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Protein Synthesis · Paper 2 — Free Biology SL Practice Questions

1MasterySAQ-SRole of mRNA, tRNA, and ribosomes7 marksPaper 2~11 min
The below shows a ribosome during translation in a prokaryotic cell. The mRNA sequence is 5′-AUGGCUAAC-3′5'\text{-AUGGCUAAC-}3'. A large and small ribosomal subunit are shown with mRNA threaded between them. Three tRNA binding sites are visible: the E site (empty), the P site (occupied by a tRNA with anticodon 3′-UAC-5′3'\text{-UAC-}5' carrying methionine and a growing peptide), and the A site (approached by a tRNA carrying alanine). The codons are 5′-AUG-3′5'\text{-AUG-}3', 5′-GCU-3′5'\text{-GCU-}3', and 5′-AAC-3′5'\text{-AAC-}3'.
(a)
State the anticodon sequence, written 3′→5′3' \to 5', of the tRNA that binds to the third codon AAC. [1 mark]
(b)
Describe how the structure of tRNA enables it to carry a specific amino acid and to recognise a specific mRNA codon. [2 marks]
(c)
The third codon is 5′-AAC-3′5'\text{-AAC-}3'. Calculate the total number of hydrogen bonds formed between this codon and its complementary anticodon, using the rules A–U=2\text{A–U} = 2 bonds and G–C=3\text{G–C} = 3 bonds. [2 marks]
(d)
The wobble hypothesis states that the third base of anticodon can form non-Watson–Crick base pairs with the third base of a codon. Explain one advantage this gives to a cell during translation. [2 marks]
diagram

Solutions

2ChallengeSAQ-LTranscription and translation processes7 marksPaper 2~11 min
A study was conducted on the expression of the β\beta-globin gene in human erythroblast precursor cells. Researchers isolated mRNA from these cells and used it as a template for complementary DNA (cDNA) synthesis. The cDNA was then sequenced. The following data were obtained from the coding strand of the β\beta-globin gene and the corresponding mature mRNA transcript. - Length of β\beta-globin gene (coding strand): 1600 bp1600\,\text{bp} - Length of β\beta-globin mature mRNA (after processing): 626 nt626\,\text{nt} - Number of amino acids in the final β\beta-globin polypeptide: 146146 - The gene contains three exons and two introns.
(a)
Calculate the number of nucleotides in the combined introns of the β\beta-globin gene. Show your working. [2 marks]
(b)
Explain why the number of nucleotides in the mature mRNA (626 nt626\,\text{nt}) is greater than three times the number of amino acids in the polypeptide. [2 marks]
(c)
Evaluate whether the data support the conclusion that all exonic sequences in the β\beta-globin gene are translated into the polypeptide. [3 marks]
diagram

Solutions

3ChallengeSAQ-LTranscription and translation processes7 marksPaper 2~11 min
In an investigation of translation efficiency, researchers used an in vitro translation system containing ribosomes, tRNAs, amino acids, ATP, and GTP. They added a synthetic mRNA with the sequence 5′-AUG GUA CCC UAA GCA UGG-3′5'\text{-AUG GUA CCC UAA GCA UGG-}3'.
(a)
Determine the amino acid sequence of the polypeptide produced from this mRNA. Use three-letter amino acid abbreviations and show your codon assignments. [3 marks]
(b)
Explain the molecular events at the ribosome that prevent the codons GCA and UGG from being translated. [2 marks]
(c)
A mutation in a tumour suppressor gene introduces a premature stop codon near the middle of the coding sequence. Evaluate whether the resulting truncated protein is likely to be functional, referring to protein structure and function. [2 marks]
diagram

Solutions

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