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Biochem gene expression regulation(29.6.10)

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Biochem gene expression regulation(29.6.10)

  1. 1. Gene Regulation
  2. 2. How and Why Genes Are Regulated <ul><ul><li>Four of the many different types of human cells: </li></ul></ul><ul><ul><ul><li>They all share the same genome. </li></ul></ul></ul><ul><ul><ul><li>What makes them different? </li></ul></ul></ul>
  3. 3. <ul><ul><li>In cellular differentiation, </li></ul></ul><ul><ul><ul><li>Certain genes are turned on and off. </li></ul></ul></ul><ul><ul><ul><li>Cells become specialized in structure and function. </li></ul></ul></ul>
  4. 5. DIFFERENT GENE CATEGORIES Housekeeping genes Genes turned “on” in all cells at all times (e.g. transcription machinery, translation machinery, energy conversion, etc.). Cell type specific genes Genes that are turned “on” in each cell that give a cell its special properties and function. Developmental regulatory genes Genes specific to certain stages during growth & development of a person. Inducible genes Genes not normally expressed but can be in response to external stimuli (e.g. hormone).
  5. 6. Gene Regulation in Bacteria <ul><ul><li>Bacteria can alter gene expression based on environmental factors. </li></ul></ul><ul><ul><li>Control sequences </li></ul></ul><ul><ul><ul><li>Are stretches of DNA that coordinate gene expression. </li></ul></ul></ul><ul><ul><li>An operon </li></ul></ul><ul><ul><ul><li>Is a cluster of genes with related functions, including the control sequences. </li></ul></ul></ul>
  6. 7. <ul><ul><li>A promoter </li></ul></ul><ul><ul><ul><li>Is a control sequence. </li></ul></ul></ul><ul><ul><ul><li>Is the site where the transcription enzyme initiates transcription. </li></ul></ul></ul><ul><ul><li>An operator </li></ul></ul><ul><ul><ul><li>Is a DNA sequence between the promoter and the enzyme genes. </li></ul></ul></ul><ul><ul><ul><li>Acts as an on and off switch for the genes. </li></ul></ul></ul>
  7. 10. THE lac OPERON © 2007 Paul Billiet ODWS
  8. 11. Operons <ul><li>An operon is a group of genes that are transcribed at the same time . </li></ul><ul><li>They usually control an important biochemical process. </li></ul><ul><li>They are only found in prokaryotes . </li></ul>Jacob, Monod & Lwoff
  9. 12. The lac Operon <ul><li>The lac operon consists of three genes each involved in processing the sugar lactose </li></ul><ul><li>One of them is the gene for the enzyme β-galactosidase </li></ul><ul><li>This enzyme hydrolyses lactose into glucose and galactose </li></ul>
  10. 13. Adapting to the environment <ul><li>E. coli can use either glucose, which is a monosaccharide, or lactose, which is a disaccharide </li></ul><ul><li>However, lactose needs to be hydrolysed (digested) first </li></ul><ul><li>So the bacterium prefers to use glucose when it can </li></ul>
  11. 14. Four situations are possible <ul><li>When glucose is present and lactose is absent the E. coli does not produce β-galactosidase. </li></ul><ul><li>When glucose is present and lactose is present the E. coli does not produce β-galactosidase. </li></ul><ul><li>When glucose is absent and lactose is absent the E. coli does not produce β-galactosidase. </li></ul><ul><li>When glucose is absent and lactose is present the E. coli does produce β-galactosidase </li></ul>
  12. 15. 1. When lactose is absent <ul><li>A repressor protein is continuously synthesised. It sits on a sequence of DNA just in front of the lac operon, the Operator site </li></ul><ul><li>The repressor protein blocks the Promoter site where the RNA polymerase settles before it starts transcribing </li></ul>Regulator gene lac operon Operator site z y a DNA I O Repressor protein RNA polymerase Blocked
  13. 16. 2. When lactose is present <ul><li>A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site ( allosteric site ) </li></ul><ul><li>This causes the repressor protein to change its shape (a conformational change ). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site </li></ul>z y a DNA I O
  14. 17. 2. When lactose is present <ul><li>A small amount of a sugar allolactose is formed within the bacterial cell. This fits onto the repressor protein at another active site ( allosteric site ) </li></ul><ul><li>This causes the repressor protein to change its shape (a conformational change ). It can no longer sit on the operator site. RNA polymerase can now reach its promoter site </li></ul>© 2007 Paul Billiet ODWS Promotor site z y a DNA I O
  15. 18. 3. When both glucose and lactose are present <ul><li>This explains how the lac operon is transcribed only when lactose is present. </li></ul><ul><li>BUT….. this does not explain why the operon is not transcribed when both glucose and lactose are present. </li></ul>
  16. 19. <ul><li>When glucose and lactose are present RNA polymerase can sit on the promoter site but it is unstable and it keeps falling off </li></ul>Promotor site z y a DNA I O Repressor protein removed RNA polymerase
  17. 20. 4. When glucose is absent and lactose is present <ul><li>Another protein is needed, an activator protein . This stabilises RNA polymerase. </li></ul><ul><li>The activator protein only works when glucose is absent </li></ul><ul><li>In this way E. coli only makes enzymes to metabolise other sugars in the absence of glucose </li></ul>Promotor site z y a DNA I O Transcription Activator protein steadies the RNA polymerase

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