A codon is a fundamental unit of the genetic code, defined as a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or a stop signal during the process of protein synthesis.
The Genetic Code: A Language Bridge
Life's essential processes rely on a crucial translation: converting the information stored in nucleic acids (DNA and RNA) into the diverse functions performed by proteins. While DNA and RNA molecules are built from a language of only four distinct nucleotide bases (Adenine (A), Cytosine (C), Guanine (G), and Thymine (T) in DNA, or Uracil (U) in RNA), the language of proteins encompasses 20 different amino acids. Codons serve as the dictionary that bridges these two languages.
Because there are four possible nucleotides at each of the three positions in a codon, there are 4 x 4 x 4 = 64 possible unique codons. This number is more than sufficient to code for the 20 amino acids and the necessary stop signals.
Key Codons: Start and Stop Signals
Among the 64 possible codons, several hold special significance in regulating protein synthesis:
- Start Codon: The codon AUG is universally recognized as the start codon. It signals the beginning of protein synthesis and also codes for the amino acid methionine (or N-formylmethionine in prokaryotes).
- Stop Codons: Three codons—UAA, UAG, and UGA—are known as stop codons (or nonsense codons). They do not code for any amino acid but instead signal the termination of protein synthesis, indicating that the polypeptide chain is complete.
Degeneracy of the Genetic Code
A notable feature of the genetic code is its degeneracy, also known as redundancy. This means that more than one codon can specify the same amino acid. For example, the amino acid Leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This degeneracy provides a layer of protection against mutations, as a change in a single nucleotide within a codon might still result in the same amino acid, thus preventing a change in the protein's structure or function.
Here are a few examples of codons and their corresponding amino acids or signals (using mRNA codons):
| Codon (mRNA) | Amino Acid / Signal | Function / Note |
|---|---|---|
| AUG | Methionine | Start codon |
| UUA | Leucine | Example of degeneracy |
| UCA | Serine | |
| GGU | Glycine | Example of degeneracy |
| UAA | Stop | Terminates synthesis |
Codons in Action: Protein Synthesis
The journey from a codon to a protein primarily involves two types of RNA:
- Messenger RNA (mRNA): During gene expression, the DNA sequence of a gene is transcribed into an mRNA molecule. The mRNA carries the genetic information in the form of a sequence of codons from the nucleus to the ribosomes in the cytoplasm.
- Transfer RNA (tRNA): At the ribosome, each mRNA codon is "read." A specific type of RNA molecule called transfer RNA (tRNA) has an anticodon loop that is complementary to an mRNA codon. Each tRNA molecule also carries a specific amino acid. When a tRNA's anticodon matches an mRNA codon, the amino acid it carries is added to the growing protein chain.
This precise interaction, dictated by the codon sequence, ensures that proteins are assembled with the correct sequence of amino acids, which is critical for their proper folding and biological function.
