Controls Over Genes

1. Controls Over Genes

2. More on Transcription • Promoters are regions on DNA that show where RNA Polymerase must bind to begin the Transcription of RNA • Called the TATA box • Transcription factors are also involved (proteins that mediate the binding of RNA polymerase) • Specific base sequences act as signals to stop • Called the termination signal

3. mRNA Processing • After the DNA is transcribed into RNA, editing must be done to the nucleotide chain to make the RNA functional • Introns, non-functional segments of DNA are snipped out of the chain (RNA splicing)

4. mRNA Editing • Exons, segments of DNA that code for proteins, are then rejoined by the enzyme ligase • A guanine triphosphate cap is added to the 5” end of the newly copied mRNA • A poly A tail is added to the 3’ end of the RNA • The newly processed mRNA can then leave the nucleus

5. Result of Transcription New Transcript Tail CAP

6. mRNA Transcript • mRNA leaves the nucleus through its pores and goes to the ribosomes

7. Why Control Gene Expression? • Some genes are “on” (being transcribed) almost all the time • Called housekeeping genes • Examples: ribosome components, enzyme for basic metabolic pathways • Many genes are only turned on when they are needed

8. Why Control? • Transcribing genes that are not needed is a waste of energy and may interfere with the status of the cell

9. Regulation • Respond to a range of stimuli • Prokaryotes respond to external stimuli (food, enzymes turned on) • Eukaryotes also respond to internal stimuli (hormones, growth factors)

10. Regulation • Developmentally regulated • Multicellular organisms progress through developmental stages • Different genes expressed at different times during development • Cell specialization • Different genes expressed in different cells

11. The strategy behind regulation.. • Gene control is control over amount of gene produced (RNA or protein) in cell • Multiple ways to control the amount of gene product in a cell

12. Controlling gene product amount • Rate of transcription – rate mRNA is produced; faster produced = more product • mRNA degradation – rate mRNA is broken down; faster broken down = less product

13. Controlling gene product amount • mRNA processing – capping, splicing; slower processing = less product • Translation – rate of translation or # of ribosomes translating; fast/more = more product Although control probably involves all of these, the most understood are changes in the rate of transcription

14. Gene Control – lac operon • Lac operon is a gene in bacteria • Bacteria have 3 genes in a row (operon) that involve breaking down lactose for energy • In order to be efficient, these genes should not be expressed unless lactose is present

15. Lac Operon - vocab • Regulatory protein – control transcription, translation, and gene products by interacting with DNA, RNA, or proteins • Repressor – protein that binds with an operator on prokaryotic DNA to prevent transcription • Operator – short base sequence between a promoter and genes; a binding site for repressors

16. Lac operon – vocab. • Promoter – piece of DNA where RNA polymerase can bind and start transcription • Negative control – regulatory protein that slows down gene activity • Positive control – regulatory protein that enhances gene activity

17. Lac operon vocab. • Operon – a promoter and a pair of operators that control a bacterial gene • Activator – protein that exerts positive control over an operon

18. Figure 15.3aPage 241 operator regulatory gene operator gene 1 gene 2 gene 3 transcription, translation promoter lactose operon repressor protein

19. Lac operon • Goal 1 – transcription low when lactose is absent • Lac I (gene upstream from operon) produces a repressor which binds to promoter region • Binding of repressor prevents RNA polymerase from binding and transcribing genes

20. Lac operon • Goal 2 – increase transcription when lactose is present • Allolactose will bind to the repressor, changing its conformation and causing it to fall off the promoter site • Promoter site now available for RNA polymerase to bind; transcription of lac genes begins

21. Lac operon • Goal 3 – turn off transcription when lactose is used up • Allolactose metabolizes, freeing up the repressor • The free repressor is available to bind the promoter site and stop transcription

22. Control of lac operon • Negative control – glucose present - repressor inactivates the lac operon • Positive control – lactose present – activator protein (called CAP) makes promoter more favorable for RNA polymerase to bind and begin transcription

23. Low Lactose • Repressor binds to operator • Binding blocks promoter • Transcription is blocked Figure 15.3bPage 241

24. High Lactose allolactose lactose mRNA RNA polymerase gene 1 operator promoter operator Figure 15.3cPage 241

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

26. Homeotic Genes • Occur in all eukaryotes • Master genes that control development of body parts • Encode homeodomains (regulatory proteins) • Homeobox sequence can bind to promoters and enhancers

27. X Chromosome Inactivation • In female mammals, in all cells one of the two Xchromosomes is completely inactivated • Inactivation is random • Inactivated chromosome can be observed in the interphase nucleus as Barr body • Genes on the inactivated chromosome are not expressed