Control of Gene Expression Year 13 Biology

Exceptions to the usual Protein Synthesis Some viruses contain RNA and no DNA. RNA is therefore replicated by RNA. RNA is the permanent genetic code in some viruses so RNA is transcribed into DNA instead. This is called reverse transcription and is controlled by the enzyme reverse transcriptase

Control of transcription and translation Why does it need to be controlled?  Continual production of proteins is wasteful of energy.  Sometimes the products can be harmful.

Control of transcription and translation in Prokaryotes – The Operon Theory.

The Operon Theory Operator gene – activates the structural gene. Promotor gene – site where RNA polymerase can bind to the DNA and begin transcribing genes Regulatory gene – produces a repressor substance that turns the operator gene off. Inducers – binds to the repressor so that it cannot bind to the operator/DNA

How does the theory work? When RNA polymerase binds directly to the promotor region, transcription of the structural genes can occur. During transcription, repressor molecules are prevented from acting by being bound to another molecule. When the transcribed proteins build up to a certain level the repressor molecule is released and it binds to the operator region. This prevents any further transcription as the RNA polymerase is blocked.

Repressors and Inducers – why are they important? Inducer – removes the repressor and allows polymerase to bind and begin transcription Repressor – occupies part of the promotor and prevents polymerase being able to begin transcription. This is important as it prevents the cell making a protein when it is not needed. This maximises the efficiency of the cell.

Control of Transcription and Translation Prokaryotes  The operon theory  ns/content/lacoperon.html ns/content/lacoperon.html

Control of transcription and translation in Eukaryotes.

Unlike prokaryotes, there is no operons. Transcription factors must bind to the promotor site. RNA polymerase then binds to these transcription factors at the promotor site.

Control of transcription and translation in Eukaryotes. Thousands of bases away, there is an enhancer site. Transcription factors also bind to this site. This site is important as it allows the promotor sequence to be in the correct position to initiate transcription and allows the cell to respond to changes in the environment

Control of transcription and translation in Eukaryotes. The DNA then bends, bringing the enhancer and promotor sites close together. It is only when these two regions are close that transcription is initiated.