lectures 10-13

lectures 10-13 - Upcoming Office Hours: Friday 10/14...

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Upcoming Office Hours: Friday 10/14 2:30-4:30 Thursday 10/20 2:30-3:30 Friday 10/21 2:30-3:30 No office hours week of 10/24 Dr. Baker begins lecturing Mon 10/24 Midterm 2 (Aparicio material only) is Wed 10/26
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Haploid-Diploid Cell Cycles of Eukaryotic Microorganisms Diploid cells divide and form colonies through mitotic divisions. Gametes (two mating types) are produced by meiosis, triggered by starvation. Gametes usually fuse (mate) at the first opportunity to produce a diploid. However, if mating is prevented, the haploid gametes of unicellular organisms can grow by mitosis, facilitating generation and analysis of recessive mutations.
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Mating-Type Determination in Yeast Yeast cells contain three genetic loci with mating-type genes. At the MAT locus, either the “a” or “ α ” gene(s) will be present, and expression of these genes determines the mating-type of the cell. At the HML and HMR loci, the α and a genes also reside, but are not expressed due to a “silent” chromatin structure. All three loci share homology flanking the genes.
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Mating-Type Switching Involves a Specialized Recombination Event The HO endonuclease specifically creates a double-strand break adjacent to the MAT gene(s). One 3’ DNA end at MAT finds homologous DNA sequence at HML or HMR to undergo strand exchange and DNA replication using HML or HMR as template. A complementary strand is also produced by lagging strand synthesis. Thus, the gene sequence at HML or HMR replaces the original sequence at MAT.
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Figure 7.8 The Biology of Cancer (© Garland Science 2007) Loss of Heterozygosity (LOH) by Mitotic Recombination dsDNA break repair in mitosis usually involves the sister chromatid, which does not result in LOH, but sometimes occurs between homologs LOH may also occur through gene conversion, chromosome loss or mutation
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Making Mutations in Haploid and Diploid Organisms In haploid cells, the effect of a mutation can be observed instantly because there is no second wild-type gene copy. In diploid cells, mutations usually do not cause immediately observable phenotypes because the second functional copy of the gene compensates for the lack of function of the mutant allele. This is called complementation .
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Dominant and Recessive Relationship of Alleles The term dominant and recessive are applied to mutant alleles in comparison to the wild-type allele and reflect the phenotype of the diploid cell. In a heterozygous cell, the phenotype of the dominant allele will be observed. The phenotype of a recessive (mutant) allele is observed only if both alleles carried are recessive mutations (homozygous). The ability of one copy of the wild-type allele to compensate for the lack of function of a second recessive mutant allele is called complementation .
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Knocking-Out a Gene in Yeast by Homologous Recombination A DNA construct with a selectable marker flanked by homology to the gene of interest is transformed into yeast. Transformants that have inserted the DNA
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lectures 10-13 - Upcoming Office Hours: Friday 10/14...

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