Meiosis
Overview of Meiosis Sexual reproduction Gametes, with half the # of chromosomes, are produced 2n1n Ensures that the next generation will have a combination of traits from both parents
Overview of Meiosis Homologous chromosomes: similarly constructed chromosomes with the same shape and that contain genes for the same traits (homologues) Just like in mitosis, meiosis occurs after interphase, when the cell grows, DNA is replicated, and chromosomes are duplicated.
Genetic recombination: production of offspring with combinations of traits that differ from those found in either parent. Crossing over Independent assortment
Meiosis I Prophase I Synapsis occurs (pairing of homologous chromosomes) Spindle appears Nuclear membrane fragments Nucleolus dissappears
Meiosis I Prophase I Synapsis Homologous chromosomes line up side by side Crossing-over: exchange of genetic material between non-sister chromatids Result= chromatids held together by centromere are no longer identical Daughter cells will receive chromosomes with recombined genetic material
Meiosis I Metaphase I Homologous pairs line up at equator Maternal and paternal member may be oriented toward either pole. In humans, results in 223 (8,388,608) combinations of maternal and paternal chromosomes in gametes
Anaphase I Homologous chromosomes separate, pulled to opposite poles by spindle fibers Independent assortment: all possible combinations of chromosomes occur in the daughter nuclei.
Telophase I Nuclear membrane reforms Nucleoli appear Cytokinesis produces two haploid daughter cells
Interkinesis: between meiosis I and II No DNA replication since chromosomes are already dublicated
Prophase II: spindle appears, nuclear membrane disassembles, nucleolus disappears Metaphase II: chromosomes attach to spindles and align at equator Anaphase II: sister chromatids separate becoming daughter chromosomes Telophase II: spindle disappears, nuclei form, nuclear membrane reforms Cytokinesis occurs Meiosis results in four haploid daughter cells
Nondisjunction Failure of homologous chromosomes to separate during anaphase I, or sister chromatids to separate at anaphase II Results in gametes with abnormal number of chromosomes Causes various syndromes Trisomy 21 (Down Syndrome)—extra copy of chromosome 21 Turner Syndrome—absence of second X chromosome
Daughter cells mature into gametes (eggs, sperms) that fuse during fertilization Restores the diploid number of chromosomes in a cell that will develop into a new individual.