Post-translational modification and degradation
Post-translational modification is the alteration of proteins and polypeptides after they have been synthesised.
In many cases, the polypeptide chain (amino acid sequence) produced after translation is not yet a functional protein. It must undergo further modification.
Post-translational modifications include adding functional groups, folding and covalently modifying the protein.
Protein degradation (the breakdown of proteins) is also involved in gene expression. If a certain type of protein becomes too prevalent, digestive enzymes are able to break them down.
Degradation can reduce the levels of certain proteins in cells if the environment changes, or if proteins become old or damaged.
All of these processes are part of the control of gene expression. They ensure that only the required amount of protein is present within the cell.