Sexual Life Cycles:Reproductive System Notes

Sexual Life Cycles:Reproductive System Notes - Meiosis...

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Meiosis & Sexual Life Cycles Notes Quiz 10 – questions 2, 4, 7, Quiz 11 – questions 3, 6, 8, 9, 11 MEIOSIS Takes place only in gonads Produces haploid gametes (eggs and sperm) from a diploid parental cell Human gametes carry 23 chromosomes During fertilization, haploid sperm and egg fuse to form a diploid zygote. Cells of zygote divide by mitosis to produce an adult organism A & B = diploid C & D = haploid DNA content halved in both meiosis I and meiosis II Ploidy level changes from diploid to haploid in meiosis I (separation of homologous chromosomes) and remains haploid in meiosis II MEIOSIS I: reductional division Prophase I: Starting cell is diploid (2n = 4). Duplicated homologous chromosomes pair up and exchange segments (crossing over/synapsis) chiasmata = sites of crossing over. centromere = point of attachment between two duplicated chromosomes. Metaphase I: Homologous pairs (tetrads) line up at the metaphase plate. 4 chromosomes in total. Anaphase I: Homologous pairs separate. Sister chromatids remain together. Telophase I: Results in 2 haploid cells (n=2). Each chromosome has two (non- identical) sister chromatids.
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MEIOSIS II: equational division Prophase II: Same as mitotic prophase except cells are haploid Metaphase II: Same as mitotic metaphase except cells are haploid Anaphase II: Sister chromatids separate and migrate to opposite poles Telophase II: 4 haploid cells present at end of telophase II and cytokinesis What are sources of genetic variability in sexual reproduction?? Crossing over (synapsis) at prophase I Independent assortment at metaphase I (separate at anaphase I) Random fertilization GAMETOGENESIS (spermatogenesis & oogenesis) Process by which gametes are formed in humans
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Germ cells produce cells that divide by mitosis and then undergo meiosis to form gametes SPERMATOGENESIS (review pg. 1022) Continuous and prolific Occurs in seminiferous tubules Spermatogonia divide by mitosis and differentiate into primary spermatocytes At puberty, hormones stimulate the primary spermatocytes to undergo meiosis I to form secondary spermatocytes, which then undergo meiosis II to form spermatids Spermatogonial stem cell  spermatogonium  primary spermatocyte (2n)  secondary spermatocyte (n)  early spermatid  (differentiation… sertoli cells provide nutrients)  sperm cell Sperm mature + become motile in epididymis Acrosome contains enzymes that help sperm penetrate egg Mitochondria provide ATP for movement Path of sperm: seminiferous tubule  epididymis  vas deferens  urethra OOGENESIS Prolonged Immature eggs form in ovary of female embryo but do not complete development until years/decades later Primordial germ cell undergoes mitotic divisions  diploid oogonium Diploid undergoes more mitotic divisions  primary oocyte At birth there are 100,000 primary oocytes, which are arrested in prophase I of meiosis I At puberty, egg follicle releases a secondary oocyte – meiosis
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