Lac repressor active by itself inducer inactivates

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Lac repressor active by itself - inducer inactivates the repressor (allolactose) Differential gene expression : expression of different genes by cells with same genome (specialization) Chromatin structure: location of gene’s promoter/genes on inside of heterochromatin might not get transcribed/expressed Histone acetylation : acetyl groups attached to lysines in histone tails (proteins DNA wraps in nucleosomes) Neutralizes charge - histone tails no longer bind to neighboring nucleosomes - prevents folding of chromatin into more compact structure - transcription proteins have easier access to acetylated region Addition of methyl groups to histone tails promotes condensation of chromatin DNA methylation : long stretches of inactivated DNA usually more methylated than regions of actively transcribed genes - DNA methylation involved in long-term inactivation of genes Epigenetic inheritance : inheritance of traits transmitted by mechanisms not directly involved in nucleotide sequence (variations even in identical genomes) Regulation of Transcription Initiation: chromatin-modifying enzymes make regions of DNA more/less able to bind to transcription machinery
Control elements : segments of noncoding DNA that serve as binding sites for proteins called transcription factors, which in turn regulate transcription Only when complete transcription initiation complex has assembled can polymerase II begin moving along DNA template General transcription factors essential for transcription of all protein-coding genes Specific transcription factor transcription of particular genes depend on them Proximal control elements: control elements located close to promoter Distal control elements: Enhancers : generally associated with only one gene Protein-mediated bending of DNA bring bound activators into contact with mediator proteins, which in turn interact with proteins at promoter Protein-protein interactions help assemble and position initiation complex on promoter Repressors can bind directly to control element DNA, blocking activator binding, or even turning off transcription when activators are bound Enhancers have ~10 control elements - particular COMBINATION of control elements associated with gene regulates transcription Recognize cell signals regardless of chromosomal location - coordinately linked genes get transcribed Silencing: repressors recruit proteins that deacetylate histones, leading to reduced transcription Transcription factories: different loops from same chromosome and loops from other chromosomes congregate in sites rich in RNA polymerases and other transcription-associated proteins RNA processing in nucleus and export of mature RNA to cytoplasm = opportunity for gene expression Alternative RNA splicing : different mRNA molecules can be created from same primary transcript depending on what regions are treated as exons Lifespan of mRNA: usually determined by nucleotide sequences untranslated regions at 3’ end of RNA

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