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Chapter 9

BIOL 1000 Chapter 9 - Genetic Recombination.docx

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BIOL 1000
Jennifer Steeves

Chapter 9 – Genetic Recombination 9.1 – Mechanism of Genetic Recombination • Alleles are different versions of the same Gene • Genetic recombination requires:  Two DNA molecules that differ from one another  Mechanism for bringing the DNA molecules into close proximity  Collection of enzymes to: cut, exchange, and paste DNA back together. • Homologous – Similar • Homology allows DNA on different molecules to line up and recombine precisely • Enzymatic cutting and pasting of both DNA backbones from each of the two DNA molecules is required for each recombination event 9.3 – Genetic Recombination in Eukaryotes: Meiosis Plants and Some Fungi • An alternating-generations life cycle  Either the haploid or the diploid phases are multicellular  Both divide by mitosis • The diploid Sporophytes are produced by fertilization • The haploid Gametophytes are produced by mitotic divisions of the Spores formed by meiosis • Meiosis occurs in some of the diploid Sporophytes and produces a generation of haploid spores • These spores then divide my mitosis to produce multicellular Gametophytes • Gametes are formed from mitotic division of specific Sporophyte tissues Most Fungi • A haploid life cycle  The haploid phase is multicellular  The diploid phase is limited to a single cell produced by fertilization which then immediately undergoes meiosis Animals • A Diploid life cycle  The diploid phase is multicellular  The haploid phase is unicellular • The products of Meiosis are haploid Gametes • The diploid phase is restored when one gamete fuses with another at fertilization Meiosis • The Function  To reduce chromosome # (from diploid to haploid)  To generate diversity in sexually reproducing organisms • Occurs only in Eukaryotes that are at least Diploid – have at least 2 copies of each chromosome Premeiotic Interphase • DNA replicates and the Chromosomal proteins are duplicated  Producing two copies, the sister chromatids, of each chromosome Prophase I • The spindle forms in cytoplasm • Condensation of Chromosomes – Begin to condense into thread-like structures • Synapsis – Homologous chromosomes come together and pair to form a Tetrad • Recombination/Crossing-Over at a Chiasma (pl. chiasmata)  Break the covalent bonds of the DNA backbones  Match complementary sequence on sister chromatic  Exchanging the ends  Restoring the bonds Prometaphase I • The nuclear envelope breaks down and the Spindle enters the area of the former nucleus • The Kinetochore microtubules from opposite spindle poles attach to the kinetochores of each homologous chromosome Metaphase I • Spindle Microtubule movements align the tetrads on the Metaphase Plate Anaphase I • The Spindle Microtubules separate the two chromosomes of each homologous pair and move them to opposite spindle poles • The poles now contain the haploid # of chromosomes  Each chromosome at the poles still contains two chromatids Telophase I • The chromosomes undergo little or no change; little decondensation/unfolding in some species • The spindle of the First Meiotic Division disassembles • Two new spindles form for the Second Division Interkinesis • Brief Transitory Stage – No DNA replication between First and Second Meiotic Division Prophase II • The chromosomes condense and the Spindle reorganize
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