The Control of Gene Expression

1. The Control of Gene Expression Chapter 11

2. Cloning to the Rescue? • Cloning has been attempted to save endangered species • A clone is produced by asexual reproduction and is genetically identical to its parent • Dolly the sheep was the first cloned mammal • Endangered animals that were cloned include cows, oxen, sheep, wildcats, and wolves • Disadvantages of cloning • Does not increase genetic diversity • Cloned animals may have health problems related to abnormal gene regulation

3. Gene Regulation • The process by which genetic information flows from genes to proteins is called gene expression • A gene is turned ‘on’ is being transcribed into specific protein molecules, a gene that is turned ‘off’ is not actively being transcribed • The turning off and on of transcription is the main way in which gene expression is regulated

4. Gene Expression • E. coli was first first studied because it does not require intercellular gene expression • Found that the bacterium changes its gene expression according to its environment • Gene expression is controlled by several parts • Promoter-before the gene, where the RNA polymerase attaches and starts transcription • Operator- between the promoter and the gene, determines whether the RNA polymerase can attach

5. Gene Expression • Regulatory Gene- turns off transcription by turning off and on the operator • Genes for related enzymes are often controlled together by being grouped into regulatory units called operons • Regulatory proteins bind to control sequences in the DNA and turn operons on or off in response to environmental changes

6. Gene Expression • Types of operon control • Inducible operon (lac operon) • Active repressor binds to the operator • Inducer (lactose) binds to and inactivates the repressor • Repressible operon (trp operon) • Repressor is initially inactive • Corepressor (tryptophan) binds to the repressor and makes it active • For many operons, activators enhance RNA polymerase binding to the promoter

7. OPERON Regulatorygene Promoter Operator Lactose-utilization genes DNA mRNA RNA polymerasecannot attach topromoter Activerepressor Protein OPERON TURNED OFF (lactose absent) DNA RNA polymerasebound to promoter mRNA Protein Inactiverepressor Lactose Enzymes for lactose utilization OPERON TURNED ON (lactose inactivates repressor) Gene Expression

8. Gene Expression in Eukaryotes • Cell differentiation results from selective gene expression • Different types of cells make different kinds of proteins • Different combinations of genes are active in each type

9. Gene Expression in Eukaryotes

10. DNAdoublehelix(2-nmdiameter) Histones “Beads ona string” Nucleosome(10-nm diameter) Tight helical fiber(30-nm diameter) Supercoil(200-nm diameter) 700nm Metaphase chromosome Gene Regulation in Eukaryotes • A chromosome contains a DNA double helix wound around clusters of histone proteins • DNA/ histone (8) complex is called nucleosome • DNA further ‘supercoils’ • DNA packing tends to block gene expression

11. Chromosome Inactivation • In females, one X chromosome per somatic cell is inactivated early in embryonic development • So coiled it cannot be read • The inactivation is inherited by its decedents • A female that is heterozygous for genes on the X chromosome has cells that express different alleles • Calico cat

12. Chromosome Inactivation Early embryo Two cell populations in adult Cell division and random X chromosome inactivation Orange fur Active X X chromosomes Inactive X Inactive X Allele for orange fur Black fur Active X Allele for black fur

13. Regulation of Eukaryotic Transcription • Regulate by making DNA more or less available for transcription • Regulatory proteins • Have more than prokaryotic organisms • Each gene has its own promoter and other control sequences • Transcription factors facilitate correct attachment of RNA polymerases • Enhancers and silencers bind to DNA • Coordinated effort to transcribe RNA

14. Enhancers Promoter Gene DNA Activatorproteins Transcriptionfactors Otherproteins RNA polymerase Bendingof DNA Transcription Regulation of Eukaryotic Transcription

15. RNA Splicing Exons 4 1 3 2 5 DNA • Once RNA is transcribed, the introns are spliced out • With-holding splicing or the way an RNA is spliced may be a way for regulating gene expression 4 1 3 2 RNA transcript 5 RNA splicing or 4 1 2 1 5 3 2 mRNA 5 • Can get more than one polypeptide from one gene

16. Regulation During Translation • Breakdown of mRNA • Enzymes in the cytoplasm break mRNA down quickly • Initiation of Translation • There are many proteins involved in the start of photosynthesis • Protein Activation • After translation polypeptides may need alteration to become functional (folding etc.) • Protein Breakdown • Selective breakdown of proteins after translation

17. Chromosome DNA unpackingOther changes to DNA GENE TRANSCRIPTION GENE Exon RNA transcript Intron Addition of cap and tail Splicing Tail Cap mRNA in nucleus NUCLEUS Flowthroughnuclear envelope mRNA in cytoplasm CYTOPLASM Breakdown of mRNA Translation Broken-down mRNA Polypeptide Cleavage/modification/activation ACTIVE PROTEIN Breakdownof protein Broken-down protein Gene Regulation • Each stage of eukaryotic expression offers an opportunity for regulation • The process can be turned on or off, speeded up, or slowed down • The most important control point is usually the start of transcription

18. Eye Antenna Head of a normal fruit fly Leg Head of a developmental mutant Genetic Control of Embryonic Development • Experiments in the embryonic development of fruit flies have shown the relationship between gene expression and development • A cascade of gene expression involves genes for regulatory proteins that affect other genes • It determines how an animal develops from a fertilized egg • Problems with gene expression can lead to mutations

