Protein Synthesis Processing
Location: Outside U.S.
Date: Winter 2012-2913
How does a cell know that is time to execute transcription?
Signals come from outside of the cell usually and lock onto receptors that cause an action to occur inside the cell. Then a cascade of signals inside the cell takes place with the final signal in the pathway telling a protein called a transcription factor, found in the nucleus, to lock onto the DNA and "turn on" the gene. This "recruits" RNA Polymerase to come over and read the gene. Of course, it is much more complicated than this. If you want to learn more, google "signal transduction pathways". Also, this research was given last year's Nobel Prize in Physiology and Medicine. To learn more, go to nobelprize.org.
Since transcription is started to ultimately synthesize a particular protein needed at a specific time therefore the incumbent signal is the indicator of this process. For example, if lactose is added to bacterial culture that was previously subsisting on glucose, enzymes for lactose would be transcribed. In this case lactose itself binds with the repressor protein that is bound with the promotor ( operator) region. And hence makes the promotor region available for transcription to start.
This is an excellent question! Cellular regulation of DNA transcription into messenger RNA for protein production is a complex, heavily studied subject. The fundamental differences between, say, a muscle cell, a nerve cell, and a skin cell are rooted in which proteins the cell produces, which in turn is controlled by DNA transcription. All cell types have the same genetic code (with some minor exceptions), so it is how the genetic code is expressed which makes one different from another.
The simple answer to your question is that transcription of a gene is initiated by special proteins in the nucleus called transcription factors. These transcription factors, when activated, can bind to specific DNA sequences. These can then act as activators, 'anchor' points for the start of transcription by RNA polymerase complexes. Alternatively, some can act as repressor proteins, blocking the RNA polymerase from being able to transcribe the gene.
These transcription factors are activated or deactivated by many other cellular processes. For example, if the cell receives an external signal from a hormone or other protein/chemical signal, it often causes what is called a signal transduction pathway. This pathway is the successive activation of many proteins in the cell which respond to the hormone signals. Some of these activated proteins are transcription factors, which control the synthesis of new proteins. Cells can also spontaneously activate and deactivate transcription factors in response to environmental stresses like sudden increases in heat.
Transcription can also be regulated by other processes in the nucleus. Some proteins don't directly affect RNA polymerase, but instead chemically modify DNA and histone proteins, which are the proteins that DNA is coiled around in a chromosome. Special proteins can methylate DNA, which 'silences' it and blocks transcription, while others can acetylate histone proteins, which makes the DNA more easily accessible for transcription. These processes can also be reversed by other proteins. If you study cellular and molecular biology further, you will learn of more exotic types of transcription regulation. Scientists are still trying to understand all of the different ways in which genes are regulated, so there are likely to be more ways to control transcription that we do not fully understand.
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Update: November 2011