What is a codon?

A codon is defined as a sequence of three nucleotide bases in messenger RNA (mRNA) that corresponds to a specific amino acid or a stop signal during the process of translation in protein synthesis. Each set of three nucleotides, or triplet, translates to one of the 20 different amino acids that make up proteins. This system of codons is essential for the genetic code, dictating how the sequence of nucleotides is converted into the sequence of amino acids in a protein.

The other options describe different concepts in molecular biology. The regulation of gene expression refers to mechanisms that control the timing and amount of a gene's expression. The ribosome structure includes ribosomal RNA and proteins, but does not define a codon. Lastly, while RNA plays a critical role in protein synthesis, not all types of RNA correspond directly to the definition of a codon; it is specifically mRNA that contains codons.