8.1 What is gene expression?

Gene expression is a term used to describe the contribution of a gene to the overall functions and phenotype of a cell through the activity of the molecular products, which are encoded in the specific nucleotide sequence of the gene. RNA is the primary product encoded in a gene, which is transcribed in the nucleus of a cell. A class of RNA molecules, messenger RNAs, are transported from the nucleus to the cytoplasm, where the translation machinery of the cell translates the nucleotide sequence of the mRNA into proteins. The functional protein repertoire in a given cell is the primary factor that dictates the shape, function, and phenotype of a cell. Due to the prime roles of proteins for a cell’s fate, most molecular biology literature is focused on protein-coding genes. However, a bigger proportion of a eukaryotic gene repertoire is reserved for non-coding genes, which code for RNA molecules that are not translated into proteins, yet carry out many important cellular functions. All in all, the term gene expression refers to the combined activity of protein-coding or non-coding products of a gene.

In a cell, there are many layers of quality controls and modifications that act upon a gene’s product until the end-product attains a particular function. These layers of regulation include epigenetic, transcriptional, post-transcriptional, translational, and post-translational control mechanisms, the latter two applying only to protein-coding genes. A protein or RNA molecule, is only functional if it is produced at the right time, at the right cellular compartment, with the necessary base or amino-acid modifications, with the correct secondary/tertiary structure (or unstructure wherever applicable), among the availability of other metabolites or molecules, which are needed to form complexes to altogether accomplish a certain cellular function. However, traditionally, the number of copies of a gene’s products is considered a quantitative measure of a gene’s activity. Although this approach does not reflect all of the complexity that defines a functional molecule, quantification of the abundance of transcripts from a gene has proven to be a cost-effective method of understanding genes’ functions.