Meiosis
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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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 1
Prophase I
Metaphase I
Anaphase I
Telephase I
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Meiosis
Both Meiosis 1 and 2 proceed in 4 phases
Meiosis 2
Prophase II
Metaphase II
Anaphase II
Telephase II
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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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Cell biology
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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Cell biology
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 2
Metaphase II
In metaphase II, the chromosomes line up in the center of the cell.
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Meiosis
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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 2
Telophase II
In telophase II, the cells divide, new cell walls formed and there are four haploid cells.
Cell biology
Meiosis
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Meiosis 2
Telophase II
The four newly formed haploid cells are called a tetrad.
Cell biology
Meiosis
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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
Haploid nucleus
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