ZOL141 1st Edition Lecture 8Outline of Last Lecture II. meiosisA. Meiosis 1B. Meiosis 2Outline of Current Lecture III. Meiosis: New Gene CombinationsA. Random assortmentB. Crossing overI.Formation of Gametesa.Malesb.FemalesCurrent LectureMeiosis: New Gene CombinationsTwo mechanisms produce new combinations:- Random Assortment:o Chromosome pairs can line up differently (in metaphase 1 of meiosis)o If 2n=6, 8 different gameteso Maternal and paternal chromosomes align independentlyo For 3 chromosome pairs, 8 possible gametes=2^3, possibilities: 2n- Crossing Overo Alleles: what is it? One variant of a gene Sometimes, a different allele can result in a different phenotype Sister chromatids are genetically identical, so they will always be the same Homologous chromosomes can differ Gene can have many alleles but each individual has only two alleles One from mom, one from dado Chromosome arms (non sister chromatids) physically exchange genetic materialo Combination of alleles is changedo Increases genetic variation in humanso Chromosome combos of offspring theoretically possible: 8x10^23Formation of Gametes- Males: spermatogenesis (begins at puberty) (64 days)o One spermatocyte generates four spermatozoao Spermatogonia in the testes divide by mitosis to produce spermatocyteso Spermatocytes undergo meiosis to form spermatids (haploid)o Spermatids undergo structural changes to become functional spermo 2n-2n-n-n-no seminiferous tubule: where it takes place- Females: oogenesis (begins during embryogenesis) (12-50 years)o Oogenia divide by mitosis to form primary oocyteso Primary oocytes undergo meiosis to form secondary oocytes (haploid)o Secondary oocyte: produced by the first meiotic divisiono Unequal division of cytoplasm: One larger functional gamete (ovum), two or three smaller polar bodieso Ovum: the haploid cell produced by meiosis that becomes the functional gamete
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