Homework #2 is posted and due 10/17 Exams should be returned on W 10/3

Each step in gene expression presents an opportunity to regulate when and how much of a gene product will be produced.

Why change gene expression? Different cells need different components Responding to the environment Replacement of damaged/worn-out parts

Two points to keep in mind: Cellular components are constantly turned-over. Gene expression takes time: Typically more than an hour from DNA to protein. Most rapidly 15 minutes.

DNA damage inhibits rRNA transciption Fig 1a The ATM repair pathway inhibits RNA polymerase I transcription in response to chromosome breaks Nature Vol 447 pg 730-734 (7 June 2007)

In bacteria, transcription and translation occur simultaneously In bacteria, transcription and translation occur simultaneously. So most regulation of gene expression happens at transcription. Fig 8.11

Transcription initiation in prokaryotes: sigma factor binds to the -35 and -10 regions and then the RNA polymerase subunits bind and begin transcription Fig 8.8

Operon: several genes whose expression is controlled by the same promoter Fig 10.4+.5

E. coli lactose metabolism Fig 10.5 E. coli lactose metabolism

In the absence of lactose, the lac operon is repressed. Fig 10.6 In the absence of lactose, the lac operon is repressed.

Fig 10.6 Lactose binds to the repressor, making it inactive, so that transcription can occur.

Fig 10.6

Glucose is a better energy source than lactose Fig 10.13

Low glucose leads to high cAMP cAMP binds to CAP which increases lac operon transcription Fig 10.13

High glucose leads to low cAMP low cAMP, CAP inactive, low lac operon transcription Fig 10.13

The lac operon: one example of regulating gene expression in bacteria Fig 10.4+.5