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MEIOSIS
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Meiosis cell division that occurs as a part of
sexual reproduction in which the daughter cells
receive the haploid number of chromosomes in
varied combination
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Diploid The full number of chromosomes
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Haploid Half the diploid number of chromosomes
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Chromosome gene carrying structure in the
nucleus
Homologous Chromosome Similarly made
chromosomes with the same shape and genes
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Sister Chromatid one of two genetically
identically chromosomal units that are the result of
DNA replication (they are attached together)
Tetrad two homologous chromosomes which have
paired during meiosis
Synapsis pairing of homologous chromosomes
during prophase I of meiosis
Crossing Over exchange of segments of genetic
material between non-sister chromatids of
homologous chromosomes during synapsis of
meiosis I
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Gamete Haploid sex cell (the egg or sperm)
Zygote  Diploid cell formed by the union of
sperm and egg (the product of fertilization)
Mitosis  Cell division where the daughter cells
are genetically identical to the parental cells in
chromosome number and structure (occurs
during growth and repair)
FIGURE 13.4 DESCRIBING CHROMOSOMES
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
OVERVIEW OF MEIOSIS: HOW MEIOSIS REDUCES
CHROMOSOME NUMBER (END RESULTS)
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
FIGURE 13.8 THE MEIOTIC DIVISION
OF AN ANIMAL CELL
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Centrosomes
(with centriole pairs)
Sister
chromatids
Nuclear
envelope
Chiamata
Spindle
Metaphase
plate
Homologous
Microtubule
chromosomes
attached to
Chromatin
separate
kinetochore
Tetrads line up
Chromosomes duplicate Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
Tetrad
Pairs of homologous
chromosomes split up
FIGURE 13.8 THE MEIOTIC DIVISION
II: Separates sister chromatids
OF AN ANIMAL MEIOSIS
CELL
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
Sister chromatids
Separate (not identical)
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
FIGURE 13.9 A COMPARISON OF
MITOSIS AND MEIOSIS
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Sister chromatids
separate during
anaphase
Anaphase
Telophase
2n
Tetrads
positioned at the
metaphase plate
Metaphase I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
n
FIGURE 13.10 THE INDEPENDENT ASSORTMENT OF
HOMOLOGOUS CHROMOSOMES IN MEIOSIS
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination
4
23 = 8
223 = 8 million
FIGURE 13.11 THE RESULTS OF CROSSING OVER DURING
MEIOSIS
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
an average of one to
three times per
chromosome pair.
A big contribution to
diversity?
Metaphase II
3. Random fertilization.
Daughter
cells
No chance of siblings to
be identical?
Recombinant
chromosomes