Gene expression is the process by which genetic instructions are used to synthesise gene products. Measuring this gene expression is a key element in the study of life sciences. For example, understanding the level of gene expression in a cell, tissue or organism can provide valuable information in terms of identifying viral infection, determining cancer susceptibility or testing whether a bacteria is resistant to penicillin.
The significance of these applications is described in more detail below:
Identifying viral infection
When a virus infects a human cell, it successfully incorporates its DNA or RNA into the genome and forces the cell to manufacture viral proteins through expression of the viral genes. Measuring this gene expression can aid understanding of the viral mechanism as well as providing a useful tool in the development of vaccines.
Testing penicillin resistance
Antibiotics can become ineffective if bacteria develop ways of destroying the antibiotic before it can kill the bacteria. For example, many bacteria produce an enzyme called penicillinase that can deactivate the antibiotic penicillin. Measuring the expression of penicillinase genes can help to predict whether a particular bacteria would be responsive or resistant to penicillin.
Measuring the expression level of cancer-causing genes (oncogenes) can help to determine a person’s susceptibility to cancer. For example, tumor suppressor genes code for a protein that can prevent the development of tumors. When these genes are under-expressed, the cancer risk is increased.
Gene expression measurement is usually achieved by quantifying levels of the gene product, which is often a protein. Two common techniques used for protein quantification include Western blotting and enzyme-linked immunosorbent assay or ELISA. However, the gene expression level can also be inferred by measuring the level of mRNA, which is achieved using a technique called Northern blotting.
Another technique for measuring mRNA is reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR). Here, a DNA template is made from mRNA using reverse transcription. This template, which is called cDNA (complimentary DNA) is then amplified. As the DNA amplification proceeds, hybridization probes emit changing levels of fluorescence, which can be used to measure the original number of mRNA copies.