Cell biology
Meiosis
Meiosis is a type of cell
division - like mitosis, but it
results in four haploid cells
with one-half the number of
chromosomes as the original
diploid cell.
Meiosis occurs in plants only
during sexual reproduction
in
specialized
cells
to
produce a haploid egg cell.
Female egg cell in pine.
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Cell biology
Meiosis
Meiosis differs from mitosis in
several important ways.
1.
The result of a mitotic
division is two diploid cells,
while meiosis results in four
haploid gametes.
2.
Mitosis requires one cell
division, while meiosis requires
two divisions.
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Cell biology
Meiosis
Meiosis 1
Prophase I
Early in prophase I the
chromosomes
become
visible as thin threads
within the nucleus.
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Meiosis
Meiosis 1
Prophase I
Early in prophase I, the
chromosomes become visible
as thin threads within the
nucleus.
Just as in mitosis, the
chromosomes have doubled
during interphase and the
chromosomes appear as two
chromatids attached at the
centromere.
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Meiosis
Meiosis 1
Prophase I
The two chromatids now
appear as single condensed
threads attached at the
centromere.
Homologous
pairs
of
chromosomes
become
associated and are lined up
at their centromeres. Each
pair is called a bivalent.
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Meiosis
Meiosis 1
Prophase I
An
important
aspect
of
prophase I is that the
bivalents
become
tightly
intertwined and pieces of one
chromatid can cross-over to
the other chromosome.
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Meiosis
Meiosis 1
Prophase I
In the last stages of
prophase I, you can again see
the two chromatids attached
to a common centromere.
However, the chromatids are
now
different
because
crossing-over
has moved
genetic material from one
homologous chromosome to
the other.
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Meiosis
Meiosis 1
Metaphase I
During metaphase I, the
paired chromosomes move to
the middle of the cell in
preparation for division.
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Meiosis
Meiosis 1
Anaphase I
In anaphase I, the chromosomes
separate and move to opposite
ends of the cell.
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Meiosis
Meiosis 1
Telophase I
In telophase I, the cell divides
and the chromosomes again
appear thread-like.
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Meiosis
Meiosis 1
Telophase I
Later in telophase I, cells
divide, but no cell plate is
made as occurs in mitosis.
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Meiosis
Meiosis 2
Prophase II
Prophase II starts the second
division stage of meiosis. The
chromosomes become more
distinct again.
Each chromosome has two
chromatids, but notice how
each chromatid is no longer
identical because of crossingover.
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Meiosis
Meiosis 2
Metaphase II
In metaphase II, the
chromosomes line up in
the center of the cell.
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Meiosis
Meiosis 2
Anaphase II
In
anaphase
II,
the
chromatids pull away from
each other. Each has its own
centromere.
The separated chromosomes
move to opposite ends of the
cell.
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Meiosis
Meiosis 2
Telophase II
In telophase II, the
divide, new cell walls
formed and there are
haploid cells. These four
are called a tetrad.
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Meiosis
Meiosis 2
Telophase II
In telophase II, the
divide, new cell walls
formed and there are
haploid cells. These four
are called a tetrad.
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Meiosis
Meiosis takes place in the reproductive cells in the flower.
The result of meiosis in the female megagametophyte is an ovule typically
with 8 haploid nuclei.
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Meiosis
Typical arrangement of nuclei in an
embryo sac. A male nucleus will fuse
with the polar nuclei to form the
endosperm and another with the egg
cell to form the embryo.
Haploid nuclei in a lily
embryo sac.
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Meiosis
Pollen grains
Haploid
nucleus
Meiosis in the male part of the flower leads to the production of sperm cells
located in the pollen grains. After flower pollination the haploid sperm cell
fuses with the female egg cell leading to a fertilized diploid cell that grows into
the embryo located in the seed.
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