Chapter 13: Gene Regulation

The Big Picture… A cell contains more genes than it expresses at any given time – why? Why are cells in multicellular organisms specialized and only express certain genes? Gene expression in cells must be regulated – From inside the cell – From outside the cell: by other nearby cells, or the environment

Bacterial Gene Regulation Unicellular with very short life spans Control gene regulation with control of transcription: transcriptional-level control Functionally related genes are controlled together in gene complexes called operons

The bacterial lac operon: an example Helps E. coli bacteria metabolize lactose Requires 3 different enzymes – the lac operon has these genes all closely linked on the DNA strand Transcription for all 3 enzymes produces 1 mRNA strand with coding for each enzyme Without the presence of lactose, a repressor protein prevents mRNA transcription of the lac operon In the presence of lactose, this protein is altered and can no longer prevent transcription This is an inducible operon – it is usually off but can be turned on but the presence of an inducer molecule

Other forms of bacterial regulation Repressible operons: genes are usually turned on but are turned off under certain conditions Both inducible and repressible operons are negative controls: the regulatory protein is a repressor that turn off transcription Positive controls: use an activator protein that binds to DNA and stimulates transcription

Other forms of bacterial regulation… Translational controls: the speed of mRNA  tRNA  protein is controlled Posttranslational controls: enzymes are turned off and on, rather than genes – Feedback inhibition

Eukaryotic Gene Regulation Cells must respond to changes in the environment Cells must become specialized and fulfill the specific roles of the tissues in the body Eukaryotic cells rely on transcriptional, translational, and posttranslational controls Eukaryotic cells generally do not have operons

Eukaryotic Transcriptional Regulation Transcription requires the transcription initiation site plus the promoter – in eukaryotes this is called a TATA box and is required for transcription to begin RNA polymerase binds to the TATA box Some promotes also have upstream promoter elements (UPEs) – usually the more of these that are present, the more strongly a particular gene is expressed

Additional transcriptional regulation DNA sequences called enhancers increase the rate of transcription Transcription factors – DNA-binding proteins that regulate transcription RNA interference – small bits of RNA permanently shut down sections of DNA DNA methylation – DNA has been chemically altered by enzymes that add methyl groups to cytosine

Eukaryotic Posttranscriptional Control After transcription mRNA is further processed before translation These event provide points of control between transcription and translation Some proteins are further modified after translation by adding or removing functional groups