Paternal homologue Maternal homologue Diploid zygote€¦ · chromosomes begin to condense, and the...
Transcript of Paternal homologue Maternal homologue Diploid zygote€¦ · chromosomes begin to condense, and the...
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Haploid sperm
Haploid egg
Diploid zygote
Fertilization Paternal homologue
Maternal homologue
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n 2n
FERTILIZAOTIN MEIOSIS MEIOSIS
Sperm (haploid) n
Egg (haploid) n
Zygote (diploid) 2n
Somatic cells
Germ-line cells
Adult male (diploid) 2n Adult female
(diploid) 2n
MITOSIS
MITOSIS
Germ-line cells
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a.
b. c.
Sister chromatids
Homologues
Kinetochore Diploid cell
Chromosome duplication
Meiosis I
Meiosis II
Haploid cells
Centromere
Synaptonemal complex
Synaptonemal complex
Homologous chromosomes
138 nm b: Reprinted, with permission, from the Annual Review of Genetics, Volume 6 © 1972 by Annual Reviews, www.annualreviews.org
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Site of crossover
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Meiosis I Mitosis
Metaphase I
Anaphase I
Metaphase
Anaphase
Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere.
Microtubules pull the homologous chromosomes apart, but sister chromatids are held together.
Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere.
Microtubules pull sister chromatids apart.
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MEIOSIS I Prophase I Metaphase I Anaphase I Telophase I
Chromosome (replicated) Spindle
Chiasmata
Kinetochore microtubule
Homologous chromosomes
Sister chromatids Chromosome Nonidentical sister chromatids
In prophase I of meiosis I, the chromosomes begin to condense, and the spindle of microtubules begins to form. The DN A has been replicated, and each chromosome consists of two sister chromatids attached at the centromere. In the cell illustrated here, there are four chromosomes, or two pairs of homologues. Homologous chromosomes pair up and become closely associated during synapsis. Crossing over occurs, forming chiasmata, which hold homologous chromosomes together.
In metaphase I, the pairs of homologous chromosomes align along the metaphase plate. Chiasmata help keep the pairs together and produce tension when microtubules from opposite poles attach to sister kinetochores of each homologue. A kinetochore microtubule from one pole of the cell attaches to one homologue of a chromosome, while a kinetochore microtubule from the other cell pole attaches to the other homologue of a pair.
In anaphase I, kinetochore microtubules shorten, and homologous pairs are pulled apart. One duplicated homologue goes to one pole of the cell, while the other duplicated homologue goes to the other pole. Sister chromatids do not separate.This is in contrast to mitosis, where duplicated homologues line up individually on the metaphase plate, kinetochore microtubules from opposite poles of the cell attach to opposite sides of one homologue's centromere, and sister chromatids are pulled apart in anaphase.
In telophase I, the separated homologues form a cluster at each pole of the cell, and the nuclear envelope re-forms around each daughter cell nucleus. Cytokinesis may occur . The resulting two cells have half the number of chromosomes as the original cell: In this example, each nucleus contains two chromosomes (versus four in the original cell). Each chromosome is still in the duplicated state and consists of two sister chromatids, but sister chromatids are not identical because crossing over has occurred.
Paired homologous chromosomes
Homologue pair on metaphase plate
Homologous chromosomes
Sister chromatids
© Clare A. Hasenkampf/Biological Photo Service
40 µm 40 µm 40 µm 40 µm
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MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II
Sister chromatids
Sister chromatids Spindle
Nuclear membrane breaking down Chromosome
40 µm 40 µm 40 µm 40 µm
Following a typically brief interphase, with no S phase, meiosis II begins. During prophase II, a new spindle apparatus forms in each cell, and the nuclear envelope breaks down. In some species the nuclear envelope does not re-form in telophase I removing the need for nuclear envelope breakdown.
In metaphase II, a completed spindle apparatus is in place in each cell. Chromosomes consisting of sister chromatids joined at the centromere align along the metaphase plate in each cell. No w , kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids.
When microtubules shorten in anaphase II, the centromeres split, and sister chromatids are pulled to opposite poles of the cells.
In telophase II, the nuclear membranes re-form around four di f ferent clusters of chromosomes. After cytokinesis, four haploid cells result. No two cells are alike due to the random alignment of homologous pairs at metaphase I and crossing over during prophase I.
Nuclear membrane re-forming
Kinetochore microtubule
© Clare A. Hasenkampf/Biological Photo Service 8
Parent cell (2n)
MEIOSIS I
Prophase I Metaphase I Anaphase I Telophase I
Prophase Metaphase Anaphase T elophase
Homologous chromosomes do not pair.
Individual homologues align on metaphase plate.
Paternal homologue
Homologous chromosomes
Chromosome replication
Chromosome replication
Homologous chromosomes pair; synapsis and crossing over occur.
Paired homologous chromosomes align on metaphase plate.
Maternal homologue
MITOSIS
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Sister chromatids separate, cytokinesis occurs, and two cellsresult, each containing theoriginal number of homologues.
Two daughter cells (each 2n)
Homologous chromosomes separate; sister chromatids remain together.
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MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II
Chromosomes align, sister chromatids separate, and four haploid cells result, each containing half the original number of homologues.
Four daughter cells (each n)
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