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Cell Reproduction

6-3 How Gametes Form: Meiosis














Home | 6-1 Chromosomes | 6-2 Mitosis and Cell Division | The Cell Cycle | 6-3 How Gametes Form: Meiosis | Vocabulary





We know that most cells have 46 chromosomes. Gametes are the exception. The are haploid which means they only have 23 chromosomes. Gamete is a reproductive cell, an egg or sperm. Gametes are only in organisms that reproduce sexually, as do most animals. In sexual reproduction, two gametes fuse to form the first cell of an individual.

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Gametes are formed by a tpe of muclear division called meiosis. Meiosis is a two-stage form of nuclear division in which the chromosomes number is cut in half.

  1. Diploid (2n) cell nuclei here have four chromosomes. One member of each chromosome pair is from one parent; the other is from another parent.
  2. First the amount of DNA doubles. The the similar chromosomes pair with one another. Occasionally, paired chromosomes with exchange segments in a process called crossing-over.
  3. The first meiotic division (meiosis 1) seperates the members of each pair of chromosomes. Meiosis 1 reduces the number of chromosomes in the cell by half.
  4. The second meiotic division (meiosis 2) seperates the two copies of each chromosomes (the chromatids). Cell division now occurs, producing four haploid (n) cells.
  5. Sperm Formation- the four cells containing these nuclei develop heads and tails.

       Egg Formation- only one of the cells containing these nuclei becomes a mature egg; the other three usually die.

 

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When two haploid gametes unite in fertilization, the resulting cell is diploid. It received one of each type of chromosome from the egg of the female partent and one of each from the sperm of the male parent. During the first division of meiosis, the two members of each chromosome pair move together and line up side by side. Proteins hold the two pairs of chromosomes so closely together that you can't tell them apert.
 
While they are together they may exchange segments of DNA. This is called crossing-over. The importance of crossing-over is in its consequences: the two are now not identical. Now each chromosomes cantains genetic materail from both parents. Crossing-over allows for natural selection to occur.

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"Biology is the only science in which multiplication means the same thing as division."