Gene expression, also called protein expression or often simply expression is the process by which a gene's DNA sequence is converted into the structures and functions of a cell.
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.
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 quantitated using standard techniques.
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.
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