The genetic code and protein synthesisEdexcel A-Level Biology A: Subtopic test

1. 1
   The base sequence of a short section of the template (antisense) strand of a gene is TACCGAGGTATT. This section is transcribed to make mRNA that begins with a start codon and ends with a stop codon.

   (a)

   What is the base sequence of the mRNA transcribed from this section?

   [1 mark]

   • AATGGCTCCATAA
   • BUACCGAGGUAUU
   • CAAUACCUCGGUA
   • DAUGGCUCCAUAA

   (b)

   How many amino acids are in the polypeptide made from this mRNA?

   [1 mark]

   • A3
   • B4
   • C12
   • D2

   (c)

   The codons GCU and GCC both code for the amino acid alanine. State the term used to describe this property of the genetic code and explain what it means.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During translation in a pancreatic cell, a ribosome moves along an mRNA molecule with the base sequence AUG UUC GGA CAU UGA. The codon UGA is a stop codon.

   (a)

   Which anticodon on a tRNA molecule pairs with the codon UUC?

   [1 mark]

   • AUUC
   • BAAG
   • CTTC
   • DAAC

   (b)

   What causes translation of this mRNA to stop?

   [1 mark]

   • AThe ribosome reaches the start codon AUG
   • BThe cell runs out of amino acids
   • CThe ribosome reaches the stop codon UGA, which has no tRNA, so the polypeptide is released
   • DA tRNA with the anticodon ACU binds and ends translation

   (c)

   Explain how tRNA molecules make sure that the correct amino acid is added at each codon of the mRNA.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   RNA polymerase transcribes a gene in the nucleus of a cell. A section of the DNA template (antisense) strand has the base sequence TACCAGTTCACT.

   (a)

   Deduce the base sequence of the mRNA transcribed from this section, and describe the role of RNA polymerase in forming it.

   [3 marks]

   (b)

   Use your answer to (a) to explain the roles of the start codon and the stop codon in the mRNA, and describe what happens to the mRNA after transcription.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cell in the pancreas makes a hormone that is a polypeptide of 18 amino acids. The gene for the hormone is in the nucleus of the cell, and the hormone is made on ribosomes in the cytoplasm.

   (a)

   Describe how the sequence of bases in the gene determines the sequence of amino acids in the hormone.

   [6 marks]

   (b)

   Explain why the genetic code must be a triplet code, and explain what is meant by degenerate and non-overlapping. Calculate the minimum number of nucleotides in the mRNA that codes for this hormone, including a stop codon.

   [6 marks]

   Total for question 4: 12 marks

End of questions