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.
Megaspore mother cell in pine.
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Megaspore mother cell in lily.
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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 called meiosis 1
and meiosis 2.
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Meiosis
Both Meiosis 1 and 2 proceed in 4 phases
Meiosis 1
Meiosis 2
Prophase I
Prophase II
Metaphase I
Metaphase II
Anaphase I
Anaphase II
Telephase I
Telephase II
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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
Late in telophase I, separate
cells can be identified, but no
cell plate is formed between
cells as would occur 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 in meiosis 1.
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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
cells divide, new cell
walls formed and there
are four haploid cells.
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Meiosis
Meiosis 2
Telophase II
The four newly
formed haploid cells
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 within an egg sac.
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Meiosis
The typical arrangement of nuclei in an embryo sac
includes three antipodals, a central cell with two
polar nuclei, two synergids and an egg cell.
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.
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Haploid nuclei in a lily
embryo sac.
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Meiosis
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.
Pollen grains
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Haploid
nucleus
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