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The genetic code and transcription questions

The genetic code as triplet, degenerate, non-overlapping and near-universal, with the consequence of each property; genes, introns and exons; transcription by RNA polymerase step by step; the splicing of pre-mRNA into mature mRNA; and why degeneracy makes many base substitutions silent.

5 original questions · 18 marks · the genetic code and transcription notes · Nucleic acids, genomes and protein synthesis

Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the points are much easier to see when you have something of your own to compare against.

• 5 marks · Describe

  Describe how a molecule of pre-mRNA is produced from a gene in the nucleus of a eukaryotic cell.

  Mark scheme

  1. B1 RNA polymerase binds to a promoter region at the start of the gene
  2. B1 the enzyme unwinds a short section of the double helix, breaking the hydrogen bonds between the paired bases
  3. B1 only one of the two exposed strands, the template strand, is read
  4. B1 free RNA nucleotides align against the template by complementary base pairing, with uracil pairing to adenine wherever thymine would have gone
  5. B1 RNA polymerase joins the aligned nucleotides by condensation into phosphodiester bonds, moving along the gene until it reaches a stop signal and releases the pre-mRNA

• 4 marks · Explain

  Explain why a mature mRNA molecule is shorter than the gene it was transcribed from, and explain how one gene can give rise to more than one polypeptide.

  Mark scheme

  1. B1 the pre-mRNA is a copy of the whole gene, introns included
  2. B1 spliceosomes cut the introns out and join the exons together, so the introns are not represented in the mature mRNA
  3. B1 in alternative splicing, different combinations of exons are retained in the mature mRNA
  4. B1 so several different mRNA molecules, and therefore several different polypeptides, can come from the same gene

• 4 marks · Suggest

  A single base is inserted into the coding sequence of a gene close to its start. The polypeptide produced is much shorter than normal and its amino acid sequence bears no relation to the original beyond the point of insertion. Suggest why, referring to two properties of the genetic code.

  Mark scheme

  1. B1 the code is read in threes from a fixed starting point and is non-overlapping, so each base belongs to one triplet only
  2. B1 inserting a base pushes every subsequent base one place along, so every triplet after the insertion is read differently, which is a frame shift
  3. B1 the amino acid sequence after that point is therefore completely altered rather than altered in one place
  4. B1 a shifted reading frame produces stop codons at positions where there were none, so translation ends early and the polypeptide is short

• 3 marks · Give

  The coding strand of a short section of a gene reads GAT CCA TTG. Give the base sequence of the template strand, and give the base sequence of the mRNA transcribed from this section.

• 2 marks · State

  State what is meant by saying that the genetic code is degenerate, and state what is meant by saying that it is non-overlapping.

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