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Lecture 15

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Western University
Biology 1002B
Tom Haffie

Lecture 15: Epigenetics Identical Twins are Not  Clicker: Which of the following signals in DNA is “understood” by the cell through protein binding to nucleic acid? o Polyadenylation signal is a site that cuts the mRNA off so it is understood  Gets recognized by protein to get cut after it gets transcribed into mRNA o Operator o Enhancer  Know figure 14.6, in the textbook  Clicker: if identical twin sisters each have a son with identical twin brothers, the kids will be identical o False , gametes are different, sexual recombination  Clicker: what does more than 50% heritability of schizophrenia mean for Alice? (identical twins) o Over 50% means, over that 50% variability attributed from genes, so that if they’re identical, she has a very high chance of getting schizophrenia  What is different about identical twins o They make different gametes o Gene expression might be different (might have same gene but different expression) o There are mutations within them because from 1 to trillions of cells there bound to be mistakes within replication, random mutations  Environmental mutagens o Only females: different active X in different tissues o  Random X chromosome inactivation creates genetic mosaics o Rainbow was a cat, and they cloned the cat and they look very different because the inactivation of X  Same genotype, different phenotype o At about 100 cells the both X chromosomes are active but then the cells undergo inactivation. The cells will inactivate paternal X or maternal X, at random, so that they’re all at random, mosaics o All the descends of the cells will keep the same inactive X, the cell line will all keep the same inactive X, so there are about 100 cell lines and each with random inactive X  That’s why they’re mosaic  Xist RNA binds along the inactive X chromosome, silencing most genes o Very condensed X chromosome called bar body o Xist -> X inactive specific transcript o They keep the X chromosome inactive o It codes the chromosome and keep it away from replication machineries to make it inactive  Stable through the cell’s entire cell line o This is called epigenetics, binding of the non-coding DNA  Xist and tsix genes are transcribed in opposite direction at same locus o Xist turns chromosome off and tsix turns the chromosome on o Xist is on the top and the bottom strand left to right is called Tsix (antisense), the expression of Tsix (antisense) will shut down Xist  They’re both RNAs o None coding RNA can regulate gene regulation  What if we compare genomes and X inactivation in Alice and Beth (twins)? o The two women have same nuclear DNA, DNA finger prints are the same o Another gel is prepared to see which X chromosome is inactivated, it digests the inactive X  One X, the restriction enzyme will make a big piece and the other X will make a small piece  If small is inactive you get big piece, if big piece is inactive you get small piece  Some are 50% 50% expressing maternal and paternal X and some can only express their maternal or paternal X  DNA is packaged around nucleosome and coiled into solenoid fibers o DNA are associated with proteins called Histones o Nucleosome: DNA wound around core of 2 molecules of H2A, H2B, H3, H4  4 histones form a nucleosome an
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