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Protein synthesis, the genetic code and gene mutationsEdexcel A-Level Biology B: Revision notes

Section 1

mRNA and tRNA

mRNA is a single-stranded polynucleotide: nucleotides (ribose, phosphate, base A, U, G or C) joined by phosphodiester bonds into a sugar-phosphate backbone. It is a short copy of one gene, so the DNA can stay in the nucleus. Hydrogen bonds form between mRNA codons and tRNA anticodons.

tRNA is a single strand folded into a cloverleaf, held in shape by hydrogen bonds between paired bases. One end binds a specific amino acid; a loop carries the anticodon, a triplet complementary to an mRNA codon. RNA contains uracil instead of thymine.

Key termsmRNAtRNAcodonanticodon
Common mistake

RNA has uracil, not thymine. Do not write T in an mRNA sequence.

Section 2

Transcription

Transcription happens in the nucleus.

  1. The hydrogen bonds between the DNA strands break and the helix unwinds at the gene.
  2. The anti-sense (template) strand is copied. The sense strand has the same base sequence as the mRNA (with T instead of U).
  3. RNA polymerase joins free RNA nucleotides to the template by complementary base pairing.
  4. The mRNA leaves the nucleus through a nuclear pore.

Worked example: anti-sense TAC GGA CTT → mRNA AUG CCU GAA.

Key termstranscriptionanti-sense strandsense strand
Exam tip

Write the mRNA by complementary pairing from the anti-sense strand: A→U, T→A, C→G, G→C.

Section 3

Translation

Translation happens at a ribosome.

  1. mRNA binds to the ribosome.
  2. A tRNA with the anticodon complementary to the first codon binds by hydrogen bonds, bringing its amino acid.
  3. A second tRNA binds the next codon and a peptide bond forms between the amino acids.
  4. The ribosome moves along the mRNA, one codon at a time, and the first tRNA is released.
  5. At a stop codon translation ends and the polypeptide is released.
Key termstranslationribosome

Section 4

The genetic code

  • Triplet code: three bases (one codon) code for one amino acid.
  • Degenerate: most amino acids have more than one codon.
  • Non-overlapping: each base is part of only one codon.
  • Start codon (AUG) begins translation; stop codons end it and code for no amino acid.
  • Not all of the genome codes for proteins: much of it is non-coding.

Worked example: a coding region of 900 nucleotides including a stop codon has 300 codons, so 299 amino acids.

Key termstriplet codedegeneratenon-overlappingstop codon

Section 5

Gene mutations and sickle cell anaemia

A gene mutation is a change in the base sequence of a gene. Types:

  • Substitution: one base replaced by another. It changes at most one amino acid, or none because the code is degenerate.
  • Deletion: a base is lost.
  • Insertion: a base is added.

Deletion or insertion of one or two bases causes a frameshift, changing every later codon, so the protein is usually non-functional.

Sickle cell anaemia: a substitution changes the sense triplet GAG to GTG, so mRNA GAG becomes GUG and valine replaces glutamic acid in β-globin. This changes the haemoglobin's bonding and shape, so it forms fibres at low oxygen concentration and red blood cells become sickle-shaped.

Key termsgene mutationsubstitutiondeletioninsertionframeshift
Common mistake

A substitution does not change the reading frame. Only insertions or deletions that are not multiples of three cause a frameshift.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Protein synthesis, the genetic code and gene mutations

  1. A short section of the anti-sense (template) strand of a gene has the base sequence TAC GGA CTT. This section is transcribed in the nucleus of a cell and the mRNA produced is translated at a ribosome.
    State the anticodon of the tRNA that binds to the first codon of the mRNA and explain how the tRNA ensures the correct amino acid is added.2 marks
  2. A patient has a genetic condition caused by a gene mutation within the coding region of a gene. Sequencing shows that a single nucleotide has been deleted close to the start of the coding region of the gene.
    Explain why the deletion is likely to have a more serious effect on the protein than a substitution of one base would.2 marks
  3. Sickle cell anaemia is caused by a mutation in the gene for the β-globin polypeptide of haemoglobin. In the normal allele a triplet on the sense strand of the DNA is GAG, which codes for the amino acid glutamic acid. In the sickle cell allele this triplet is GTG, which codes for the amino acid valine.
    Explain how this mutation leads to the production of a different haemoglobin protein.3 marks
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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).