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The genetic code and protein synthesisEdexcel International A Level Biology: Revision notes

Section 1

The genetic code

A gene is a sequence of bases in DNA that codes for the sequence of amino acids in a polypeptide.

The genetic code has three features:

  • Triplet: a sequence of three bases, a codon, codes for one amino acid. Four bases taken three at a time give 43=644^3 = 64 combinations, enough for 20 amino acids.
  • Non-overlapping: each base is part of only one codon, so the code is read in separate groups of three.
  • Degenerate: there are 64 codons but only 20 amino acids, so most amino acids are coded for by more than one codon.

AUG is the start codon (it also codes for methionine). UAA, UAG and UGA are stop codons and do not code for an amino acid.

Key termsgenecodondegeneratenon-overlappingstop codon
Exam tip

For a polypeptide of nn amino acids, the mRNA read by the ribosome has 3n3n bases, plus 3 more for the stop codon.

Section 2

Template strand, codons and anticodons

Only one DNA strand is copied in transcription. This is the template (antisense) strand. The other strand, the coding (sense) strand, has the same base sequence as the mRNA, except that it has T where the mRNA has U.

The mRNA is complementary to the template strand. Its triplets are codons. A tRNA molecule has an anticodon, a triplet of bases complementary to a codon, and carries the amino acid that codon codes for.

Key termstemplate (antisense) strandanticodon
Common mistake

Do not write thymine in mRNA or in an anticodon. RNA contains uracil, so A in the template pairs with U.

Section 3

Transcription

Transcription makes mRNA from a gene.

  1. RNA polymerase attaches to the gene and breaks the hydrogen bonds between the bases, unwinding the DNA.
  2. Only the template strand is used. Free RNA nucleotides pair with the exposed bases by complementary base pairing (A with U, T with A, C with G, G with C).
  3. RNA polymerase joins the adjacent RNA nucleotides by phosphodiester bonds.
  4. The single-stranded mRNA separates from the DNA, which rewinds. The mRNA leaves the nucleus through a nuclear pore.
Key termstranscriptionRNA polymerasemRNA

Section 4

Translation

Translation makes a polypeptide at a ribosome.

  1. The ribosome binds to the mRNA at the start codon AUG.
  2. A tRNA with the complementary anticodon, carrying its specific amino acid, pairs with the codon by complementary base pairing. A second tRNA pairs with the next codon.
  3. A peptide bond forms between the two amino acids in a condensation reaction, and the first tRNA is released.
  4. The ribosome moves along the mRNA one codon at a time.
  5. At a stop codon there is no tRNA, so translation ends and the polypeptide is released.
Key termstranslationribosometRNApeptide bond
Exam tip

The anticodon matches the codon, not the DNA. Work from the mRNA codon to its complementary anticodon.

Section 5

Worked example

A template strand reads TACCAGATT.

  • mRNA: AUG GUC UAA
  • Anticodons: UAC (pairs with AUG) and CAG (pairs with GUC)
  • AUG codes for methionine and GUC for valine. UAA is a stop codon.
  • The product is a dipeptide (two amino acids).

To go from template to mRNA, change each base to its complement and use U for A's partner. To go from mRNA to anticodon, use the complement again.

Must know

  • The code is triplet, non-overlapping and degenerate
  • Start codon AUG; stop codons UAA, UAG, UGA
  • Transcription: RNA polymerase unwinds the gene and joins RNA nucleotides complementary to the template strand
  • Translation: ribosome, mRNA codons, tRNA anticodons, peptide bonds
  • A polypeptide of nn amino acids needs 3n3n bases plus a stop codon

That's the notes covered.

Carry on to the next subtopic.

Exam questions on The genetic code and protein synthesis

  1. The template (antisense) strand of a short section of a gene has the base sequence TACGGATTC. The section is transcribed into messenger RNA.
    The codon CCU in the mRNA codes for the amino acid proline. A mutation in the gene changes this codon to CCC, which also codes for proline. Explain why the polypeptide is unchanged.2 marks
  2. A ribosome is translating a short mRNA with the codon sequence AUG GCU UUC UAA. Transfer RNA molecules, each carrying a specific amino acid, deliver the amino acids to the ribosome.
    State the number of amino acids in the polypeptide made from this mRNA and explain your answer.2 marks
  3. A student is studying a bacterial gene whose polypeptide product has 150 amino acids, including the first, methionine. The student is also learning why the genetic code uses sequences of three bases.
    Calculate the minimum number of bases in the mRNA that is read by the ribosome to make this polypeptide. Explain your answer.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).