Final answer:
A codon is a sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid in protein synthesis. Codons are part of the genetic code, which is universal and degenerate. For example, the amino acid leucine has several codons like CUG and CUU.
Step-by-step explanation:
A codon is defined as a consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid during protein synthesis. This is part of the genetic code, which is a universal code that utilizes three-base codons to encode genetic instructions for the amino acid sequences of proteins.
Each group of three nitrogen bases in nucleic acids makes up a 'word' of the genetic code. For instance, the codon for leucine (Leu) can be represented by several codons such as CUG, CUU, CUC, CUA, UUA, and UUG. This is due to the degenerate nature of the genetic code, where more than one codon can specify the same amino acid. The code is read in a 5' to 3' direction, and specific codons such as AUG are known to initiate protein synthesis.
Moreover, there are codons that don't specify any amino acid and serve as termination codons or stop codons; these are UAA, UAG, and UGA. When these codons are encountered during protein synthesis, the process stops. This system ensures that proteins are synthesized with a precise primary structure as encoded by the gene.