“executioner enzymes”):
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Cells deprived of blood supply swell, the membrane
ruptures, and the cell bursts (necrosis).
Mitochondria membranes become permeable to proteins
and other products.
Programmed cell death:
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Physiological process responsible for remodeling of
tissues during embryonic development and tissue
turnover in the adult.
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