More than one celled organisms grow by way of mitosis and the cytoplasmic division of body cells. On the other hand, meiosis occurs only in germ cells, which are put aside for the formation of gametes (sperm and egg). Reproduction by meiosis allows for species survival and it increases genetic variability.
The process, during which the germ cells are generated is called meiosis. It represents nature's solution to the problem of chromosome doubling that would occur, if two diploid cells, i.e. two cells with a double set of chromosomes would fuse. Accordingly does meiosis produce haploid germ cells, with maternal and paternal germ cell fusing at fertilization and thus generating a diploid fusion product, the zygote. Meiosis is made up by two subsequent processes, both of which resemble mitosis. In the first process are the homologous chromosomes separated. It has an unusually long prophase that is subdivided into different stages They are followed by metaphase, anaphase and telophase.
Two nuclei fuse upon fertilization, so that the number of chromosomes does necessarily double. If this thought is spun further, would an greater growth of the number of chromosomes from generation to generation have to be expected. This is not the case, because the chromosomes are reduced to half their normal number in germ cell production. This process is called meiosis. It consists of two successive mitosis-like divisions: in the first division is the number of chromosomes reduced to their half, the second is a normal mitosis.
Each germ cell contains a complete set of chromosomes, a haploid set. Accordingly are the cells haploid and zygotes and the body cells that stem from them are diploid, because they contain two equal sets of chromosomes, one from the mother and one from the father. They exist, especially in plants.
At the beginning of meiosis, in prophase 1 the plate breaksdown and chromosomes become visible as in mitosis (1). The chromosomes have replicated but individual chromatids are not visible. Instead of lining up on a metaphase, as in mitosis, chromosomes come together in pairs (2). Each chromosome in a pair is similar in structure (homologous), but would have come originally from different parents. Later in prophase the homologous pairs twist round each other and chromatids may cross over (3). Breaks occur at these crossovers or chiasmata, and pieces of chromatid are exchanged (4). Chromosome pairs line up across the equator of the spindle at metaphase I (5). In anaphase I the chromosomes separate and travel to opposite ends of the spindle. The chromosomes migrate to the equators of two new spindles for metaphase 2 (7). Next the chromatids are pulled apart in anaphase 2 to form four clusters of chromosomes in telophase 2. The nuclear envelopes reform around four haploid nuclei that will give rise gamete
The leptotene. This phase differs only slightly from the early stages of mitosis. Usually are the cells and nuclei of meiotic tissues bigger than that of their neighbouring tissues and often do they seem to be longer and are longitudinally structured. At regular intervals can thickenings be found, like beads on a string: the chromomeres. Their number, size and positioning is constant in each species.
The zygotene. During this phase begins the pairing of homologous chromosomes. It is also called synapsis and the resulting structure synaptic complex. Directly after initiation of the process does the pairing spread like a zipper across the whole length of the chromosome.
The pachytene. During the pachytene does the pairing stabilize. The number of synaptic complexes corresponds to the number of chromosomes in a haploid set of the respective species. The pairs are also called bivalents. The diplotene. The bivalents separate again. During this does it become visible that each chromosome is built of two chromatids, so that the whole complex stands still, four strands during the separation. Normally...
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