The nucleotides of DNA are grouped into three-letter combinations known as what?

The correct answer is codons, which are three-letter combinations of nucleotides in DNA (or RNA) that correspond to specific amino acids or serve as stop signals during protein synthesis. Codons are crucial for the genetic code, as they dictate the sequence in which amino acids are added to a growing polypeptide chain, ultimately determining the structure and function of proteins.

The use of the term "triplets" is a common way to refer to the three nucleotide sequences, but in the specific context of genetic coding, "codons" is the precise terminology used to describe these functional units. Amino acids are the building blocks of proteins and are encoded by codons, but they are not the nucleotide sequences themselves. Similarly, genes are longer sequences of DNA that comprise multiple codons, encompassing the entire blueprint for a specific protein or functional RNA. Therefore, while triplets might describe the grouping of nucleotides, codons specifically refer to those groups that have a direct role in coding for proteins, which makes "codons" the most accurate answer in this context.