Regulation of Gene Activity and Gene Mutations

1. Regulation of Gene Activity and Gene Mutations Chapter 15

2. Mader: Biology, 9th Ed. Prokaryotic Regulation Operon – group of structural and regulating genes that function together as a unit Regulator genes – located outside of the operon, codes for a repressor that controls whether the operon is active or not

3. Mader: Biology, 9th Ed. Prokaryotic Regulation Operon Model consists of the following components: Promoter – short DNA sequence where RNA polymerase first attaches Operator – short DNA sequence where active repressor binds Structural Genes – 1 to several genes coding for enzymes of metabolic pathway that are translated as a unit

4. Mader: Biology, 9th Ed. Operon

5. Mader: Biology, 9th Ed. trp Operon Regulator codes for a repressor that ordinarily is unable to attach to the operator RNA polymerase binds to the promoter, the structural genes are expressed, and the product enzymes are produced

6. Mader: Biology, 9th Ed. trp Operon If tryptophan 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 already present, the enzymes are not needed and the operon is turned off Repressible operon Tryptophan is the corepressor, binds to repressor Blocks synthesis of enzymes and tryptophan

7. Mader: Biology, 9th Ed. trp Operon

8. Mader: Biology, 9th Ed. lac Operon 3 structural genes are adjacent to one another on the chromosome and are under control of a single promoter and single operator Regulator gene codes for a lac operon repressor that ordinarily binds to the operator and prevents transcription

9. Mader: Biology, 9th Ed. lac Operon When lactose is present, it binds to the repressor which undergoes a change, preventing it from binding to the operator Lactose is the inducer of the lac operon The enzymes are inducible enzymes, and the entire unit is an inducible operon If lactose is absent, repressor attaches to the operator Expression is normally “off”

10. Mader: Biology, 9th Ed. lac Operon

11. Mader: Biology, 9th Ed. Eukaryotic Regulation A variety of mechanisms regulates gene expression in eukaryotic cells Five primary levels of control: Chromatin packing Transcriptional Control Posttranscriptional Control Translational Control Posttranslational Control

12. Mader: Biology, 9th Ed. 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 Presence limits access to DNA, and euchromatin becomes genetically active when histones no longer bar access to DNA

13. Mader: Biology, 9th Ed. Organization of Chromatin

14. Mader: Biology, 9th Ed. Chromatin Structure Organization of Chromatin Euchromatin Loosely coiled DNA Transcriptionally active Heterochromatin Tighly packed DNA Transcriptionally inactive, genes are not being expressed in that particular cell Example of heterochromatin in a cell Barr Bodies - Inactive X chromosome in females This chromosome is not producing gene products

15. Mader: Biology, 9th Ed. X-Inactivation

16. Mader: Biology, 9th Ed. Transcriptional Control Transcription Factors – DNA-binding proteins that control transcription Different combination is believed to regulate activity of any particular gene Enhancers - Regions 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

17. Mader: Biology, 9th Ed. Transcription Factors

18. Mader: Biology, 9th Ed. Posttranscriptional Control Posttranscriptional control begins once there is a primary mRNA transcript Processing of primary transcript Differential excision of introns Differential splicing of exons Varies the mature mRNA that leaves the nucleus Speed of mRNA transport from nucleus to cytoplasm Affect number of transcripts arriving at ER Can determine the amount of gene product realized per unit time following transcription

19. Mader: Biology, 9th Ed. Processing of mRNA Transcripts

20. Mader: Biology, 9th Ed. 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 Masking of mRNA Life of mRNA Influence of hormones

21. Mader: Biology, 9th Ed. Posttranslational Control Posttranslational Control - Affects the activity of a protein product Activation of protein product Degradation rate of a protein

22. Mader: Biology, 9th Ed. Genetic Mutations Effect of Mutations on Protein Activity Point Mutations Involve change in a single DNA nucleotide Therefore a change in specific codon Nonsense mutations – introduction of a stop codon as a result of mutation Affects on protein vary Nonfunctional Reduced functionality Unaffected

23. Mader: Biology, 9th Ed. Point Mutation

24. Mader: Biology, 9th Ed. Genetic Mutations Effect of Mutations on Protein Activity Frameshift Mutations - Occur because one or more nucleotides are either inserted or deleted from DNA; renders proteins nonfunctional Normal: THE CAT ATE THE RAT After deletion: THE ATA TET HER AT After insertion: THE CCA TAT ETH ERA T

25. Mader: Biology, 9th Ed. Nonfunctional Proteins Single nonfunctional protein can have a dramatic effect on the phenotype Androgen insensitivity due to faulty receptor for androgens (male sex hormones) Cells are unable to respond to male hormones such as testosterone

26. Mader: Biology, 9th Ed. Carcinogenesis Development of cancer involves a series of various types of mutations Proto-oncogenes – stimulate cell cycle Tumor-suppressor genes – inhibit cell cycle Mutation in oncogene and tumor-suppressor gene Stimulates cell cycle Leads to tumor formation

27. Mader: Biology, 9th Ed. Carcinogenesis

28. Mader: Biology, 9th Ed. Causes of Mutations Replication Errors 1 in 1,000,000,000 replications DNA polymerase Proofreads new strands Generally corrects error

29. Mader: Biology, 9th Ed. Causes of Mutations Environmental Mutagens – environmental agent that increases the chance of mutation Carcinogens - Mutagens that increase the chances of cancer UV Radiation Tobacco Smoke

30. Mader: Biology, 9th Ed.