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.

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 need

for 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

Lactose

• The lac operon 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-galactosidase permease• lacA has an unknown function

Lac Operon

• 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

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.

Trp OPERON-a corepressor

5.5 - Summary

5.5 Vocabulary