Mitosis, Meiosis and Gametogenesis Mitosis is the mechanism by which somatic (body) cells in higher organisms replicate & divide. each cell has 2 copies of each of each chromosome, one that is of paternal origin, the other of maternal. These cells are said to be diploid or 2N, where N is the number of chromosomes per set (one full set is a genome). after mitosis both daughter cells have the same genetic composition. Mitosis is dynamic, but is divided into stages for descriptive purposes Interphase; chromosomes invisible, between divisions Prophase: chromosomes condense and become visible Each can be seen to have already doubled; the duplicates are referred to as chromatids Chromatids are held together by a common centromere (kinetochore) The number of chromosomes = the number of centromeres Centriole splits, migrates apart to form poles and spindle fibers form Metaphase: all chromosomes are aligned along the equator, attached at the centrommere to spindle fibers from both poles Anaphase: spindle fibers from each pole that are attached to the centromeres begin to contract and separate the daughter chromatids
Telophase; two new nuclei form following cytokinesis (division of the cytoplasm) animal cells "pinch" into two daughters plant cells divide by laying down a dividing plate Useful reviews of the stages of mitosis can be seen at several sites. Several include "animations" to show the changes that occur. http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html http://gened.emc.maricopa.edu/bio/bio181/biobk/biobookmito.html http://www.csuchico.edu/~jbell/biol207/animations/mitosis.html http://galileo.physiology.uiowa.edu/animations/mitosis.htm
Meiosis occurs in specialized cells in "germline" tissues ovaries produce eggs testes produce sperm (in plants, anthers produce pollen) As in mitosis, chromosomes have already replicated when the visible events begin The products of meiosis have one of each of the chromosomes; that is they are haploid (1N) Two meiotic divisions follow one replication Meiosis I (MI): the first division reduces the number of chromosomes to 1N, although each is still doubled Meiosis II (MII): the centromeres split and daughter chromosomes separate Prophase I: homologous chromosomes pair side by side. This pairing is called synapsis. Metaphase I: homologous chromosomes remain paired when they line up at the equator. The paired chromosomes are often called a tetrad, since the chromosome arms have already doubled, and each chromosome can be seen to exist as a pair of chromatids, which are joined at the centromere. Anaphase I: homologues centromeres are pulled to opposite poles, effectively halving the number of chromosomes, but as indicated previously, each chromosome is still doubled. Telophase I may be very short, cytokinesis occurs but nuclei may not be fully re-formed.
Meisosis II occurs in the daughter cells of meiosis I without another round of replication. The steps are analogous to those in mitosis. Some good reviews of meiosis, some with animations or videos can be found at: http://gened.emc.maricopa.edu/bio/bio181/biobk/biobookmeiosis.html http://mentor.lscf.ucsb.edu/mcdb4a/meiosis.htm The movement of one member of each homologous pair of chromosomes in MI to opposite poles explains Mendels law of segregation; that is each gamete gets one allele from each gene. Since only chance determines which member of any pair of chromosomes goes to a specific pole, genes on different chromosomes assort independently, accounting for Mendel's second law (independent assortment)
Gametogenesis Meiosis is just the beginning of gamete formation; using humans as an example: In males, all four products of meiosis can develop into sperm meiosis is continuous in many cells after sexual maturity takes about 74 days to convert to functional sperm tail comes from centriole head contains nucleus and acrosome from golgi midpart is packed with mitochondria In females, each meiotic division is followed by unequal division of cytoplasm; one nucleus is shunted into a non-functional polar body Meiosis is initiated in ovaries before birth of female, but arrests at prophase I after pberty, one ovarian follicle per month resumes division Secondary oocyte bursts into Fallopian tube The second division of meiosis is not completed unless the oocyte is penetrated by a sperm