Codon — Science definition

A group of three mRNA bases that codes for one specific amino acid (or a start/stop signal).

Where it is taught

In use

The ribosome reads the mRNA three bases at a time, and each three-base 'word' is called a codon.

Protein Synthesis

Questions using codon

Problem 1

Each amino acid is coded by a codon of 3 mRNA bases. How many mRNA bases are needed to code for a protein that is 15 amino acids long?
Answer and explanation

45 bases

3 bases per codon x 15 amino acids = 45 bases.

Problem 2

A single-base mutation changes a codon, but because the genetic code is redundant, the new codon still specifies the same amino acid. This kind of mutation is described as
  1. nonsense — it creates an early stop signal
  2. a frameshift that shifts every later codon
  3. always fatal to the cell
  4. silent — the protein is unchanged
Answer and explanation

silent — the protein is unchanged

When a changed codon still codes for the same amino acid, the protein is unchanged, so the mutation is 'silent' — possible because several codons can specify the same amino acid.

Problem 3

An RNA codon changes but still specifies the same amino acid. What kind of protein-sequence effect is possible? Show the reasoning, not just the conclusion.
Hints

Specify template versus coding strand before converting a sequence.

Answer and explanation

It can be synonymous, leaving that amino acid unchanged; other regulatory effects are still possible.

Problem 4

Why must a codon table be read using RNA rather than a DNA template triplet directly?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Translation reads mRNA; the template must first be converted to complementary RNA with correct orientation.

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