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Biology, 9th ed,Sylvia Mader

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1 Biology, 9th ed,Sylvia Mader
Chapter 15 Chapter 15 Gene Regulation Gene Regulation

2 Prokaryotic Regulation:
Biology, 9th ed,Sylvia Mader Prokaryotic Regulation: Chapter 15 Gene Regulation Bacteria do not need the same enzymes and other proteins all of the time. - They need only: 1. The enzymes required to break down the nutrients available to them or 2. The enzymes required to synthesize whatever metabolites are absent under the present circumstances.

3 Prokaryotic Regulation: The Operon Model (Jacob & Monod 1961)
Biology, 9th ed,Sylvia Mader Prokaryotic Regulation: The Operon Model (Jacob & Monod 1961) Chapter 15 Gene Regulation An operon consists of three components: 1. Promoter DNA sequence where RNA polymerase first attaches Short segment of DNA 2. Operator DNA sequence where active repressor binds

4 Prokaryotic Regulation: The Operon Model (Jacob & Monod 1961)
Biology, 9th ed,Sylvia Mader Prokaryotic Regulation: The Operon Model (Jacob & Monod 1961) Chapter 15 Gene Regulation 3. Structural Genes One to several genes coding for enzymes of a metabolic pathway Translated simultaneously as a block Long segment of DNA A regulator gene is located outside of the operon. It codes for a repressor that controls whether the operon is active or not.

5 Repressible Operons: The trp Operon - Normally turned ON
Biology, 9th ed,Sylvia Mader Repressible Operons: The trp Operon - Normally turned ON Chapter 15 Gene Regulation 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 Transcription & translation occur Enzymes for synthesis of tryptophan are produced Tryptophan will be produced by E. coli

6 Biology, 9th ed,Sylvia Mader
Chapter 15 The trp Operon Gene Regulation

7 Repressible Operons: The trp Operon - Genes repressed
Biology, 9th ed,Sylvia Mader Repressible Operons: The trp Operon - Genes repressed Chapter 15 Gene Regulation If tryptophan IS present enzymes are not needed and following occurs: Tryptophan combines with repressor, causing it to change shape, thus acting as a corepressor Repressor becomes functional Blocks transcription & synthesis of enzymes and tryptophan is NOT produced

8 Biology, 9th ed,Sylvia Mader
Chapter 15 The trp Operon Gene Regulation

9 Summary of repressible trp operon
Biology, 9th ed,Sylvia Mader Summary of repressible trp operon Chapter 15 Gene Regulation Operon usually ON, must be turned OFF Repressor Transcription bound ? occurs? NO > YES YES > NO *** Corepressors are frequently the products in the pathway. In this case, tryptophan is the corepressor.

10 Inducible Operons: The lac Operon - Normally turned OFF
Biology, 9th ed,Sylvia Mader Inducible Operons: The lac Operon - Normally turned OFF Chapter 15 Gene Regulation When E. coli is denied glucose & is given lactose instead, it immediately begins to make three enzymes needed for the metabolism of lactose. These enzymes are encoded by three structural genes which are adjacent to one another on the chromosome. They are controlled by one regulator gene that codes for a one repressor.

11 Inducible Operons: The lac Operon - Normal OFF state
Biology, 9th ed,Sylvia Mader Inducible Operons: The lac Operon - Normal OFF state Chapter 15 Gene Regulation If lactose (a sugar that can be used for food) is absent: Repressor attaches to the operator RNA polymerase cannot bind to promoter Transcription of structural genes is blocked Enzymes needed to digest lactose NOT made

12 Biology, 9th ed,Sylvia Mader
Chapter 15 The lac Operon Gene Regulation

13 Inducible Operons: The lac Operon - Induced state
Biology, 9th ed,Sylvia Mader Inducible Operons: The lac Operon - Induced state Chapter 15 Gene Regulation If lactose IS present: It combines with repressor and renders it unable to bind to operator by causing shape of repressor to change RNA polymerase binds to the promoter Transcription of genes occurs The three enzymes necessary for lactose catabolism are produced Lactose will be digested by enzymes

14 Biology, 9th ed,Sylvia Mader
Chapter 15 The lac Operon Gene Regulation

15 Summary of inducible lac operon
Biology, 9th ed,Sylvia Mader Summary of inducible lac operon Chapter 15 Gene Regulation Operon usually OFF, must be turned ON Repressor Transcription bound ? occurs? YES > NO NO > YES *** Inducers are frequently the reactants in the pathway. In this case, the lactose is the inducer.

