Gene RegulationChapter

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Gene Regulation 2

Outline

Prokaryotic Regulation trp Operon lac Operon

Eukaryotic Regulation Transcriptional Control Posttranscriptional Control Translational Control Posttranslational Control

Genetic Mutations Cancer

Gene Regulation 3Prokaryotic Regulation:

The Operon Model

Operon consist of three componentsPromoter­DNA sequence where RNA polymerase first attaches­ Short segment of DNA

Operator­DNA sequence where active repressor binds­ Short segment of DNA

Structural Genes­One to several genes coding for enzymes of a metabolic pathway­ Translated simultaneously as a block­ Long segment of DNA

Gene Regulation 4Repressible 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

5The trp Operon

Gene Regulation 6Inducible 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

7The lac Operon

8Action 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

10Regulation of Gene Expression:

Levels of Control in Eukaryotes

Gene Regulation 11

Chromatin Structure

Eukaryotic DNA associated with histone proteins Together make up chromatinAs seen in the interphase nucleus

Nucleosomes:DNA wound around balls of eight molecules of histone proteins

Looks like beads on a stringEach bead a nucleosome

The levels of chromatin packing determined by degree of nucleosome coiling

12Levels 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

14X-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

16Lampbrush Chromosomes

17Initiation 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

19Processing of mRNA Transcripts

Gene Regulation 20

Translational Control

Translational Control - Determines degree to which mRNA is translated into a protein productPresence of 5′ cap

Length of poly-A tail on 3′ end

Posttranslational Control - Affects the activity of a protein productActivation

Degradation rate

Gene Regulation 21Effect 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

22Point Mutation

23Androgen Insensitivity

Gene Regulation 24

Carcinogenesis

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

25Carcinogenesis

26Achondroplasia andXeroderma Pigmentosum

Gene Regulation 27

Causes of Mutations

Replication Errors1 in 1,000,000,000 replicationsDNA polymerase­ Proofreads new strands­Generally corrects errors

Environmental MutagensCarcinogens - Mutagens that increase the chances of cancer­Ultraviolet Radiation­ Tobacco Smoke

Gene Regulation 28

Review

Prokaryotic Regulation trp Operon lac Operon

Eukaryotic Regulation Transcriptional Control Posttranscriptional Control Translational Control Posttranslational Control

Genetic Mutations Cancer

Gene RegulationEnding Slide Chapter

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