Protein synthesis, the genetic code and gene mutationsEdexcel A-Level Biology B: Subtopic test

1. 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.

   (a)

   What is the base sequence of the mRNA transcribed from this section of the anti-sense strand?

   [1 mark]

   • AATG CCT GAA
   • BUAC GGA CUU
   • CAUG CCU GAA
   • DTAC GGA CTT

   (b)

   A different mRNA has a coding region of 1,200 nucleotides, including the stop codon. How many amino acids are in the polypeptide it codes for?

   [1 mark]

   • A1,200
   • B400
   • C401
   • D399

   (c)

   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]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which type of mutation is most likely to alter every amino acid after the point of the mutation?

   [1 mark]

   • ADeletion of a single base
   • BSubstitution of one base for another
   • CSubstitution that gives a codon for the same amino acid
   • DInsertion of a whole triplet of bases

   (b)

   Which feature of the genetic code explains why some base substitutions do not change the amino acid that is coded for?

   [1 mark]

   • AIt is non-overlapping
   • BIt is degenerate
   • CIt uses stop codons
   • DIt is triplet-based

   (c)

   Explain why the deletion is likely to have a more serious effect on the protein than a substitution of one base would.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Explain how this mutation leads to the production of a different haemoglobin protein.

   [3 marks]

   (b)

   Deduce the base sequences of the anti-sense triplet and the mRNA codon for both the normal allele and the sickle cell allele, and name the type of mutation involved.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Cells of the pancreas make large amounts of the polypeptide hormone insulin. The insulin gene is in the nucleus, but the polypeptide is assembled in the cytoplasm at ribosomes.

   (a)

   Describe how the insulin gene is transcribed and how the mRNA is translated into the polypeptide.

   [6 marks]

   (b)

   Explain how the structures of mRNA and tRNA, and the properties of the genetic code, ensure that the correct polypeptide is made. Include a justified conclusion about why the code being degenerate is advantageous.

   [6 marks]

   Total for question 4: 12 marks

End of questions