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DNA and RNA: four letters and a rule about pairing questions

The three parts of a nucleotide, the sugar-phosphate backbone made by condensation, complementary base pairing with two hydrogen bonds between adenine and thymine and three between guanine and cytosine, what 5 prime and 3 prime mean, why the strands are antiparallel, how the structure suits the storage of information, and DNA compared with messenger, transfer and ribosomal RNA.

5 original questions · 16 marks · the dna and rna: four letters and a rule about pairing notes · Nucleic acids, genomes and protein synthesis

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• 4 marks · Compare

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

  Mark scheme

  1. B1 DNA contains the pentose sugar deoxyribose, whereas mRNA contains ribose
  2. B1 DNA contains thymine, whereas mRNA carries uracil in its place
  3. B1 DNA is two antiparallel strands wound into a helix, whereas mRNA is a single strand
  4. B1 DNA can be hundreds of millions of nucleotides long, whereas an mRNA molecule is tens to a few thousand nucleotides long

• 4 marks · Suggest

  A nucleic acid extracted from a virus contains adenine 31%, guanine 19%, cytosine 27% and thymine 23%. Suggest what these figures tell you about the structure of the molecule, and suggest one further measurement that would test your conclusion.

  Mark scheme

  1. B1 in a double-stranded molecule adenine would equal thymine and guanine would equal cytosine, because each base has a fixed partner opposite it
  2. B1 here adenine at 31% does not equal thymine at 23%, and guanine at 19% does not equal cytosine at 27%
  3. B1 the molecule cannot therefore be double-stranded; it is single-stranded DNA, since thymine rather than uracil is present
  4. B1 any one workable further measurement: heat the sample and look for a temperature at which strands separate, since a single-stranded molecule has no paired bases to separate

• 3 marks · Explain

  Explain why every base pair in a DNA molecule is the same width, and explain what this means for the shape of the molecule as a whole.

  Mark scheme

  1. B1 adenine and guanine are purines, built from two fused rings, while thymine and cytosine are pyrimidines, built from one
  2. B1 the pairing rule puts a purine opposite a pyrimidine in every pair, so each rung spans the same distance
  3. B1 the two sugar-phosphate backbones therefore stay a constant distance apart along the whole molecule, whereas two purines would bulge and two pyrimidines would pinch

• 3 marks · Calculate

  A sample of double-stranded DNA contains 3200 bases in total, of which 960 are guanine. Calculate the number of adenine bases in the sample.

• 2 marks · Name

  Name the three components of a single DNA nucleotide, and name the bond that joins one nucleotide to the next along a strand.

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