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11-4 Meiosis
Each organism must inherit a single copy of every
gene from each of its “parents.”
Gametes are formed by a process that separates the
two sets of genes so that each gamete ends up with
just one set.
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Copyright Pearson Prentice Hall
just one set.
11-4 Meiosis Chromosome Number
Chromosome Number
All organisms have different numbers of chromosomes.
A body cell in an adult fruit
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A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.
11-4 Meiosis Chromosome Number
These two sets of chromosomes are homologous.
Each of the 4 chromosomes that came from the male
parent has a corresponding chromosome from the
female parent.
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11-4 Meiosis Chromosome Number
A cell that contains both sets of homologous
chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is
sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can be
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For Drosophila, the diploid number is 8, which can be
written as 2N=8.
11-4 Meiosis Chromosome Number
The gametes of sexually reproducing organisms
contain only a single set of chromosomes, and
therefore only a single set of genes.
These cells are haploid. Haploid cells are
represented by the symbol N.
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represented by the symbol N.
For Drosophila, the haploid number is 4, which can
be written as N=4.
11-4 Meiosis Phases of Meiosis
What happens during the process of meiosis?
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11-4 Meiosis Phases of Meiosis
Phases of Meiosis
Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half
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chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
11-4 Meiosis
Meiosis involves two divisions, meiosis I and
meiosis II.
By the end of meiosis II, the diploid cell that
entered meiosis has become 4 haploid cells.
Phases of Meiosis
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11-4 Meiosis Phases of Meiosis
Meiosis I
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Prophase I Metaphase I Anaphase I Telophase Iand
Cytokinesis
Interphase I
Meiosis I
11-4 Meiosis Phases of Meiosis
Cells undergo a round of
DNA replication, forming
duplicate chromosomes.
Interphase I
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11-4 Meiosis Phases of Meiosis
Each chromosome pairs
with its corresponding
homologous
chromosome to form a
tetrad.
MEIOSIS I Prophase II
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There are 4 chromatids in
a tetrad.
11-4 Meiosis Phases of Meiosis
When homologous chromosomes form tetrads in
meiosis I, they exchange portions of their
chromatids in a process called crossing-over.
Crossing-over produces new combinations of
alleles.
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alleles.
11-4 Meiosis Phases of Meiosis
Spindle fibers attach to
the chromosomes.
MEIOSIS I Metaphase I
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11-4 Meiosis Phases of Meiosis
MEIOSIS I Anaphase I
The fibers pull the
homologous
chromosomes toward
opposite ends of the
cell.
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11-4 Meiosis Phases of Meiosis
MEIOSIS I Telophase I and Cytokinesis
Nuclear membranes form.
The cell separates into two
cells.
The two cells produced by
meiosis I have
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meiosis I have
chromosomes and alleles
that are different from each
other and from the diploid
cell that entered meiosis I.
11-4 Meiosis Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a
second meiotic division.
Unlike meiosis I, neither cell goes through
chromosome replication.
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Each of the cell’s chromosomes has 2 chromatids.
11-4 Meiosis Phases of Meiosis
Meiosis II
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Telophase II
and Cytokinesis
Prophase II
Metaphase II Anaphase IITelophase I and Cytokinesis I
Meiosis II
11-4 Meiosis Phases of Meiosis
Meiosis I results in two
haploid (N) daughter
cells, each with half the
number of chromosomes
as the original cell.
MEIOSIS IIProphase II
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11-4 Meiosis Phases of Meiosis
The chromosomes line
up in the center of cell.
MEIOSIS II Metaphase II
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11-4 Meiosis Phases of Meiosis
The sister chromatids
separate and move
toward opposite ends of
the cell.
MEIOSIS II Anaphase II
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11-4 Meiosis Phases of Meiosis
Meiosis II results in four
haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis
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11-4 Meiosis Gamete Formation
Gamete Formation
In male animals, meiosis results in four equal-sized gametes called sperm.
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11-4 Meiosis Gamete Formation
In many female animals, only one egg results from
meiosis. The other three cells, called polar bodies,
are usually not involved in reproduction.
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11-4 Meiosis Comparing Mitosis and Meiosis
How is meiosis different from mitosis?
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11-4 Meiosis Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different
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produces four genetically different haploid cells.
11-4 Meiosis Comparing Mitosis and Meiosis
Mitosis
• Cells produced by mitosis have the same number
of chromosomes and alleles as the original cell.
• Mitosis allows an organism to grow and replace
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cells.
• Some organisms reproduce asexually by mitosis.
11-4 Meiosis Comparing Mitosis and Meiosis
Meiosis
• Cells produced by meiosis have half the number
of chromosomes as the parent cell.
• These cells are genetically different from the
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diploid cell and from each other.
• Meiosis is how sexually reproducing organisms
produce gametes.
11-4
If the body cells of humans contain 46
chromosomes, a single sperm cell should have
a. 46 chromosomes.
b. 23 chromosomes.
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c. 92 chromosomes.
d. between 23 and 46 chromosomes.
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During meiosis, the number of chromosomes
per cell is cut in half through the separation of
a. daughter cells.
b. homologous chromosomes.
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c. gametes.
d. chromatids.
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The formation of a tetrad occurs during
a. anaphase I.
b. metaphase II.
c. prophase I.
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c. prophase I.
d. prophase II.
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In many female animals, meiosis results in the
production of
a. only 1 egg.
b. 1 egg and 3 polar bodies.
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c. 4 eggs.
d. 1 egg and 2 polar bodies.
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Compared to egg cells formed during meiosis,
daughter cells formed during mitosis are
a. genetically different, while eggs are
genetically identical.
b. genetically different, just as egg cells are.
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b. genetically different, just as egg cells are.
c. genetically identical, just as egg cells are.
d. genetically identical, while egg cells are
genetically different.