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Purpose of Regulating Gene Expression
Purpose of Regulating Gene Expression
We know by now that most of the cells in your body have the exact same DNA. So what makes a cell in your toe different from a cell in your liver? If they contain the same information, shouldn't they do the same things? The answer to this mystery is all about gene expression - which genes each cell turns on and which ones it turns off. In fact, this is how stem cells differentiate into different cells. Stem cells are basically blank canvases on which anything (or almost anything) can be painted. Depending on what genes get expressed via transcription and translation, two cells with vastly different jobs and structures can have the same DNA.
In addition, cells need to be able to change things when conditions change. Last I checked, the world was an unpredictable place, and cells must be able to acclimate. For instance, if a new food source becomes available, a cell would benefit from being able to properly digest it. But, if that food was absent, a cell shouldn't waste the energy producing the enzymes to break it down.
Turning Genes On and Off
Turning Genes On and Off
So how do cells actually 'turn on' and 'turn off' genes? How can a cell ensure that some genes are transcribed and others ignored? The primary ways we will be investigating are through regulatory sequences and transcription factors.
Regulatory Sequences
Regulatory Sequences
Regulatory sequences are stretches of DNA that interact with regulatory proteins to control transcription. There are multiple types of regulatory sequences, but we will focus on promoters, terminators, and enhancers primarily.
A. Promoters
A. Promoters
Promoters are sections of DNA that initiate transcription of a particular gene. They are found upstream of the gene near the starting site of transcription.
The promoter has a specific sequence that allows RNA polymerase to bind to it and begin transcription.
B. Terminators
B. Terminators
Terminators are sections of DNA that signal the end of a gene. This section is responsible for terminating, or stopping, transcription and the release of the new mRNA.
Terminators are after the gene, so are said to be downstream of the gene.
C. Enhancers and Transcription Factors
C. Enhancers and Transcription Factors
Enhancers are short sections of DNA that bind to transcription factors. If a transcription factor is bound, this increases the likelihood that transcription will occur for that gene (and thus it will be more likely to be expressed).
There are two kinds of transcription factors: positive and negative. Positive transcription factors make a gene more likely to be expressed. Negative transcription factors make a gene less likely to be expressed.
So a cell can release more positive or negative transcription factors in response to a stimulus in order to better deal with that stimulus. Thus, it will generate more or less of some protein that is necessary for its response.
That might have seemed overwhelming, so let's look at some examples of these regulatory sequences in action. These regulatory sequences (such as the promoter) and the gene(s) make up an operon.
Operons
Operons
An operon is a section of DNA under control of one promoter. So, this section of DNA can be more easily turned on or off. There are two major types of operons: inducible and repressible.
A. Inducible Operons
A. Inducible Operons
An inducible operon is, as the name suggests, an operon that can be induced, or 'turned on'. So, this operon (which is just a gene, some proteins, and the areas of DNA around it) is normally shut off and must be turned ON to be transcribed.
Under normal circumstances when expression isn't occurring [so it is turned OFF] (left side of diagram), there is a repressor active that essentially blocks the RNA polymerase (shown in white) from transcribing the gene. In order to transcribe the gene, that repressor must be removed. Think of the repressor as a big roadblock on the railroad track that is the DNA.
Inducible operons are most common for proteins that are used to break down food (catabolic processes).
When the gene is expressed, an inducer binds to the repressor, making the repressor inactive. Thus, the roadblock is removed and the gene can be transcribed (expressed).
So, to summarize, inducible operons are normally OFF (not expressed), and must be turned ON if they are to be expressed.
Example: The lac Operon
Example: The lac Operon
The lac operon is incredibly well-studied and is found in bacteria. It contains genes for an enzyme that breaks down lactose into glucose and galactose. It is also generally the example used on tests (including the AP test) for inducible operons, so being familiar with it is recommended.
You do not need to memorize this operon or any of its components, but you should be able to recognize it as an inducible operon and provide evidence to support that assertion.
Take a few minutes to analyze the two provided situations: one in which lactose (the inducer) is absent and one in which it is present. Compare and contrast this inducible operon with a repressible operon.
B. Repressible Operons
B. Repressible Operons
A repressible operon, as its name suggests, can be repressed. So, a repressible operon is generally ON (being expressed) and must be shut OFF to stop expression.
Under normal circumstances when expression is occurring [so it is turned ON] (left side of diagram), there is a repressor, but it is inactive. It is simply floating around waiting for its missing piece.
Repressible operons are most common for proteins that are used to build up organic molecules (anabolic processes).
When the gene is not expressed, a corepressor binds to the repressor, making the repressor active. Thus, the roadblock is instated and the gene can no longer be transcribed (expressed).
So, to summarize, repressible operons are normally ON (expressed), and must be turned OFF.
Example: The trp Operon
Example: The trp Operon
The trp operon is a classic example of a repressible operon and the most likely one for you to encounter on a test.
You do not need to memorize this operon or any of its components, but you should be able to recognize it as an repressible operon and provide evidence to support that assertion.
Take a few minutes to analyze the two provided situations: one in which tryptophan (the corepressor) is absent and one in which it is present. Compare and contrast this repressible operon with an inducible operon.
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