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epigenetic modification.docx

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McMaster University
Bhagwati Gupta

October 15 , 2013 Biology 2C03: Genetics Epigenetic Modifications Epigenetics - The colours on a human chromosome indicate the chromatin density like a heat map (red to blue equals high to low density, respectively) Does Chromosome Structure Affect the Regulation of Gene Expression? - Transcriptional regulation of the X-chromosome - Problem: how does the cell compensate for this imbalance as females would always have twice the amount of proteins transcribed for the X-chromosome  Female: 2 X-chromosomes – two copies of each gene or two doses  Male: 1 X-chromosome – one copy of each gene or one dose - How can a cell compensate for this imbalance? Dosage Compensation - Dosage compensation is a way of equalizing gene expression in the face of different gene dosage - This can occur in different ways: 1) Increasing expression from the chromosome (increase X chromosome expression in males) 2) Decreasing expression rom the two X chromosomes (females) 3) Turning off expression from one X chromosome (females) - While mechanisms of dosage compensation vary, all involve modifications to chromosome structure and gene expression, perhaps through histone modification – not changing the genetic code just the structure - Dosage compensation is an example of an epigenetic phenomenon - Epigenetic phenomenon: a heritable change in gene expression that does not involve a change in DNA sequence - Structure of the chromosome, not changes in the DNA code, which is inherited X-Inactivation - One of the X-chromosomes is inactivated = barr body (increased intensity/focus) - Interphase nucleus: female cell - Yellow: fluorescent probe that recognizes the X-chromosome - Murray Barr and Ewart Bertam, 1949: Condensed mass in nucleus of female cells in cats, not male - Susumu Ohno, 1960: Proposed that the mass was the X-chromosome - X-chromosomes were extremely condensed compared to other autosomal chromosomes Lyon Hypothesis and X-Inactiavation - Mary Lyon proposed the random inactivation of one female X chromosome  If a cell contains more than 2 X chromosomes, all but one of them are inactive - Therefore, females are functionally hemizygous (only one allele present) for X-linked genes at the cellular level  Approximately 50% cells express one allele, and 50% cells will express the other cells (only the alleles found on those chromosomes = mosaic) - Inactive X = Barr body - Random process that occurs during development What happens if there are Genes on the X-chromosome? - Genetics – Calico cats  Orange gene is one the X-chromosome  O = orange  o = black - White determined by another gene on an autosome. White is dominant - Orange versus black: determined by the gene, O - This cat is heterozygous, genotype: Oo - Phenotype: hemizygous  Some cells are orange (Ox)  Some cells are black (xo)  X indicates an allele that is present, but is not contributing to the phenotype X-Inactivation - Early in development, one X chromosome is inactivated in each cell – the process is random, it is equally likely that the maternal or paternal X chromosome will be inactivated - All descendent cells maintain the inactivation pattern  All descendent cells are expressing the B phenotype = black fur  All descendent cells are expressing the b phenotype = white fur - Females that are heterozygous for X-linked traits are genetic mosaics Testing the Hypothesis at the Cellular Level (Davidson, Nitowsky and Childs, 1963) - Glucose-6-phosphate dehydrogenase (G-6-PD) gene is on the X-chromosome in humans - Two protein variants, both functional despite differences in protein sequence and differences in structure - On a protein gel, one variant migrates further than another:  See both protein variants migrating in heterozygous females  What’s happening on organism level  How do we prove whats happening at the cellular level - Mix of cells expresses both protein variants - Take tissues from heterozygous females - Isolate single cells and grow colonies of cells  Plated cells to obtain single colonies (single cell that continues to grow and divide and is genetically identical to the ancestral cell) - Extract proteins from clonal colonies  Control: represents the alleles found in the entire tissue = 2 bands
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