Lecture 11: Meiosis

Meiosis Which cells divide by meiosis? Somatic cells (all cells but germ cells) divide by mitosis Germ cells = precursors of eggs and sperm Germ cells divide by meiosis Unicellular eukaryotic organisms (yeast and others) can divide by meiosis What is meiosis? Meiosis is necessary for sexual reproduction From meioun (Greek) = make smaller During meiosis diploid (2N) germ cells generate haploid (1N) cells for reproduction Occurs in all eukaryotes, except for few which lost their ability for sexual reproduction Does not happen in prokaryotes Why is meiosis beneficial? Mitosis produces identical copy of the cell Sexual reproduction, part of which is meiosis produces a cell which is not the same Greater chance to produce an organism which will survive in unpredictably variable environment What happens in meiosis? Meiosis is not a part of cell cycle, as it does not happen again with the same cell Meiosis still consists of the similar phases G1 - growth S - DNA replication G2 - growth Meiotic division I (prophase I, metaphase I, anaphase I, telophase I) Meiotic division II (prophase II, metaphase II, anaphase II, telophase II) Two divisions of meiosis First we had a germ cell, which is 2N After S phase it became 4N (DNA doubled) After first division: 2 cells with 2N After second division: 4 cells with 1N Mitosis and meiosis division I Bivalent forms of meiotic prophase I In mitosis both daughter cells get one paternal and one maternal chromosomal In meiotic division I one cells gets two maternal, another gets two paternal Chromatids in two daughter cells do not separate in meiosis How many of each chromosomes are there in a cell that enters meiotic division I? • A. One paternal and one maternal • B. Two paternal and two maternal Bivalent Bivalent is a complex of 4 chromatids: 2 pairs of sister chromatids Sister chromatids are two identical copies, which resulted from replication in S-phase Homologous chromatids are not the same, they came from father and mother Which chromosomes go to different daughter cells in mitosis? • A. Homologs • B. Sisters (identical replication) What is the purpose of bivalent? Recombination happens between homologs at several spots along the chromosomes Cross-overs (also called chiasma, plural - chiasmata) are the results of recombination events Chromosomes exchange parts of chiasmata Crossovers hold homologs together in meiosis I Meiotic prophase I Leptotene - (Greek - thin shreds). Sister chromatids form visible strands within the nucleus Zygotene - (paired threads). Synapses (pairing/coming together) of homologous chromosomes, facilitated by assembly of the synaptonemal complex Pachytene - (thick threads). Tetrad of the chromosomes forms a bivalent. Chromosomal crossovers (chiasmata) form Diplotene - (two threads). The synaptonemal complex degrades and homologous chromosomes remain tightly bound at chiasmata. All human eggs remain in this stage until they ovulate, one egg at a time Diakinesis- (moving through). Resembles prometaphase of mitosis: the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form What holds 4 chromatids together? Two pairs of sisters are held together by chiasmata Sister wishing each pair are glued with cohesins Kinetochores of sister chromatids function as one At anaphase, the sister chromatids are still connected at centromeres with other special kind of cohesins Two sources of genetic variation during meiosis Maternal and paternal chromosomes are combined in different ways in gametes Exchange of genes between paternal and maternal chromosomes How many different gamete’s can form in organisms with 3 chromosomal pairs? • A. 3 • B. 6 • C. 8 • D. 9 Two major mechanisms for reassortment of genetic material during meiosis to increase genetic variability in population Random distribution (independent assortment) of maternal and paternal chromosomes yields 2^n different haploid gametes for organism with n chromosomes Crossing-over during meiotic prophase I exchanges segments of maternal and paternal homologous chromosomes