Control Mechanisms

42 000 genes exist that code for proteins in humans not all proteins are needed at all times – e.g. it would be inefficient to transcribe and translate the insulin gene when glucose levels are low

Housekeeping genes Some genes are always needed in a cell, and these are constantly transcribed and translated (i.e. they’re always on)

Transcription factors turn genes on when required (they are proteins that bind to DNA and help RNA polymerase to bind) Gene regulation is vital to an organism’s survival Regulation of genes occurs at 4 levels in eukaryotic cells:

1) Transcriptional Regulates which genes are transcribed (DNA to mRNA) or controls rate at which transcription occurs

2) Posttranscriptional The mRNA molecules undergo changes in the nucleus Introns are removed and exons are spliced together

3) Translational Controls how often and rapidly mRNA transcripts will be translated into proteins This control affects the length of time it takes for mRNA to be activated, and the speed at which enzymes in the cytoplasm destroy mRNA

4) Posttranslational Before many proteins become functional, they must pass through the cell membrane A number of control mechanisms affect the rate at which a protein becomes active and the time it remains functional

Examples of control mechanisms in prokaryotes The lac Operon The trp Operon

The lac Operon 3 genes in E. coli that code for proteins involved in the metabolism of lactose E. coli use enzyme ß-galactosidase to degrade lactose to glucose and galactose, but do not want to produce enzyme when lactose not present LacI protein blocks the transcription of the ß-galactosidase gene by binding to the lactose operator and blocking RNA polymerase (“roadblock”) Presence of lactose is an inducer (it removes the roadblock)

The trp Operon Tryptophan is an amino acid used by E. coli for the production of protein, but it can absorb tryptophan from its environment trp operon is repressed when high levels of tryptophan are present When levels drop, the shape of the trp repressor protein changes and it falls off the trp operator RNA polymerase is free to transcribe trp operon genes, resulting in tryptophan production