16 The lac Operon - Further control
Biology, 9th ed,Sylvia Mader The lac Operon - Further control Chapter 15 Gene Regulation E. coli preferentially break down glucose. Thus, they have a way to ensure that the lac operon is only turned on maximally when glucose is absent. This involves use of cyclic AMP which is abundant when glucose is absent. - Cyclic AMP binds to a molecule called catabolite activator protein (CAP).

17 The lac Operon - Further control (2)
Biology, 9th ed,Sylvia Mader The lac Operon - Further control (2) Chapter 15 Gene Regulation The cAMP-CAP complex then binds to a CAP binding site next to the lac operon promoter. • When CAP binds to DNA, the DNA bends. - This exposes the promoter to RNA polymerase which is now better able to bind to the promoter.

18 The lac Operon - Further control (2)
Biology, 9th ed,Sylvia Mader The lac Operon - Further control (2) Chapter 15 Gene Regulation When glucose IS present: There is little cAMP in the cell - CAP is not activated by cAMP - lac operon does NOT function maximally and cell will preferentially use glucose as its food source.

19 Biology, 9th ed,Sylvia Mader
Chapter 15 Action of CAP Gene Regulation

20 Animations for the Operons
Trp Operon lac Operon

21 Eukaryotic Regulation
Biology, 9th ed,Sylvia Mader Eukaryotic Regulation Chapter 15 Gene Regulation A variety of mechanisms to control gene expression: Five primary levels of control: Nuclear levels Chromatin Packing Transcriptional Control Posttranscriptional Control Cytoplasmic levels Translational Control Posttranslational Control

22 Regulation of Gene Expression: Levels of Control in Eukaryotes
Biology, 9th ed,Sylvia Mader Regulation of Gene Expression: Levels of Control in Eukaryotes Chapter 15 Gene Regulation

23 Biology, 9th ed,Sylvia Mader
Chromatin Structure Chapter 15 Gene Regulation 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

24 Levels of Chromatin Structure
Biology, 9th ed,Sylvia Mader Levels of Chromatin Structure Chapter 15 Gene Regulation

25 Biology, 9th ed,Sylvia Mader
Chromatin Packing Chapter 15 Gene Regulation Euchromatin Loosely coiled DNA Appears lightly stained in micrographs Transcriptionally active - capable of being transcribed Heterochromatin Tightly packed DNA Appears darkly stained in micrographs Transcriptionally inactive

26 Biology, 9th ed,Sylvia Mader
Chromatin Packing Chapter 15 Gene Regulation Barr Bodies Females have two X chromosomes, but only one is active Other is tightly packed along its entire length Inactive X chromosome is called a Barr body Inactive X chromosome does not produce gene products

27 X-Inactivation in Mammalian Females
Biology, 9th ed,Sylvia Mader X-Inactivation in Mammalian Females Chapter 15 Gene Regulation

28 Transcriptional Control
Biology, 9th ed,Sylvia Mader Transcriptional Control Chapter 15 Gene Regulation Transcription controlled by DNA-binding proteins called transcription factors Bind to a promoter adjacent to a gene Transcription activators bind to regions of DNA called enhancers. Might be brought near region of promoter by hairpin loops in DNA. Always present in cell, but most likely have to be activated before they will bind to DNA

29 Lampbrush Chromosomes
Biology, 9th ed,Sylvia Mader Lampbrush Chromosomes Chapter 15 Gene Regulation

30 Initiation of Transcription
Biology, 9th ed,Sylvia Mader Initiation of Transcription Chapter 15 Gene Regulation

31 Transcriptional Control (2)
Biology, 9th ed,Sylvia Mader Transcriptional Control (2) Chapter 15 Gene Regulation Transposons are specific DNA sequences that have the ability to move within and between chromosomes. Their movement to a new location sometimes alters neighboring genes by decreasing their expression - Thus, they can act like regulator genes - They also can be a source of mutations.

