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Learn7402Paper 1 · Paper 1: Topics 1–4Genetic information, variation and relationships between organismsTranscription and translation
Genetic information, variation and relationships between organisms · Subtopic 3 of 3

Transcription and Translation

A gene is a sequence of DNA bases coding for a polypeptide. The code is triplet (3 bases = 1 amino acid), degenerate (most amino acids have several codons), non-overlapping and universal. Transcription (nucleus): RNA polymerase reads the template strand, building pre-mRNA; introns are spliced out, leaving exons. Translation (ribosome): mRNA codons pair with anticodons on tRNA carrying specific amino acids; peptide bonds form; the ribosome moves along until a stop codon. Gene mutations: substitution (may be silent, missense or nonsense), insertion or deletion (frameshift, usually serious). Sickle cell: one base substitution changes glutamic acid to valine in the β chain.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe features of the genetic code.
  • Describe transcription and splicing.
  • Describe translation.
  • Explain effects of different mutations.
Pause and think

Why does a single base deletion usually have a bigger effect than a single base substitution?

Section 2 of 14

Core idea

Start hereFrom gene to protein

A gene is just a sequence of bases. Transcription and translation turn it into a working protein.

The big idea

DNA is transcribed into mRNA in the nucleus; ribosomes translate mRNA codons into amino acids using tRNA.

Protein synthesis

Two stages, two places.

Worked exampleMinimum mRNA length

A polypeptide has 150 amino acids. Minimum mRNA bases including the stop codon?

Goal
Bases.
Strategy
3 bases per amino acid + stop.
  1. 150×3=450150\times3=450
    Coding codons.
  2. 450+3=453450+3=453
    Stop codon.
Answer453 bases.
✓ CheckIntrons already removed from mRNA ✓.
Common misconceptionFrameshift Insertions and deletions shift every following codon; substitutions change at most one.

Summary

  • Triplet code.
  • Transcription, splicing, translation.
  • Mutations.
Key words
Codon
Three-base sequence on mRNA.
Intron
Non-coding region removed from pre-mRNA.
HistoryCracking the code. Marshall Nirenberg showed in 1961 that poly-U RNA made a chain of phenylalanine: UUU was the first codon decoded.
Fun factThe code is (almost) universal. The same codons mean the same amino acids in bacteria, plants and humans, which is why bacteria can make human insulin.
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