1. Control Mechanisms (Prokaryote) SBI4U

2. Controlling Expression  When a gene is being used by a cell, it gets transcribed, and then the mRNA is translated to produce the polypeptide. We say that the gene has been expressed.  How does a cell control, or regulate, which genes are expressed, and when they are expressed?

3.  Gene regulation involves turning certain genes on or off  depends on whether their products are required  Regulators are the proteins that switch genes on or off  activators, or  repressors

4. Recall… Protein synthesis occurs in two distinct stages: 1. Transcription, in the _________ ◦ Product: ____ 2. Translation, in the ______________ ◦ Product: __________

5. Four Levels of Control  Transcriptional – which genes are transcribed  Post-transcriptional – mRNA modifications  Translational – ribosome activity vs. mRNA degradation  Post-translational – regulation of protein activity

6. Transcriptional regulation Two classic examples:  lac operon – negative regulation  trp operon – positive regulation

7. Operons  Cluster of genes, which generally code for the enzymes of a biochemical pathway,  all genes are under the control of one set of regulatory sequences  promoter and operator sequences  regulation is dependent on the presence or absence of effector molecules  The genes are transcribed as a single piece of RNA, and are translated simultaneously. In this way, the entire biochemical pathway becomes active at the same time. only in prokaryotic cells

9. Operator sequence  Sequence that the regulator protein binds to  – the operon’s “ON-OFF” switch.  Depending on the regulator, binding to the operator causes either:  activation of transcription  repression of transcription

10. Effector Acts on the regulator protein  Can be an inducer  stimulates transcription  Can be a co-repressor  inhibits transcription

11. Lac operon  lactose = glucose + galactose  E. coli: ß-galactosidase catalyses the cleavage of the bond in lactose Not economical to produce ß-galactosidase when lactose is not present

12. Structure of lac operon Cluster of genes: lacZ, lacy, lacA  Each codes a different part of the enzyme  ALL are under the control of one promoter

13. lac Operon

14. WATCH THIS!!

15. http://www.sumanasinc.com/webcontent/an imations/content/lacoperon.htmlwww.sumanasinc.com/webcontent/an imations/content/lacoperon.html

16. Summary: Regulation of lac operon lactose is absentlactose is present LacI binds to lac promoter. No transcription. Lactose binds to LacI protein. LacI changes conformation → unable to bind lac operator. RNA polymerase can access lac promoter; transcription occurs.

17.  What is the effector molecule? _______________  It acts as an inducer – it induces transcription by deactivating the repressor protein.

20. The trp operon

21. WATCH THIS!! WATCH THIS TOO!!

22.  What is the effector molecule? _______________  It acts as a co-repressor. It binds to a repressor to activate it in order to repress transcription.

23. Summary – Regulation of trp operon trp absenttrp present trp operon is transcribed (default) trp itself will bind to the repressor protein. conformational shape occurs allows repressor to bind to the trp operator → shuts off transcription

24. SUMMARY

25. Recap: Vocabulary Basic terms:  Gene regulation: Describes any process that alters the rate of gene expression.  Operon: A cluster of closely-related genes. They are all controlled by one set of regulatory sequences.

26. Types of molecules involved:  Regulators: Molecules that carry out translational gene regulation.  Classified as either activators, or repressors.  Example: LacI repressor; trp repressor  Effectors: Substances to which regulator proteins respond.  Inducers or Co-repressors  Example: Lactose, Tryptophan

27. Regulatory DNA sequences:  Promoter: DNA sequence to which RNA polymerase binds to begin transcription.  Operator: DNA sequence to which a transcription factor binds in order to alter transcription. Close to the promoter.

28. Credits  http://commons.wikimedia.org/wiki/File:La c-operon.jpeg http://commons.wikimedia.org/wiki/File:La c-operon.jpeg  http://commons.wikimedia.org/wiki/File:Tr poperon.svg http://commons.wikimedia.org/wiki/File:Tr poperon.svg