Gene Regulation Chapter 15

Gene Regulation 2Outline Prokaryotic Regulation  trp Operon  lac Operon Eukaryotic Regulation  Transcriptional Control  Posttranscriptional Control  Translational Control  Posttranslational Control Genetic Mutations  Cancer

Gene Regulation 3 Prokaryotic Regulation: The Operon Model Operon consist of three components  Promoter ­DNA sequence where RNA polymerase first attaches ­Short segment of DNA  Operator ­DNA sequence where active repressor binds ­Short segment of DNA  Structural Genes ­One to several genes coding for enzymes of a metabolic pathway ­Translated simultaneously as a block ­Long segment of DNA

Gene Regulation 4 Repressible Operons: The trp Operon The regulator codes for a repressor If tryptophan (an amino acid) is absent:  Repressor is unable to attach to the operator (expression is normally “on”)  RNA polymerase binds to the promoter  Enzymes for synthesis of tryptophan are produced If tryptophan is present:  Combines with repressor as corepressor  Repressor becomes functional  Blocks synthesis of enzymes and tryptophan

5 The trp Operon

Gene Regulation 6 Inducible Operons: The lac Operon The regulator codes for a repressor If lactose (a sugar that can be used for food) is absent:  Repressor attaches to the operator  Expression is normally “off” If lactose is present:  It combines with repressor and renders it unable to bind to operator  RNA polymerase binds to the promoter  The three enzymes necessary for lactose catabolism are produced

7 The lac Operon

8 Action of CAP

Gene Regulation 9 Eukaryotic Regulation A variety of mechanisms Five primary levels of control:  Nuclear levels ­Chromatin Packing ­Transcriptional Control ­Posttranscriptional Control  Cytoplasmic levels ­Translational Control ­Posttranslational Control

10 Regulation of Gene Expression: Levels of Control in Eukaryotes

Gene Regulation 11 Chromatin Structure Eukaryotic DNA associated with histone proteins  Together make up chromatin  As seen in the interphase nucleus Nucleosomes:  DNA wound around balls of eight molecules of histone proteins  Looks like beads on a string  Each bead a nucleosome The levels of chromatin packing determined by degree of nucleosome coiling

12 Levels of Chromatin Structure

Gene Regulation 13 Chromatin Packing Euchromatin  Loosely coiled DNA  Transcriptionally active Heterochromatin  Tightly packed DNA  Transcriptionally inactive Barr Bodies  Females have two X chromosomes, but only one is active  Other is tightly packed along its entire length  Inactive X chromosome is Barr body

14 X-Inactivation in Mammalian Females

Gene Regulation 15 Transcriptional Control Transcription controlled by proteins called transcription factors  Bind to enhancer DNA  Regions of DNA where factors that regulate transcription can also bind  Always present in cell, but most likely have to be activated before they will bind to DNA

16 Lampbrush Chromosomes

17 Initiation of Transcription

Gene Regulation 18 Posttranscriptional Control Posttranscriptional control operates on primary mRNA transcript Given a specific primary transcript:  Excision of introns can vary  Splicing of exons can vary  Determines the type of mature transcript that leaves the nucleus May also control speed of mRNA transport from nucleus to cytoplasm  Will affect the number of transcripts arriving at rough ER  And therefore the amount of gene product realized per unit time

19 Processing of mRNA Transcripts

Gene Regulation 20 Translational Control Translational Control - Determines degree to which mRNA is translated into a protein product  Presence of 5′ cap  Length of poly-A tail on 3′ end Posttranslational Control - Affects the activity of a protein product  Activation  Degradation rate

Gene Regulation 21 Effect of Mutations on Protein Activity Point Mutations  Involve change in a single DNA nucleotide  Changes one codon to a different codon  Affects on protein vary: ­Nonfunctional ­Reduced functionality ­Unaffected Frameshift Mutations  One or two nucleotides are either inserted or deleted from DNA  Protein always rendered nonfunctional ­Normal :THE CAT ATE THE RAT ­After deletion:THE ATA TET HER AT ­After insertion:THE CCA TAT ETH ERA T

22 Point Mutation

23 Androgen Insensitivity

Gene Regulation 24Carcinogenesis Development of cancer involves a series of mutations Proto-oncogenes – Stimulate cell cycle Tumor suppressor genes – inhibit cell cycle Mutation in oncogene and tumor suppressor gene: ­Stimulates cell cycle uncontrollably ­Leads to tumor formation

25 Carcinogenesis

26 Achondroplasia and Xeroderma Pigmentosum

Gene Regulation 27 Causes of Mutations Replication Errors  1 in 1,000,000,000 replications  DNA polymerase ­Proofreads new strands ­Generally corrects errors Environmental Mutagens  Carcinogens - Mutagens that increase the chances of cancer ­Ultraviolet Radiation ­Tobacco Smoke

Gene Regulation 28Review Prokaryotic Regulation  trp Operon  lac Operon Eukaryotic Regulation  Transcriptional Control  Posttranscriptional Control  Translational Control  Posttranslational Control Genetic Mutations  Cancer

Gene Regulation Ending Slide Chapter 15