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Gene expression is a multi-step process that begins with transcription of DNA, which genes are made of, into messenger RNA. It is then followed by post transcriptional modification and translation into a gene product, followed by folding, post-translational modification and targeting. The amount of protein that a cell expresses depends on the tissue, the developmental stage of the organism and the metabolic or physiologic state of the cell.
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Patterns of gene expression
Numerous terms are used to describe patterns of gene expression, including:
- A constitutive gene is a gene that is transcribed continually compared to a facultative gene which is only transcribed when needed.
- A housekeeping gene is typically a constitutive gene that is transcribed at a relatively constant level. The housekeeping gene's products are typically needed for maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions. Examples include actin, GAPDH and ubiquitin.
- A facultative gene is a gene which is only transcribed when needed compared to a constitutive gene.
- An inducible gene is a gene whose expression is either responsive to environmental change or dependent on the position of the cell cycle.
Measurement
Indirectly, the expression of particular genes may be assessed with DNA microarray technology, which can provide a rough measure of the cellular concentration of different messenger RNAs; often thousands at a time. While the name of this type of assessment is actually a misnomer, it is often referred to as expression profiling. The expression of many genes is known to be regulated after transcription, so an increase in mRNA concentration need not always increase expression. A more sensitive and more accurate method of relative gene expression measurement is real-time polymerase chain reaction. With carefully constructed standard curve it can even produce an absolute measurement such as in number of copies of mRNA per nanolitre of homogenized tissue, or in number of copies of mRNA per total poly-adenosine RNA. Protein expression levels can be measured by fusing the desired protein to another reporter protein, such as the green fluorescent protein or the enzyme beta-galactosidase. The expression level of these reporter proteins can be directly quantified using standard techniques.
Regulation of gene expression
Regulation of gene expression is the cellular control of the amount and timing of appearance of the functional product of a gene. Any step of gene expression may be modulated, from the DNA-RNA transcription step to post-translational modification of a protein. Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation, morphogenesis and the versatility and adaptability of any organism.
Overexpression
The protein encoded for by a gene can be expressed in increased quantity. This can come about by increasing the number of copies of the gene or increasing the binding strength of the promoter region.
Often, the DNA sequence for a protein of interest will be cloned or subcloned into a plasmid containing the lac promoter, which is then transformed into the bacterium Escherichia coli. Addition of IPTG (a lactose analog) causes the bacteria to express the protein of interest. However, this strategy does not always yield functional protein, in which case, other organisms or tissue cultures may be more effective. As for example the yeast, Saccharomyces cerevisiae, is often preferred to bacteria for proteins that undergo extensive Posttranslational modification. Nonetheless, bacterial expression has the advantage of easily producing large amounts of protein, which is required for X-ray crystallography or nuclear magnetic resonance experiments for structure determination.
Gene networks and expression
Genes have sometimes been regarded as nodes in a network, with inputs being proteins such as transcription factors, and outputs being the level of gene expression. The node itself performs a function, and the operation of these functions have been interpreted as performing a kind of information processing within cell and determine cellular behaviour.
Techniques
- Primer: Used to facilitate expression
- Shuttle Vector
