Restriction enzymes, one of many tools of the molecular biology toolbox, were introduced in the previous chapter. The present chapter is devoted to another important method designed to examine the function of specific genes.

Interfering with gene expression

In order to study the biological function of a gene, the molecular biologist needs methods that allow alteration of that gene – for example, an alteration that reduces the production of the protein specified by the gene. The behaviour of a normal cell may then be compared to a cell where the gene of interest has been manipulated, thereby allowing a conclusion regarding the function of the gene of interest. In the yeast Saccharomyces cerevisiae, for instance, there are methods that allow the complete removal of genes (such a removal is often referred to as a gene knock-out ). If, for instance, the yeast gene named PSY3 is removed or inactivated, it gives rise to an increased level of mutations. This observation suggests to us that this particular gene is related to the repair of DNA damage.

For studies of human genes, you may want to examine a species more closely related to man than is yeast. You can delete or inactivate genes in mammals such as mice, but this is more technically involved than in yeast. On the other hand, there are other methods that make possible a reduction in gene expression in animals. One such important method involves RNA interference (RNAi) (also known as RNA silencing). In one common type of experiment, an mRNA is inactivated by the introduction of a small synthetic RNA which is complementary to the mRNA. In the bioinformatics example below we will see how we can design a small RNA for such an experiment. RNAi is an example of a gene knock-down method, as the expression of the target gene is reduced (compared to the gene knock-out, where a specific gene is completely removed or inactivated).