Upon fertilization, both the DNA content and the number of chromosomes doubles to 2c and 2n, respectively. Following DNA replication, the DNA content doubles again to 4c, but each pair of sister chromatids is still counted as a single chromosome a replicated chromosome , so the number of chromosomes remains unchanged at 2n. If the cell undergoes mitosis, each daughter cell will return to 2c and 2n, because it will receive half of the DNA, and one of each pair of sister chromatids. In contrast, the 4 cells that come from meiosis of a 2n, 4c cell are each 1c and 1n, since each pair of sister chromatids, and each pair of homologous chromosomes, divides during meiosis.
For simplicity, nuclear membranes are not shown, and all chromosomes are represented in a similar stage of condensation. In the future they may be used to replace cells and tissues that have been damaged or lost due to disease. If you have any other comments or suggestions, please let us know at comment yourgenome.
Can you spare minutes to tell us what you think of this website? Open survey. In: Facts In the Cell. In metaphase individual chromosomes pairs of chromatids line up along the equator. During anaphase the sister chromatids are separated to opposite poles.
During anaphase I the sister chromatids move together to the same pole. During anaphase II the sister chromatids are separated to opposite poles. Similarities Mitosis Diploid parent cell Consists of interphase, prophase, metaphase, anaphase and telophase In metaphase individual chromosomes pairs of chromatids line up along the equator.
Ends with cytokinesis. In meiosis, a cell containing the diploid number of chromosomes is converted into four cells, each having the haploid number of chromosomes. In human cells undergoing meiosis, for instance, a cell containing 46 chromosomes yields four cells, each with 23 chromosomes. Meiosis occurs by a series of steps that resemble the steps of mitosis.
Two major phases of meiosis occur: meiosis I and meiosis II. During meiosis I, a single cell divides into two. During meiosis II, those two cells each divide again. The same demarcating phases of mitosis take place in meiosis I and meiosis II—prophase, metaphase, anaphase, and telophase—but with some variations contained therein. As shown in Figure , first, the chromosomes of a cell are divided into two cells. The chromosomes of the two cells then separate and pass into four daughter cells.
The parent cell is diploid, while each of the daughter cells has a single set of chromosomes and is haploid. Synapsis and crossing over occur in the prophase I stage. Figure The process of meiosis, in which four haploid cells are formed. The members of each chromosome pair within a cell are called homologous chromosomes. Homologous chromosomes are similar but not identical. They may carry different versions of the same genetic information.
For instance, one homologous chromosome may carry the information for blond hair while the other homologous chromosome may carry the information for black hair. As a cell prepares to enter meiosis, each of its chromosomes has duplicated in the synthesis stage S of the cell cycle, as in mitosis. Each chromosome thus consists of two sister chromatids. At the beginning of meiosis I, a human cell contains 46 chromosomes, or 92 chromatids the same number as during mitosis. Meiosis I proceeds through the following phases:.
The chromatids shorten and thicken and become visible under a microscope. An important difference, however, is that a process called synapsis occurs. Synapsis is when the homologous chromosomes migrate toward one another and join to form a tetrad the combination of four chromatids, two from each homologous chromosome. A second process called crossing over also takes place during prophase I.
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