What Is a Codon?
A codon is a sequence of three consecutive nucleotides in a DNA or mRNA molecule that corresponds to a specific amino acid or a stop signal. Codons are the fundamental units of the genetic code, acting like three-letter 'words' that cells read to build proteins.
Section 2: The Genetic Code and Redundancy
The genetic code is read in these non-overlapping, three-nucleotide codons. Since there are four different bases in mRNA (Adenine, Uracil, Guanine, Cytosine), there are 64 (4³) possible codons. However, there are only 20 common amino acids. This means the code is 'degenerate' or redundant, with multiple codons often specifying the same amino acid. Special codons, like AUG (start) and UAA/UAG/UGA (stop), signal the beginning and end of protein synthesis.
Section 3: A Practical Example of Codons
Consider a segment of an mRNA molecule with the sequence: AUG-UUC-GCA. The ribosome reads this sequence one codon at a time. The first codon, AUG, signals the start and codes for the amino acid Methionine. The second codon, UUC, codes for Phenylalanine. The third codon, GCA, codes for Alanine. This process continues until a stop codon is reached, resulting in a specific protein chain.
Section 4: Why Are Codons Important?
Codons are critically important because they form the basis of translation—the process of converting the genetic information stored in nucleic acids (DNA and RNA) into the functional proteins that carry out virtually all cellular tasks. Any change or mutation within a codon can lead to the insertion of a wrong amino acid or premature termination, potentially causing a non-functional protein and leading to genetic diseases.