1. 1
   A human gene is transcribed into a primary transcript (pre-mRNA) 8000 nucleotides long. The gene has 5 exons with a combined length of 1200 nucleotides; the rest of the transcript consists of introns. Before the mRNA leaves the nucleus, a modified guanine nucleotide cap is added to its 5' end and a tail of about 200 adenine nucleotides is added to its 3' end. In cells treated with a drug that blocks addition of the adenine tail, the mRNA is broken down in the cytoplasm within minutes, instead of lasting several hours.

   (a)

   Which parts of the primary transcript are removed before the mRNA leaves the nucleus?

   [1 mark]

   • AThe exons
   • BThe 5' cap and the adenine tail
   • CThe introns
   • DThe promoter

   (b)

   What do the results with the drug show about the function of the adenine (polyA) tail?

   [1 mark]

   • AIt stabilises the mRNA so it is not broken down quickly in the cytoplasm
   • BIt is translated into lysine residues at the end of the polypeptide
   • CIt is the site where RNA polymerase binds to start transcription
   • DIt is the site where introns are removed

   (c)

   Calculate the percentage of the primary transcript that is removed as introns, and outline what happens to the exons.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Researchers tested three compounds on a cell-free system containing mRNA, ribosomal subunits, tRNAs carrying amino acids, and the enzymes needed for translation. Compound X prevents the large ribosomal subunit from joining the small subunit once it has reached the start codon. Compound Y blocks the A site of the ribosome. Compound Z allows one peptide bond to form but then prevents the ribosome from moving along the mRNA.

   (a)

   During elongation, in which site of the ribosome is the tRNA that holds the growing polypeptide chain before the next peptide bond forms?

   [1 mark]

   • AThe A site
   • BThe P site
   • CThe E site
   • DThe mRNA-binding site of the small subunit

   (b)

   Which sequence of events occurs in the initiation of translation?

   [1 mark]

   • ASmall subunit binds directly to the start codon → moves to the 5' end → large subunit attaches
   • BLarge subunit binds to the mRNA → small subunit attaches → initiator tRNA pairs with the start codon
   • CInitiator tRNA binds to the 3' end of the mRNA → small subunit moves to the start codon → large subunit attaches
   • DSmall subunit binds to the 5' end of the mRNA → moves along to the start codon → initiator tRNA pairs with the start codon → large subunit attaches

   (c)

   Explain why compound Z stops elongation after only one peptide bond has formed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In pancreatic β-cells the insulin mRNA is translated into pre-proinsulin, a polypeptide of 110 amino acids. A signal sequence of 24 amino acids at one end is removed as the polypeptide enters the rough endoplasmic reticulum, giving proinsulin, which folds and forms disulfide bonds. In the Golgi apparatus and secretory vesicles a central section of 35 amino acids, which includes the C-peptide, is cut out, leaving an A chain of 21 amino acids and a B chain of 30 amino acids held together by disulfide bonds. C-peptide is secreted with insulin, one molecule of C-peptide for every molecule of insulin. Insulin used for injections is made without C-peptide. A patient who injects insulin has a high blood insulin concentration but a very low blood C-peptide concentration.

   (a)

   Calculate the total number of amino acids removed in converting pre-proinsulin into insulin, and outline the two stages of this modification.

   [3 marks]

   (b)

   Deduce what the data suggest about the patient's own insulin production, explaining your reasoning.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The human genome has about 3 billion base pairs, but only about 1.5% of it codes for polypeptides, in roughly 20 000 genes. Human cells nevertheless produce well over 100 000 different polypeptides. Much of the rest of the genome consists of introns, regulatory sequences such as promoters, telomeres at the ends of chromosomes, and genes that are transcribed to make rRNA and tRNA. In a typical human cell, proteasomes break down thousands of protein molecules every minute and the amino acids are reused; if proteasomes are inhibited by a drug, damaged and unneeded proteins accumulate and the cell soon dies.

   (a)

   Explain how transcription of a protein-coding gene is initiated in a eukaryotic cell, and how the transcript is processed and translated to produce a polypeptide.

   [6 marks]

   (b)

   Using the information, discuss the claim that the proteome of a human cell is determined only by the number of protein-coding genes in its genome.

   [6 marks]

   Total for question 4: 12 marks

End of questions