Meiosis Reduction Division
Check us out at http://www.tutorvista.com//videos The basic number of chromosomes in the body cells of a species is called the somatic number and is designated 2n. Thus, in humans 2n = 46. We have 46 chromosomes. In the sex cells the chromosome number is n (humans: n = 23). So, in normal diploid organisms, chromosomes are present in two copies, one from each parent (23x2=46). The only exception are the sex chomosomes. In mammals, the female has two X chromosomes, and the male one X and one Y chromosome. A karyotype is the characteristic chromosome number of a eukaryote species.The preparation and study of karyotypes is part of cytogenetics, the genetics of cells. All eukaryotes that reproduce sexually use meiosis. This also includes single-celled organisms. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission. Metaphase I: The chromosomes line up along the equatorial (the central line) of the spindle fibres in homologous pairs. Anaphase I: The chromosomes are divided so that there are equal amounts on either side of the cell. As there are 46 chromosomes in a human cell, 23 end up on either side. (Cytokinesis, the division of cells into two, takes place. The cell divides.) Telophase I: The two daughter cells are completely divided, a nucleic envelope forms and the chromosomes become less visible. There are 23 chromosomes in each of these cells. The two cells prepare to divide again in a stage known as Interkinesis or Interphase II. Both of these cells will go through meiosis II. Prophase II: The chromosomes become visible, the nuclear envelope disappears and the centrioles form the spindle fibres. Metaphase II: The chromosomes line up along the middle line on the spindle fibres Anaphase II: The chromosomes get split into its two chromatids. Chromatids are the two strands of DNA (deoxyribo-nucleic acid) that make up the chromosome. They are joined by a mid-way connection called a centromere. (Cytokinesis takes place. The cell divides.) Telophase II: The cells are completely divided. The nucleic envelope reforms and four new cells with different DNA are created. In males, all four cells become sperm. In females, only one becomes a mature egg, while the remaining three become re-absorbed into the body.