Final answer:
A codon is a sequence of three nucleotides that specifies a particular amino acid in the synthesis of proteins, found in either DNA or RNA.
Step-by-step explanation:
The structure or makeup of a codon is best described as a sequence of three nucleotides. These sequences are found in DNA or RNA, which constitute the genetic code that dictates the synthesis of proteins. A codon is read from a 5' to 3' direction from mRNA. Each triplet codon corresponds to a specific amino acid during the process of translation, where the mRNA is used to direct the protein synthesis. In summary, a codon is a set of three consecutive bases in mRNA that specifies a particular amino acid, and this specification is universal across almost all organisms.