Microbial Genetics_More - Eukaryotic Gene Expression can be...

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Eukaryotic Gene Expression can be Regulated at any Stage Differential Gene Expression - Almost all the cells in an organism contain an identical genome *Subset of genes expressed in each cell is unique (allows them to do their specific job) *Differences between cell types caused by differential gene expression (not dif genes being present) ^Differential Gene Expression: Expression of different genes by cells with the same genome - Very little DNA (~1.5% in humans) codes for proteins [codes for RNA products] - In all organisms, a common control point for gene expression is at transcription *In response to signals such as hormones from outside the cell Regulation of Chromatin Structure *Location of a gene’s promoter relative to nucleosomes & sites where DNA attaches to the chromosome scaffold (nuclear lamina) affects whether a gene is transcribed *Genes within heterochromatin (highly condensed) are usually not expressed ^Genes that are inserted into heterochromatin are no longer expressed *Chemical modifications to histones & DNA of chromatin can affect chromatin structure & gene expression - Histone Modifications *Histones = proteins around which DNA is wrapped in nucleosomes *N-terminus of the histone protrudes outward ^Tails are accessible to modifying enzymes which add/remove specific chemical groups *Histone acetylation/deacetylation: addition/removal of acetyl groups (COCH3) ^Acetylation neutralization of lysines; histone tails no longer bind to neighboring nucleosomes ^Looser structure Transcription proteins have easier access to genes in an acetylated region *Some enzymes that de/acetylate are associated with/components of transcription factors ^Suggests that this may promote transcription by also recruiting components *Addition of methyl promotes condensation of chromatin *Phosphorylation of an aa next to a methylated one does opposite *Histone Code Hypothesis – Proposes that specific combinations of modifications rather than overall level of histone acetylation helps determine chromatin configuration which influences transcription - DNA Methylation *Genes are usually more heavily methylated in cells in which they are not expressed *removal of methyl can turn on genes *Some proteins bind to methylated DNA and recruit histone deacetylation enzymes *Methylation necessary for long term inactivation of genes ^Methylated genes stay that way through successive cell divisions *Genomic Imprinting – Methylation permanently regulates expression of either the maternal or paternal allele of a particular gene at the start of development - Epigenetic Inheritance *Epigenetic Inheritance = inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence [ex: modifications to chromatin] *Epigenetic variation might explain difference between identical twins [ex: one has schizophrenia,
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Microbial Genetics_More - Eukaryotic Gene Expression can be...

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