19. FERTILIZATION AND MITOSIS ZYGOTE EGG CELL WITHIN OVARIAN FOLLICLE Egg cell Egg protein signaling follicle cells 1 Translation of “head” mRNA EMBRYO Follicle cells Gradient of regulatory protein Gene expression in follicle cells 4 Follicle cell protein signaling egg cell 2 Gene expression Gradient of certain other proteins Localization of “head” mRNA 5 3 Gene expression Body segments “Head” mRNA 6 Head-Tail Polarity in the Fruit Fly

20. Developmental Genes • Homeotic Genes- A master control gene that determines the identity of a body structure of a developing organism, by controlling the developmental fate of groups of cells • Contain nucleotide sequences called homeoboxes • Are similar in many kinds of organisms • Arose early in the history of life

21. Fly chromosomes Mouse chromosomes Fruit fly embryo (10 hours) Mouse embryo (12 days) Adult fruit fly Adult mouse Developmental Genes • Fruit flies and mice have similar homeotic genes (colored boxes) • The order of homeotic genes is the same • The gene ordercorresponds toanalogous bodyregion

22. SIGNALING CELL Signal molecule 1 Plasma membrane Receptor protein 2 TARGET CELL Signal Transduction Pathways • Cell-to-cell signaling • Important for development • Coordination of cellular activities • A signal-transduction pathway that turns on a gene • The signaling cell secretes the signal molecule • The signal molecule binds to a receptor protein in the target cell’s plasma membrane

23. SIGNALING CELL Signal molecule 1 Plasma membrane Receptor protein 2 3 TARGET CELL Relay proteins 4 Transcription factor (activated) Signal Transduction Pathways • Binding activates the first relay protein, which then activates the next relay protein, etc. • The last relay protein activates a transcription factor

24. SIGNALING CELL Signal molecule 1 Plasma membrane Receptor protein 2 3 TARGET CELL Relay proteins 4 Transcription factor (activated) NUCLEUS DNA 5 Transcription mRNA New protein 6 Translation Signal Transduction Pathways • The transcription factor triggers transcription of a specific gene • Translation of the mRNA produces a protein

25. Root ofcarrot plant Plantlet Cell divisionin culture Single cell Adult plant Root cells cultured in nutrient medium Cloning • Most differentiated cells retain a complete set of genes • In general, all somatic cells of a multi-cellular organism have the same genes

26. Cloning • Researchers clone animals by nuclear transplantation • A nucleus of an egg cell is replaced with the nucleus of a somatic cell from an adult • In reproductive cloning, the embryo is implanted in a surrogate mother • In therapeutic cloning, the idea is to produce a source of embryonic stem cells • Stem cells can help patients with damaged tissues

27. Donorcell Nucleus fromdonor cell Implant blastocystin surrogate mother Clone of donoris born(REPRODUCTIVEcloning) Removenucleusfrom eggcell Add somaticcell fromadult donor Grow in culture to producean early embryo (blastocyst) Remove embryonic stem cells from blastocyst andgrow in culture Induce stemcells to formspecialized cellsfor THERAPEUTICuse Cloning

28. Cloning • Cloned animals can show differences from their parent due to a variety of influences during development • Reproductive cloning is used to produce animals with desirable traits • Agricultural products • Therapeutic agents • Restoring endangered animals • Human reproductive cloning raises ethical concerns

29. Stem Cells • Stem cells can be induced to give rise to differentiated cells • Embryonic stem cells can differentiate into a variety of types • Adult stem cells can give rise to many but not all types of cells • Therapeutic cloning can supply cells to treat human diseases • Research continues into ways to use and produce stem cells

30. Blood cells Adult stem cells in bone marrow Nerve cells Cultured embryonic stem cells Heart muscle cells Different types of differentiated cells Different culture conditions

31. The Genetic Basis of Cancer • Escape from the control mechanisms that normally limit their growth and development • Due to changes in genes that affect expression of other genes • Oncogene-gene that causes cancer • Proto-oncogene- a normal gene with the potential to become an oncogene • A mutation can change a proto-oncogene into an oncogene • An oncogene causes cells to divide excessively

32. Oncogenes • Promote cancer when present in a single copy • Can be viral genes inserted into host chromosomes • Can be mutated versions of proto-oncogenes, normal genes that promote cell division and differentiation • Converting a proto-oncogene to an oncogene can occur by • Mutation causing increased protein activity • Increased number of gene copies causing more protein to be produced • Change in location putting the gene under control of new promoter for increased transcription

33. Proto-oncogene DNA Multiple copies of the gene Gene moved tonew DNA locus,under new controls Mutation within the gene Oncogene New promoter Hyperactivegrowth-stimulatingprotein in normalamount Normal growth-stimulatingproteinin excess Normal growth-stimulatingproteinin excess Mutations and Cancer

34. Mutated tumor-suppressor gene Tumor-suppressor gene Normalgrowth-inhibitingprotein Defective,nonfunctioningprotein Cell divisionunder control Cell division notunder control Mutations and Cancer • Mutations that inactivate tumor-suppressor genes have similar effects

35. Genetic Changes and Cancer • Four or more somatic mutations are usually required to produce a cancer cell • One possible scenario for colorectal cancer includes • Activation of an oncogene increases cell division • Inactivation of tumor suppressor gene causes formation of a benign tumor • Additional mutations lead to a malignant tumor

36. Colon wall 1 2 3 Increased cell division Growth of polyp Growth of malignant tumor (carcinoma) Cellular changes: DNA changes: Oncogene activated Second tumor- suppressor gene inactivated Tumor-suppressor gene inactivated

37. Chromosomes 4 mutations 1 mutation 2 mutations 3 mutations Normal cell Malignant cell