Meiosis
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As meiosis begins, the chromosomes start to become visible. At this stage they have produced replicas of each other. The centrioles separate and move to opposite poles (ends) of the cell.
The chromosomes shorten and fatten and become clearly visible. Similar-looking, or homologous, chromosomes line up alongside each other in pairs. Each pair of matching chromosomes is called a bivalent.
Each chromosome is now seen to consist of two threads. These threads are called chromatids and each is an exact copy of the parent chromosome. The chromatids are joined together at a point called the centromere.
The paired chromosome (bivalents) migrate to the middle line, or equator, of the cell. The nuclear membrane disappears. From the centrioles, a system of fibres called the spindle, stretches across the cell.
The chromosomes separate from their homologous partners and move to opposite poles of the cell. Each side of the cell now contains just half the normal number of chromosomes.
Nuclear membranes form and the cell splits into two. The centrioles duplicate and move to the poles of the daughter cells. New spindles begin to form. In many species, complete division does not occur at this stage.
New spindles have formed and the nuclear membranes have disappeared. The chromosomes, each consisting of two chromatids joined together at the centromere, migrate to the equators of the daughter cells.
Each chromatid separates from its partner and they move to opposite poles of the cell. These chromatids are now really daughter chromosomes and the ends of each cells have the haploid number (n=3).
Nuclear membranes form round the chromosomes and each cell splits. So, a diploid (2n=6) mother cell has now been converted into four haploid (n=3) gametes.
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by: Guest
Total views: 672
Word Count: 300
Date: Thu, 9 Aug 2007 Time: 12:00 AM
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