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

1. 1
   The template (antisense) strand of a short section of a gene has the base sequence TACGGATTC. The section is transcribed into messenger RNA.

   (a)

   Which mRNA base sequence is produced from this template strand?

   [1 mark]

   • AATGCCTAAG
   • BAUGCCUAAG
   • CUACGGAUUC
   • DAUGCCUUAG

   (b)

   The mRNA is read from its first base. None of its codons is a stop codon. How many amino acids does this section of mRNA code for?

   [1 mark]

   • A9
   • B6
   • C2
   • D3

   (c)

   The codon CCU in the mRNA codes for the amino acid proline. A mutation in the gene changes this codon to CCC, which also codes for proline. Explain why the polypeptide is unchanged.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A ribosome is translating a short mRNA with the codon sequence AUG GCU UUC UAA. Transfer RNA molecules, each carrying a specific amino acid, deliver the amino acids to the ribosome.

   (a)

   Which tRNA anticodon pairs with the codon GCU?

   [1 mark]

   • ACGA
   • BCGT
   • CGCU
   • DCGU

   (b)

   Why does polypeptide synthesis stop at the codon UAA?

   [1 mark]

   • ANo ribosome can bind to the codon UAA
   • BUAA is the same as the start codon
   • CUAA is a stop codon, so no tRNA with a complementary anticodon binds
   • DUAA codes for the same amino acid as UUC

   (c)

   State the number of amino acids in the polypeptide made from this mRNA and explain your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is studying a bacterial gene whose polypeptide product has 150 amino acids, including the first, methionine. The student is also learning why the genetic code uses sequences of three bases.

   (a)

   Calculate the minimum number of bases in the mRNA that is read by the ribosome to make this polypeptide. Explain your answer.

   [3 marks]

   (b)

   There are four different bases in DNA and twenty different amino acids in proteins. Explain why a code of two bases per amino acid would not be sufficient, and why a triplet code means the genetic code is degenerate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cell is making a short peptide. The template (antisense) strand of the gene for the peptide has the base sequence TACCAGATT.

   (a)

   Explain how this template strand is used to produce the mRNA.

   [6 marks]

   (b)

   Explain how the mRNA AUGGUCUAA is translated to produce the peptide.

   [6 marks]

   Total for question 4: 12 marks

End of questions