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Gene Regulation

Gene Regulation. Chapter 13. 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

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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

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