Amino acid — Science definition

One of the 20 building blocks that link together in a specific order to form a protein.

Where it is taught

In use

Most sense codons specify an amino acid — one link in the chain — while stop codons signal termination rather than adding an amino acid, and as the ribosome moves down the message, it links the amino acids together in exactly the order the codons dictate.

Protein Synthesis

Questions using amino acid

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

Different sequence changes can have different downstream consequencesDifferent sequence changes can have different downstream consequencesOriginal: AUG GCA UAA → Met–Ala–stopSubstitution: AUG GCU UAA → Met–Ala–stopAnother change: AUG GUA UAA → Met–Val–stopAn insertion can shift downstream reading-frame groups.A sequence change and a functional effect are different claims.
Original supplied-code sequence comparisons, not pathogenic-variant identification or the full range of molecular effects.
Interpret the supplied visual. Why does GCA→GCU not change this amino acid, while GCA→GUA does?
Answer and explanation

The supplied code maps GCA/GCU to alanine and GUA to valine; different codons can share or differ in meaning.

A substitution changes a base; an insertion or deletion changes sequence length. A reading frame groups consecutive bases into codons, so a non-three-base insertion within a translated region can alter downstream grouping. Code redundancy allows a substitution to retain the same amino acid, as GCA and GCU both specify alanine in the supplied entries. Other changes alter an amino acid or termination. Effects depend on location, regulation and protein context; not every mutation is visible or every substitution harmless.

The supplied code maps GCA/GCU to alanine and GUA to valine; different codons can share or differ in meaning.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

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