A basic overview of meiosis (production of sex cells)
Uploaded by monkees4va on Nov 19, 2008
Meiosis is a two-part cell division process in organisms that sexually reproduce. Meiosis produces gametes with one half the number of chromosomes as the parent cell. The two stages of meiosis are meiosis I and meiosis II. Under normal conditions, chromosomes, (the term for a molecule of DNA ) are packed together in the nucleus. At the end of the meiotic process, four daughter cells are produced. Each of the resulting daughter cells has one half of the number of chromosomes as the parent cell.
Meiosis takes place in reproductive tissue. In animals, meiosis takes place in the testes and in females within the ovaries. In plants, meiosis occurs in the anthers to form pollen grains and within the ovules of the ovaries. In meiosis a diploid cell divides in such a way so as to produce four haploid cells. The haploid cells then form as gametes. Each gamete is genetically different, as a result of both random alignment of the homologous pairs of chromosomes (independent assortment of the chromosomes) and crossing over between members of a homologous pair.
In the first initial stage the chromosomes become visible with a light microscope as they condense ( as they shorten, coil, and thicken). Also, the spindle fibre begins to extend outward from each of the two centriole pairs. Then the nuclear membrane begins to break up and disappear and the chromosomes migrate to the equator, with the homologous chromosomes together One member of each homologous pair then is pulled by spindle fibres to each pole.
The second stage of meiosis continues this process again to split the separated homozygous chromosomes into four gametes The cell then divides into two and the nuclear membrane may reform, although it does not always happen. The centrioles then move back to the opposite poles and they are split apart by spindle fibres, creating four different gametes for reproduction.
The process whereby one or more gene alleles present in one chromosome may be exchanged with their alternative alleles on a homologous chromosome to produce a recombinant (crossover) chromosome. This may happen when in the first stage of meiosis when homologous chromosomes line up with each other on the equator to be separated by spindle fibres. This helps contribute further to variation and it is at this stage that gene mutations can occur.