What defines a codon in the context of genetics?

A codon is defined as a specific sequence of three nucleotides in the genetic code that corresponds to a particular amino acid or signals the termination of protein synthesis. This three-nucleotide sequence is crucial during translation, the process in which ribosomes synthesize proteins based on the instructions encoded in mRNA (messenger RNA). Each codon in the mRNA matches with a corresponding tRNA (transfer RNA) molecule that carries the specific amino acid, ensuring that the correct amino acids are incorporated into the growing polypeptide chain in the proper order.

In contrast, the other options describe different aspects of molecular genetics. While a sequence of amino acids constitutes a protein, it is not what defines a codon. Similarly, a segment of DNA that codes for a genetic trait may encompass one or more codons but is not limited to the three-nucleotide structure specific to codons. Lastly, certain types of RNA play various roles in gene regulation, but they do not define what a codon is. Thus, the defining characteristic of a codon is its role as a three-nucleotide sequence linked to a specific amino acid.