Which of the following describes post-translational modification?

Post-translational modification refers to the biochemical modifications that occur to a polypeptide chain after its translation into protein from messenger RNA (mRNA). These modifications can include a variety of processes such as phosphorylation, glycosylation, ubiquitination, and acetylation, among others.

These alterations are crucial for the proper functioning of proteins, as they can impact protein activity, localization, stability, and interactions with other molecules. For example, the addition of phosphate groups can activate or deactivate enzymes, while glycosylation can play a role in protein folding and stability.

The other options refer to processes that occur at different stages of the biological flow of information. The addition of nucleotides to RNA strands pertains to RNA processing during transcription, regulation of gene expression at the transcription level involves controlling when and how genes are transcribed into RNA, and the formation of ribosomal RNA is part of the ribosome assembly process, which occurs independently of the modifications seen after proteins have been synthesized. Therefore, the definition highlighted in option B accurately reflects the characteristics and importance of post-translational modifications in cellular biology.