32 Posttranscriptional Control
Biology, 9th ed,Sylvia Mader Posttranscriptional Control Chapter 15 Gene Regulation Posttranscriptional control operates within the nucleus on the primary mRNA transcript Given a specific primary transcript: Excision of introns can vary Splicing of exons can vary Thus, differing versions of the mRNA transcript might leave the nucleus

33 Posttranscriptional Control
Biology, 9th ed,Sylvia Mader Posttranscriptional Control Chapter 15 Gene Regulation Posttranscriptional control 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

34 Processing of mRNA Transcripts
Biology, 9th ed,Sylvia Mader Processing of mRNA Transcripts Chapter 15 Gene Regulation

35 Translational Control
Biology, 9th ed,Sylvia Mader Translational Control Chapter 15 Gene Regulation Translational control determines degree to which mRNA is translated into a protein product Presence of 5′ cap and the length of poly-A tail on 3′ end can determine whether translation takes place and how long the mRNA is active - Example: Long life of mRNA in RBCs that code for hemoglobin attributed to presence of 5’ cap and 3’ poly-A tail

36 Posttranslational Control
Biology, 9th ed,Sylvia Mader Posttranslational Control Chapter 15 Gene Regulation Some proteins are not immediately active after synthesis. Some need to be activated - Folding and breaking into chains must occur in bovine insulin before it is active Some are degraded quickly - Cyclin proteins that control cell cycle

37 Animations for Eukaryotic Control
Control of gene expression in eukaryotes Transcription Complex and Enhancers

38 Effect of Mutations on Protein Activity
Biology, 9th ed,Sylvia Mader Effect of Mutations on Protein Activity Chapter 15 Gene Regulation A mutation is a permanent change in the sequence of bases in DNA. Effects on proteins can range from no effect to complete inactivity Germ-line mutations Occur in sex cells; can be passed on to future generations Somatic mutations Occur in body cells; can’t be passed on to future generations Can lead to development of cancer

39 Effect of Mutations on Protein Activity
Biology, 9th ed,Sylvia Mader Effect of Mutations on Protein Activity Chapter 15 Gene Regulation Point Mutations Involve change in a single DNA nucleotide Changes one codon to a different codon Could change one amino acid for another Effects on protein vary: Drastic - completely nonfunctional Reduced functionality Unaffected

40 Effect of Mutations on Protein Activity
Biology, 9th ed,Sylvia Mader Effect of Mutations on Protein Activity Chapter 15 Gene Regulation Frameshift Mutations One or two nucleotides are either inserted or deleted from DNA Can lead to completely new codon order Protein can rendered nonfunctional Normal : THE CAT ATE THE RAT After deletion: THE ATA TET HER AT After insertion: THE CCA TAT ETH ERA T

41 Biology, 9th ed,Sylvia Mader
Chapter 15 Point Mutation Gene Regulation

42 Nonfunctional Proteins
Biology, 9th ed,Sylvia Mader Nonfunctional Proteins Chapter 15 Gene Regulation Examples of nonfunctional proteins: Hemophilia due to the transposon Alu Phenylketonuria (PKU) due to faulty code for one enzyme Cystic fibrosis due to inheritance of faulty code for a chloride ion channel Androgen insensitivity due to a faulty receptor for androgens (male sex hormones)

43 Biology, 9th ed,Sylvia Mader
Carcinogenesis Chapter 15 Gene Regulation Development of cancer involves a series of mutations: •Proto-oncogenes – Stimulate cell cycle but are usually turned off. Can mutate and become oncogenes which are turned on all the time. •Tumor suppressor genes – inhibit cell cycle Mutation in oncogene and tumor suppressor gene: Stimulates cell cycle uncontrollably Leads to tumor formation

44 Biology, 9th ed,Sylvia Mader
Chapter 15 Carcinogenesis Gene Regulation

45 Biology, 9th ed,Sylvia Mader
Causes of Mutations Chapter 15 Gene Regulation Spontaneous Errors: Happen for no apparent reason Example of spontaneous germ-line mutation is achondroplasia, a type of dwarfism Replication Errors: DNA polymerase proofreads new strands Generally corrects errors -1 in 1,000,000,000 replications error occurs

46 Biology, 9th ed,Sylvia Mader
Causes of Mutations Chapter 15 Gene Regulation Environmental Mutagens A mutagen is an environmental agent that increases the chances of a mutation Carcinogens - Mutagens that increase the chances of cancer -Many agricultural & industry chemicals -Many drugs Tobacco smoke chemicals Radiation (X-rays, gamma rays, UV)

47 Achondroplasia and Xeroderma Pigmentosum
Biology, 9th ed,Sylvia Mader Achondroplasia and Xeroderma Pigmentosum Chapter 15 Gene Regulation

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