Final answer:
A codon is a sequence of three nucleotides in mRNA that pairs with the tRNA's anticodon during protein synthesis to specify an amino acid's placement. There are 64 possible codons, with AUG also marking the start of translation.
Step-by-step explanation:
The specific three-nucleotide sequence in mRNA that binds to the anticodon of tRNA to signify the placement of an amino acid in a protein is called a codon. During the process of translation, each codon within the mRNA corresponds to a specific amino acid or a termination signal in the protein synthesis process. There are 64 possible mRNA codons—formed by combinations of the nucleotide bases adenine (A), uracil (U), guanine (G), and cytosine (C). Of these codons, three specify the termination of protein synthesis, and 61 are responsible for the addition of amino acids to the polypeptide chain. Importantly, the codon AUG acts as the start signal for protein synthesis, as well as coding for the amino acid methionine.
The genetic code is described as being degenerate because multiple codons can specify the same amino acid. The first two bases of the codon are usually the most important, with mutations in the third base often still allowing for the correct amino acid to be incorporated, due to what is known as the wobble effect. This phenomenon contributes to the resilience of the genetic code against point mutations.