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Gene Regulation

Gene Regulation

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Gene Regulation

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  1. Gene Regulation Chapter 13

  2. Prokaryotic and Eukaryotic Genomes • all of the genetic material possessed by an organism or group of organisms • both contain many 1,000's of protein coding genes • prokaryotes = most genes code for protein as usual • only a small amount of noncoding DNA • eukaryotes = much of genome does not code for protein • many noncoding regions and repetitive DNA • many coding genes are active (expressed) only part of the time • they are controlled by some mechanism • inducible vs. constitutive genes Nonprotein-coding DNA sequences in various groups

  3. Control of Gene Expression • mRNA initially contains many noncoding regions (introns) • modified before leaving nucleus • introns removed • coding regions (exons) remain • primary mRNA  mature mRNA • introns left in place  no translation • removing exons different protein • alternative RNA splicing • all genes contain two basic parts • coding region (“cistron”) • codes for mRNA and its protein • one coding region may contain several subcoding regions • produce proteins that work together • regulatory region • regulates transcription of coding region • promoter • RNA polymerase binds to begin transcription • operator • controls expression of coding reg. • turns a gene “on” or “off” • constitutive genes lack one Fig. 13.8 Processing of mRNA transcripts

  4. Prokaryotic Gene Regulation • characterized by two recurring themes • most genes contain many subcoding regions in the coding region • transcription is prevented when a repressor binds to the operator • repressor • a special protein produced by a regulatory gene • operon model of prokaryotic gene regulation • components of an operon • regulator gene  produces the repressor protein • protein coding gene (usually with subcoding regions) • has a regulatory region = promoter and operator • transcription of all subcoding regions is regulated by the repressor • repressor is bound to operator  no transcription occurs • RNA polymerase is blocked  gene is “off” • repressor is not bound to operator  transcription occurs as usual • RNA polymerase not blocked  gene is “on”

  5. Page 234 Operon model

  6. Fig. 15.2 The lacoperon

  7. Eukaryotic Gene Regulation • express only a fraction of their genes at any given time • eukaryotic genes also consist of regulatory and coding region • regulatory region = promoter and operator • coding region rarely contains subcoding reg. • changes in the chromatin itself • can affect the availability of genes • increased packaging conceals genes  makes them less accessible • decreased packaging makes genes more accessible • chromatin can also be modified chemically • DNA methylation • methyl groups (-CH3) attached to DNA bases • diminishes transcription • histoneacetylation • acetyl groups (-COCH3) attached to chromatin • increases transcription Fig. 13.4 Levels at which control of gene expression occurs in eukaryotic cells

  8. Fig. 12.10 Levels of chromatin structure

  9. initiation of transcription • transcription factors • proteins that help determine when/where genes turned “on” • bind to a gene’s promoter in response to certain stimuli • activates operator  gene turned “on” • help RNA polymerase begin transcription • control elements • noncoding segments of DNA • lie outside of regulatory region of any particular gene • can inhibit transcription – silencers • can stimulate transcription – enhancers

  10. Fig. 13.7 Initiation of transcription

  11. RNA regulators • noncoding RNA (introns) that can regulate DNA, RNA, or proteins • microRNA • amount of regulatory RNA increases with organism complexity • RNA regulators allow for an increase in complexity • translation • ribosomes and mRNA can be blocked from assembling at initiation • modification of the protein • protein processing is also subject to regulation • assembly into its various structures, etc. • mechanisms keep a protein functional or make it nonfunctional • proteasomes • degradation of mRNA itself • each mRNA has a characteristic lifespan • special sequences in the mRNA determine this • other control mechanisms increase diversity in gene expression

  12. Transposons • genes that can move from one place to another in a genome • “jumping genes” • transposition • actual mechanism is complex • more common in prokaryotes than eukaryotes • can significantly affect gene expression • jumping into the middle of a coding sequence • prevents the normal functioning of that gene • its expression is halted or altered • jumping into the regulatory region of another gene • may either increase or decrease expression • some transposons carry genes themselves • activated when they are inserted near another gene’s promoter Transposons