1. Controls Over Genes Chapter 14

2. Gene Control Which genes are being expressed in a cell depends upon: The type of cell Internal chemical conditions External signals Built-in control systems

3. Mechanisms of Gene Control Controls related to transcription Transcript-processing controls Controls over translation Post-translation controls

4. Regulatory Proteins Can exert control over gene expression through interactions with: –DNA –RNA –New polypeptide chains –Final proteins

5. Activator and Repressor Proteins Positive Control System –Activator protein enhances some activity Negative Control System –Repressor protein inhibits some activity

6. Some Controls Affect Noncoding DNA Promoter signals beginning of gene Enhancer is a binding site for activator protein

7. Chemical Modifications Methylation of DNA can inactivate genes Acetylation of histones allows DNA unpacking and transcription

8. Gene Control in Prokaryotes No nucleus separates DNA from ribosomes in cytoplasm When nutrient supply is high, transcription is fast Translation occurs even before mRNA transcripts are finished

9. The Lactose Operon

10. Negative Control – Low Lactose b In the absence of lactose, the repressor binds to two operators in DNA. It makes the DNA loop out in a way that blocks operon gene transcription; it stops RNA polymerase from binding to its promoter.

11. Negative Control – High Lactose

12. Positive Control of Lactose Operon CAP is an activator protein CAP affects promoter CAP will adhere to promoter only when in complex with cAMP; when glucose levels are high

13. Positive Control – High Glucose There is little cAMP CAP cannot be activated The promoter is not good at binding RNA polymerase The lactose-metabolizing genes are not transcribed very much

14. Positive Control – Low Glucose cAMP accumulates CAP-cAMP complex forms Complex binds to promoter RNA polymerase can now bind The lactose-metabolizing genes are transcribed rapidly

15. Cell Differentiation All cells in a multicellular organism inherited the same genes Some of those genes are used in all cell types During development, cells activate and suppress other genes in selective ways

16. Controls of Eukaryotic Gene Expression DNA pre-mRNA transcript mRNA NUCLEUS mRNA CTYOPLASM translational control protein product protein product control inactivated protein inactivated mRNA degradation control mRNA transport control transport processing control transcription control

17. Most Genes Are Turned Off Cells of multicelled organisms rarely use more than 5-10 percent of their genes at any given time The remaining genes are selectively expressed

18. Polytene Chromosomes Occur in salivary glands of midge larvae Consist of multiple DNA molecules Can produce multiple copies of transcripts

19. Chromosome Puff Portion of the polytene chromosome in which the DNA has loosened up to allow transcription Appears in response to ecdysone Translation of transcripts from puffed region produces protein components of saliva

20. Controls Over Gene Expression Work at certain stage before, during, and after transcription and translation Most controls over gene expression occur at translation

21. Selective Gene Expression All differentiated cells in a complex, multicelled body use most of their genes in much the same way Each type also uses a fraction of those genes in a unique, selective way

22. Homeotic Genes A class of master genes in most eukaryotic organisms Transcribed in specific locations in the developing embryo – products form in local tissue regions Guide formation of organs and limbs

23. X Chromosome Inactivation Mammalian females have two X chromosomes per cell One X is inactivated in each cell Inactivation is random Female is a “mosaic”

24. Barr Body Condensed X chromosome Visible in micrographs May be either the maternal or the paternal X chromosome

25. Clues to Gene Controls Research using Drosophila melanogaster has led to remarkable discoveries about how embryos develop