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Lecture 15

Lecture 15 Identical Twins or Not.docx

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Biology 1002B
Tom Haffie

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Lecture 15: Identical Twins or Not Monday, March 04, 2013 10:25 AM Today's Topics:  Identical twins or not Announcements:  Homework: Lecture Topic:   Xist (X inactive specific transcript) RNA binds along the inactive X chromosomes, silencing most genes  The genes become in inaccessible to transcription machinery by Xist RNA coating the chromosomes and making them inactive (stable throughout the life)  This is an epigenetic mechanism  What is different about identical twins?  Make different gametes  Different gene expression  Mutation  Random X chromosome inactivation creates genetic mosaics in females  All of the decedents of the original cell, maintain the same X-chromosome inactivation (maternal/paternal)   Xist and tsix are transcribed in apposite directions at same locus  Tsix shuts of Xist (there are not pns here, just RNA)   What if we compare genomes and X inactivation in Alice and Beth  Control: half inactivation of maternal/paternal  Alice is inactivating each half the time  Beth is inactivating only one of them all the time   Nucleosome: DNA wound around core of 2 molecules each of H2A, H2B, H3, H4  DNA that is more open is more available for transcription  Regulation of gene expression via packing  Nucleosomes can be moved, to "remodel" chromatin near promoter  Pack them tighter, move them around the make them more/less available for transcription (epigenetic regulation)   Histone tails can be modified to affect DNA packaging  Acetylation: relaxing of chromatin   DNA methylation can silence genes in two ways  Prevention of a transcription factor and shut down its transcription  Methylation of DNA might attract the deacetylse snzyme
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