Structure of DNA and RNAEdexcel International A Level Biology: Subtopic test

1. 1
   A technician extracts DNA from a bacterium and analyses its base composition. The DNA is a double-stranded molecule and 28% of its bases are adenine.

   (a)

   What percentage of the bases in this DNA is thymine?

   [1 mark]

   • A28%
   • B22%
   • C44%
   • D56%

   (b)

   What percentage of the bases in this DNA is cytosine?

   [1 mark]

   • A28%
   • B44%
   • C22%
   • D11%

   (c)

   Explain why the percentage of thymine in this DNA is equal to the percentage of adenine.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A virologist extracts the genetic material from a newly discovered virus. The material is made of polynucleotide and base analysis gives: adenine 23%, cytosine 27%, guanine 29% and uracil 21%.

   (a)

   Which base in the analysis is found in RNA but never in DNA?

   [1 mark]

   • AAdenine
   • BCytosine
   • CUracil
   • DGuanine

   (b)

   Part of this molecule folds back on itself so that two regions form base pairs. Which pairing would be expected between the two regions?

   [1 mark]

   • AAdenine with thymine
   • BAdenine with uracil
   • CGuanine with uracil
   • DCytosine with adenine

   (c)

   Explain why the base percentages suggest that the molecule is not double-stranded along its whole length.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   One strand of a short section of a DNA molecule has the base sequence TACGGCATT. The section is part of a double-stranded molecule.

   (a)

   Deduce the base sequence of the complementary strand. Name the bond that holds the two strands together at the bases and state which bases it joins.

   [3 marks]

   (b)

   Calculate the total number of hydrogen bonds between the two strands in this section. Hence explain why a section of DNA with a higher proportion of C–G base pairs is more stable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research group is comparing the structure of two nucleic acids from a human cell: a DNA molecule from a chromosome in the nucleus and a messenger RNA molecule copied from one gene on that chromosome.

   (a)

   Describe how mononucleotides are joined to form a polynucleotide. Calculate the number of phosphodiester bonds in a single polynucleotide strand of 120 nucleotides.

   [6 marks]

   (b)

   Compare the structure of the DNA molecule with the structure of the messenger RNA molecule.

   [6 marks]

   Total for question 4: 12 marks

End of questions