The genetic code is the rulebook cells use to turn information in nucleic acids into proteins. DNA stores the instructions, messenger RNA carries a working copy, and ribosomes read the message three bases at a time. Each three-base RNA word is a codon that usually specifies one amino acid.
Understanding codons helps explain how genes produce traits, how mutations can change proteins, and how biotechnology can design useful sequences.
A codon table shows how the 64 possible RNA codons match 20 amino acids plus start and stop signals. Translation begins at the start codon AUG, proceeds along the mRNA from 5' to 3', and stops when the ribosome reaches UAA, UAG, or UGA. Because several codons can code for the same amino acid, the genetic code is redundant, which can reduce the effect of some mutations.
To translate a sequence, convert DNA to mRNA if needed, find the first AUG in the correct reading frame, then group bases into codons and use the table to build the amino acid chain.
Key Facts
- A codon is a sequence of 3 RNA bases that codes for an amino acid or a stop signal.
- There are 4 RNA bases and 3 positions per codon, so 4^3 = 64 possible codons.
- AUG is the start codon and codes for methionine, written Met or M.
- The stop codons are UAA, UAG, and UGA, and they do not code for an amino acid.
- Translation reads mRNA in the 5' to 3' direction in non-overlapping codons.
- For a coding DNA strand, mRNA has the same sequence except U replaces T, so DNA 5'-ATG-3' becomes mRNA 5'-AUG-3'.
Vocabulary
- Genetic code
- The set of rules that matches RNA codons to amino acids and stop signals during protein synthesis.
- Codon
- A three-nucleotide sequence in mRNA that specifies one amino acid or a translation stop signal.
- Anticodon
- A three-nucleotide sequence on tRNA that pairs with a complementary mRNA codon.
- Reading frame
- The way an mRNA sequence is divided into groups of three bases starting from a chosen position.
- Redundancy
- The feature of the genetic code in which more than one codon can code for the same amino acid.
Common Mistakes to Avoid
- Reading the codon table with DNA letters instead of RNA letters is wrong because codon tables use U, not T.
- Starting translation at the first base of a sequence is wrong if the first start codon AUG occurs later in the correct reading frame.
- Changing the reading frame after finding AUG is wrong because ribosomes read codons in fixed groups of three after initiation.
- Treating stop codons as amino acids is wrong because UAA, UAG, and UGA signal the ribosome to end translation.
Practice Questions
- 1 Translate the mRNA sequence 5'-AUG GCU UUU UGA-3' into amino acids using a codon table.
- 2 A coding DNA sequence is 5'-ATG CCG TTA TAA-3'. Write the mRNA sequence and identify the amino acid sequence up to the stop codon.
- 3 A mutation changes the mRNA codon GAA to GAG, and both code for glutamic acid. Explain why the protein sequence does not change and what this shows about the genetic code.