5.5 Control Mechanisms

1. 5.5 Control Mechanisms • There are approximately 42 000 genes that exist to code for proteins in humans. • Not all proteins are required at all times. • Some genes are required at all times • Housekeeping genes • Transcription factors turn on genes when required. • Gene regulation can occur at four levels. • Transcriptional • Regulates which genes are transcribed OR controls rate of transcription • Posttranscriptional • mRNA undergoes changes in the nucleus. • Translational • How often and how rapidly mRNA transcripts are translated into proteins. • Posttranslational • Before many proteins become functional they must leave the cell.

2. Lac OPERON • Lactose is a disaccharide composed of glucose and galactose. • E. coli use lactose for energy. • Must split lactose into monomers. • β-galactosidase • When lactose is not present, no needfor production. • The gene for β-galactosidase is part of an operon. • Comprised of a cluster of structural genes, a promoter, and a short sequence of bases between the two known as an operator

3. Lactose

4. The lacoperon consists of a cluster of three genes that code for proteins in the metabolism of lactose. • lacZ, lacY, and lacA • lacZ codes for β-galactosidase • lacY codes for β-galactosidasepermease • lacA has an unknown function

5. Lac Operon

6. The LacI protein is a repressor protein that blocks the transcription of β-galactosidase gene by binding to the lactose operator and getting in the way of RNA polymerase. • When lactose is present, it binds to the LacI protein, allowing β-galactosidase to be transcribed. • Lactose acts as a signal molecule or inducer

7. Trp OPERON • Used to control Tryptophan metabolism • Transcription is blocked when there are high amounts of tryptophan present. • The level of tryptophan is the effector. • Consists of five genes. • Code for five polypeptides that make three enzymes needed to synthesize tryptophan.

9. Trp OPERON-a corepressor

11. 5.5 - Summary

12. 5.5 Vocabulary