Lecture 12 chapter 13(1) - Genomic Imprinting Because most...

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12-1 Genomic Imprinting Because most eukaryotes are diploid organisms, usually it doesn’t matter whether a particular allele of any given gene came from the mother or the father Exceptions Some genes are modified (imprinted) differently in females and males Evidence for reprogramming the male versus female gametes , altering which allele of certain genetic loci is expresses
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Role of Chromatin structure on gene expression Chromatin structure histones nucleosomes 30nm fiber higher order folding Chromatin and gene activity nucleosome positioning histone modifications chromatin remodeling
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Histones Two copies of each of the core histones make up the nucleosome Very high composition of basic amino acids H1 (and H1˚ and H5) binds the linker regions between nucleosomes
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Histones are conserved proteins which, together with DNA, form chromatin +linker histone H1
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Histone assembly Requires DNA Assembles in an ordered process through the histone-fold domain – one H3 2 H4 2 tetramer binds DNA first this assembly allows two H2A/H2B dimers to add to the complex
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07_Figure22.jpg
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DNA assembles with the histone octamer to form a nucleosome Basic unit of chromosome organization Core DNA length and the range of linker DNA lengths vary by species
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Histone octamer Among the strongest protein-DNA interactions Numerous hydrogen bonds between basic amino acid residues and the DNA’s minor groove tight binding required to hold DNA in energetically constrained form
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Histone octamer Binding through backbone and minor groove is largely sequence-independent Extensive H3/H4 interactions with DNA hold structure for subsequent H2 subunit binding Positive charge allows negatively charged DNA to wrap ~1.65 times around histone core
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Nucleosome unit Micrococcal nuclease cuts in unprotected linker region between nucleosomes Strong digestion yields the unit size of wrapped DNA of 147 bp Incomplete digestion yields a ladder of approximately 200 bp of linker + nucleosomal DNA
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Linker histone H1 H1 binds to linker DNA Not a part of the nucleosome. Locations vary along a stretch of DNA Participates in further compaction of DNA
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Chromatin structures Beads-on-a-string (10nm fiber) without H1 higher-order 30nm fiber in the presence of H1
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SV40 minichromosome
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“Solonoid model” Hollow tube model derived from X-ray crystal structure
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In tetranucleosome model, nearest neighbors in condensed structure are not linearly consecutive nucleosomes
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The 30 nm fiber may take on a tetranucleosome structure However, idealized model with assumptions not necessarily reflecting reality
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