39 Meiosis and Sexual Reproduction
Summary
OBJECTIVEDescribe the details of meiosis.
Cell division in reproductive cells is called meiosis. This two-phase process divides the chromosomes of a diploid germ cell, generating four haploid gametes. During prophase I, the nuclear envelope begins to breakdown and nuclear chromatin starts to condense into individual chromosomes made up of two sister chromatids. Then, during metaphase I, pairs of homologous chromosomes (called tetrads) move along their microtubule attachments so they are lined up along the metaphase plate. The next step is anaphase I, during which the attachments between the homologous chromosomes break down, and kinetochores pull the homologous chromosomes towards opposite poles. The final stages of meiosis I are telophase and cytokinesis, during which the cells split apart forming two daughter cells. The first phase of meiosis II is prophase II, during which the nuclear envelope breaks down and the spindles reform. During metaphase II, the chromosomes align along the metaphase plate. During anaphase II, sister chromatids (considered individual chromosomes when they separate) move towards opposite poles of the meiotic spindle. In the final stage of meiosis II, the chromosomes reach the poles, the spindle breaks down, and nuclear envelopes reform. Cytokinesis produces four haploid daughter cells from the original diploid cell.
OBJECTIVEDistinguish processes and outcomes of meiosis I and meiosis II.
Meiosis I is the reduction division, and meiosis II is more similar to mitosis in that the sister chromatids are separated. The goal of meiosis I is to separate homologous chromosomes. The goal of meiosis II is to separate sister chromatids. In meiosis II, no DNA is duplicated as in prophase I of meiosis I.
OBJECTIVERelate sexual reproductive processes to the adaptive advantage of genetic variability.
Independent assortment, crossing-over and random fertilization introduce genetic variability into the new offspring. Gametes are the result of an immense number of genetic possibilities created during independent assortment and crossing over. Humans have 23 pairs of chromosomes. Because of independent assortment during meiosis I, there are 8.4 million possible gametes that may be created even if crossing over did not occur.
Key Terms
Anaphase I:Stage of cell division in which the homologous chromosomes are moved to opposite ends of the cell.
Anaphase IIStage of cell division in which the chromosomes divide and each chromatid moves to the opposite poles of the cell.
Bivalent: Paired sister chromatids that make a group of four chromatids; also called a tetrad.
Chiasm: The point of crossover between chromosomes or other biological entities; pl. chiasmata.
Cohesin: Protein complex made of four subunits that binds sister chromatids together.
Crossing over: Exchange of genetic material between homologous chromosomes in meiosis.
Homologous chromosome: In diploid cells, the two copies of each chromosome containing the same sequence of genes, although the alleles may differ.
Meiosis: Type of cell division resulting in four haploid cells, the gametes.
Meiosis I: Homologous chromosomes separate.
Meiosis II: Sister chromatids separate.
Metaphase I: Stage of cell division in which the chromosomes align along the equator of the cell. Homologous chromosomes align parallel to each other.
Metaphase II: Stage of cell division in which the chromosomes are moved to the equator of the cell.
Prophase I: Stage of cell division in which the chromosomes coil and condense.
Prophase II: Stage of cell division in which the chromosomes attach to the reforming spindle apparatus.
Sister chromatid: One of a pair of chromatin threads; one side of a chromosome X-shape.
Synapsis: Binding of homologs during the early part of cell division.
Telophase I: Stage of cell division in which the chromosomes may uncoil and the nucleus forms.
Telophase II: Stage of cell division in which the chromosomes begin to uncoil and the nucear envelope begins to form.
Tetrad: Set of paired sister chromatids that form a group of four chromatids; also called a bivalent.
References
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