Molecular and Cell Biology II
Molecular and Cell Biology II MICRB 252
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Date Created: 11/01/15
BMBMICRB 252 Spring 05 Notes Prof P Mitchell LECTURE 25 Meiosis part 1 CHAPTER 20 GERM CELLS AND FERTILIZATION Figures covered Fig 201 through 2013 201618 Asexual vs Sexual Reproduction Most higher organisms reproduce sexually and have only a very brief haploid phase Sexual reproduction increases rate of evolution by increasing genetic variation in the population Life cycles of higher and some lower eukaryotes involve alternations between diploid and haploid generations of cells Haploid cells for sexual reproduction are generated from diploid precursor cells by mei05is Meiosis 39 The formation of eggs and sperm begins in a similar way with meiosis 39 Meiosis requires one round of replication followed by two successive cell divisions I and II Prophase of division I is the longest part of meiosis it resembles 62 of mitotic cell cycle replicated DNA maintained for prolonged time o replicated homologous chromosomes pair up at the metaphase plate as bivalents 2 pairs of tightly associated sister chromatids o complementary DNA base pairing between homologous chromosomes allows crossing over to occur between non sister chromatids o Crossing over has important role in genetic reassortment in gametogenesis 8lt has been useful to geneticists mapping relative gene locations results in chiasmata that hold the pair of homologs together during metaphase synaptonemal complex distributes recombination nodules so cross over events are spread out At anaphase I homologues not chromatids separate additional means of reassorting maternal and paternal genes At anaphase of division sister chromatids separate haploid daughter cells result 0 O Primordial germ cells and sex determination in mammals 39 Small number of cells in gastrulating mammalian embryo are signaled by neighbors to become PGCs These cells migrate into genital ridges the primordial gonads A developing gonad will become an ovary unless its somatic cells have Y chromosome Sry gene on the Y is a testes determining gene Sry is a transcription factor expressed in subset of somatic cells in the developing gonad it induces these cells to become Sertoli cells Sertoli cells produce signals that promote male development and suppress female development and induce male PGCs to commit to sperm development Terms to know sexual vs asexual reproduction haploid vs diploid BMBMICRB 252 Spring 05 Notes Prof P Mitchell meiosis vs mitosis genetic recombination gametesgerm cells germ line vs somatic cells zygote bivalent crossing over non sister chromatids chiasma synaptonemal complex recombination nodule primordial germ cell Sry gene Sertoli cells Leydig cells theca cells