The Genetic Code & Translation

Overview of Gene Expression: DNA → RNA → Protein

Gene expression requires two sequential information-conversion events.
- Transcription: information is copied from the 4-letter nucleotide language of DNA to the same 4-letter nucleotide language of RNA.
- Translation: information is converted from the 4-letter nucleotide language of mRNA to the 20-letter amino-acid language of proteins.
Conceptual flow is often summarized as DNA → RNA → Protein.

Messenger RNA (mRNA) is the final product of transcription.
After transcription:
- The mRNA “message” is read in three-letter words called codons.
- Each codon can have one of 4 possible bases in each position, giving 4 \times 4 \times 4 = 64 total codons.
Functional meaning of codons:
- Each codon either
- codes for a specific amino acid, or
- serves as a punctuation signal (START or STOP) during translation.
Redundancy (degeneracy):
- Only 20 amino acids exist, so multiple codons can specify the same amino acid.

Breakdown of the 64 codons:
- 61 codons → specify amino acids.
- 3 codons → STOP signals; they do not encode an amino acid but terminate translation.
- 1 codon (AUG) → START signal; also codes for the amino acid methionine.
- Because of this, methionine is always the first amino acid in a newly synthesized polypeptide.
The same codon table applies to all known life forms; hence the term “universal” genetic code.

Universality implies that all living organisms—from viruses and bacteria to plants and animals—share a common ancestor that used the same code.
The code’s persistence for billions of years suggests its early establishment in the first cell (or even pre-cellular life).
Hypothetical alternative codes appear to be either
- non-existent in nature, or
- represented only by minor, rare, and biologically insignificant variations.
The near-invariance of the code supports evolutionary theory by providing molecular evidence for shared heritage across all domains of life.