MCDB101Bslideset6

MCDB101Bslideset6 - 188 Mechanism of Recombination (Chpt. 6...

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1 Mechanism of Recombination – ( Chpt. 6 pg 191-200) 188 movie Co In E Analysis of Meiotic Products in Fungi suggests a mechanism for Recombination and Gene Conversion • Fungi have ascospore which contains all the haploid products of a single meiosis. • Dissection of the ascospore and analysis of spore genotype reveals occasional nonmendelian ratios in allele frequency = Gene Conversion • Gene Conversion is often associated with recombination events. 189 Co In E
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2 5.14 190 Co In E 5.20 191 Co In E
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3 5.21 192 Co In E ws + ws + ws + as if the ws allele was converted to ws + 5:3 octad • Expect 4:4 segregation in octads • In rare cases, 5:3 or 6:2 octads are seen!! • This means that what should be identical products of mitosis are NOT identical! • As if one allele was “converted” to another Gene Conversion ws + ws + ws + ws + meiosis ws ws + ws ws + mitosis + + 193 Co In E
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4 Fig. 6.21.a 194 Co In E Mismatch repair in heteroduplex region can lead to gene conversion 6.21c 195 Co In E
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5 Gene conversion: a connection with recombination • One allele changes to another = DNA change • Only seen in heterozygotes, and only converts to the other allele in the heterozygote (not a “mutation”) • Not “reciprocal”: if see one conversion from a to b, don’t see an equivalent conversion from b to a in the same octad • Often correlates with a nearby recombination event • In yeast, double-stranded DNA breaks are seen in meiotic prophase 195 Co In E Double-stranded gap repair model: Overview • Double-stranded break introduced into 1 chromatid • Break is widened • Strand invasion of the homolog to form a heteroduplex and a D-loop • Repair of the gaps using the homolog as a template, forming a double Holliday junction “Joint molecule” • Branch migration of Holliday junctions to form more heteroduplex DNA • Resolve the 2 Holliday junction “joint molecule” back to 2 separate chromatids 196 Co In E
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MCDB101Bslideset6 - 188 Mechanism of Recombination (Chpt. 6...

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