Centromeres are important structures in chromosomes during mitosis and meiosis. Chromosomes consist of two identical chromatids joined by a centromere. Mitosis produces two identical cells, while meiosis produces four gametes with half the DNA.
A centromere forms part of the structure of a chromosome during nuclear divisions, both mitosis and meiosis. Chromosomes are made up of long strands of DNA, which wind up before either type of division. This DNA coiling occurs during interphase before mitosis or meiosis occurs. It is an extremely important period as the DNA molecules make an identical copy of themselves ensuring that the offspring have the correct amount of DNA when division is complete.
When DNA has replicated, chromosomes consist of two identical arms, called chromatids. Each chromatid contains one copy of the replicated DNA and the area where the chromatids are held together is called the centromere. Centromeres can be found anywhere along the length of the chromosome, but the location is specific to any particular chromosome. The centromeres plus the two chromatids make up the structure of a single chromosome.
Mitosis is the process in which cells reproduce by dividing to form two identical offspring. Mitosis occurs in most cells and produces new cells to replace old or damaged ones or to allow the organism to grow. Meiosis occurs only in the reproductive organs and is how gametes, sperm and eggs are produced in humans. Centromeres play an important role during both mitosis and meiosis.
Mitosis consists of four stages and at the end of the first, duplicate copies of DNA are formed with the centromere joining the two identical chromatids. At the same time, a spindle consisting of protein microtubules forms across the nucleus. During metaphase, the second stage of mitosis, chromosomes line up in the center of the spindle. Each chromosome is joined to a spindle microtubule at its centromere.
During prophase, identical chromatids of each chromosome are separated. They are pulled to opposite poles of the nucleus by microtubules attached to each centromere. After each chromosome has been divided, the cell divides producing two identical cells with the same DNA in each. All cells except gametes have two copies of each chromosome. One chromosome of each pair comes from the father and one from the mother. These chromosomes have the same genetic information, genes, found in the same place and are called homologous chromosomes.
Meiosis is basically mitosis that occurs twice within the same cell, with some specific differences. First, during meiosis, homologous chromosomes line up along the spindle. When the spindle fibers attach to the centromeres, the chromosomes are separated. Two cells form, but now have only one copy of each chromosome, or half of the DNA.
The second DNA cleavage is identical to mitosis. Chromosomes line up along the spindle equator, and each microtubule joins a centromere of each chromosome. The chromatids are pulled to opposite poles of the nucleus and a new cell is formed. Since chromatids have identical copies of DNA, each final cell has a single copy of each chromosome. The end result of meiosis is the existence of four gametes with only half of the DNA.
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