Controlling Gene Expression 7.4

Control Mechanisms housekeeping genes code for proteins that are needed all the time; they are constantly being transcribed and translated not all proteins are needed by all cells at all times, so gene regulation is important to an organism's survival

Prokaryotic Cells use operons to control gene expression an operon is: –a promoter region –an operator –a cluster of genes

Lactose disaccharide found in milk or milk sugars intestinal bacteria cells metabolize lactose for energy

lac operon 3 genes control the proteins needed to metabolize lactose the lac operon contains: –a promoter –an operator –3 genes: lacZ, lacY, and lacA

lac operon animation Narrated animation: ree/ /126997/animation27.ht ml

lac operon if no lactose is present, the genes do not need to be expressed, and lacI (a repressor protein) is bound to the operator if lactose is present, it binds to lacI and inactivates it, allowing the genes to be expressed lactose is an inducer

Tryptophan a non-essential amino acid used for protein synthesis intestinal bacteria cells can obtain tryptophan from a mammalian diet, or they can synthesize it themselves

trp operon 5 genes control the biosynthesis of tryptophan the trp operon contains: –a promoter –an operator –5 genes

trp operon animation Narrated animation: cgi/pluginpop.cgi?it=swf::535::535::/site s/dl/free/ /120080/bio26.swf: :The%20Tryptophan%20Repressor

trp operon if no tryptophan is present, the genes need to be expressed and the trp repressor protein is inactive

trp operon if tryptophan is present, the genes do not need to be expressed; tryptophan binds to the trp repressor protein, which binds to the operator tryptophan is a corepressor

Summary The lac operon is inducible. The trp operon is repressible.

Eukaryotic Cells genes can be regulated: during transcription (most common) after transcription during translation after translation

Transcriptional promoters must be accessible in order for transcription to occur –chromatin remodelling –addition of acetyl group (CH 3 COO - ) to histones transcription initiation complex regulated by activators & repressors

Transcriptional methylation of genes –methyl group attached to cytosine in promoter region –causes gene to be inactive

Agouti Mice

Post-Transcriptional alternative splicing binding of masking proteins to mRNA (common in unfertilized egg cells) changes in rate of degradation of mRNA (e.g. casein in rat mammary gland cells)

Translational changes in length of poly(A) tail can increase or decrease the length of time for translation (not fully understood)

Post-Translational processing mechanisms (e.g., removal of specific sections activates certain proteins such as insulin) chemical modification (addition or deletion of chemical groups) addition or removal of tags to control rate of degradation of proteins

Cancer

Cancerous cells genes that regulate cell growth can become oncogenes, causing unregulated cell division high concentrations of telomerase cause constant lengthening of telomeres