Structure of DNA and RNAAQA A-Level Biology: Subtopic test

1. 1
   Analysis of a sample of double-stranded DNA from a bacterium showed that 22 % of its bases were guanine.

   (a)

   What percentage of the bases in the sample were adenine?

   [1 mark]

   • A22 %
   • B44 %
   • C56 %
   • D28 %

   (b)

   Which statement correctly describes the hydrogen bonding between the strands of this DNA?

   [1 mark]

   • AThree hydrogen bonds between adenine and thymine, two between cytosine and guanine
   • BTwo hydrogen bonds between adenine and thymine, three between cytosine and guanine
   • CTwo hydrogen bonds between every pair of bases
   • DHydrogen bonds between the phosphate groups of the two strands

   (c)

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

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a eukaryotic cell, DNA is held in the nucleus. A gene is copied into a molecule of messenger RNA (mRNA), which then moves to the ribosomes in the cytoplasm.

   (a)

   Which two components are found in a nucleotide of mRNA but not in a nucleotide of DNA?

   [1 mark]

   • ADeoxyribose and thymine
   • BRibose and thymine
   • CRibose and uracil
   • DDeoxyribose and uracil

   (b)

   Which bond forms between two nucleotides when they are joined by a condensation reaction?

   [1 mark]

   • AA phosphodiester bond between the phosphate group of one nucleotide and the pentose of the next
   • BA hydrogen bond between the bases of adjacent nucleotides in the same strand
   • CA peptide bond between the bases
   • DA glycosidic bond between two pentose sugars

   (c)

   Explain how the structure of mRNA makes it suitable for carrying genetic information from the nucleus to the ribosomes.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the early twentieth century many scientists doubted that DNA carried the genetic code. A synthetic double-stranded DNA fragment, 10 base pairs long, contains 3 adenine-thymine base pairs and 7 cytosine-guanine base pairs.

   (a)

   Suggest why many scientists doubted that DNA carried the genetic code.

   [3 marks]

   (b)

   Calculate, for the DNA fragment, the number of hydrogen bonds between the strands and the number of phosphodiester bonds, and explain why a fragment with more cytosine-guanine pairs needs more energy to separate its strands.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A scientist is comparing a gene in the DNA of a human liver cell with the messenger RNA (mRNA) made from that gene, which passes to the ribosomes in the same cell.

   (a)

   Explain how the structure of DNA makes it suited to storing genetic information.

   [6 marks]

   (b)

   Compare the structure of DNA with the structure of the mRNA made from it.

   [6 marks]

   Total for question 4: 12 marks

End of questions