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Nucleic Acids and Protein Synthesis: Question 2

Syllabus 6.1

Structured AS 7 marks

A student is comparing a sample of DNA extracted from the nucleus of an onion root tip cell with a sample of mRNA extracted from the same cell.

(a) Describe the structure of a single nucleotide found in a molecule of DNA. [3]

(b) State two structural differences between a molecule of DNA and a molecule of mRNA. [2]

(c) The two strands of the DNA double helix are described as antiparallel. Explain what this means. [2]

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Worked solution

Part (a): Structure of a DNA nucleotide

A nucleotide is the basic repeating unit of a nucleic acid. Each DNA nucleotide is built from three covalently joined components:

  • a phosphate group,
  • a pentose sugar. In DNA this sugar is deoxyribose,
  • a nitrogenous base, one of adenine, thymine, cytosine or guanine.

The base is attached to one carbon of the deoxyribose, and the phosphate group is attached to a different carbon of the same sugar. It is the sugar (not the base or phosphate) that links to the phosphate of the next nucleotide along the strand, via a phosphodiester bond, to build up the sugar-phosphate backbone.

Part (b): DNA versus mRNA

DNA and mRNA are both nucleic acids built from nucleotides, but they differ structurally in several ways. Any two of the following are acceptable:

  • DNA nucleotides contain the sugar deoxyribose; mRNA nucleotides contain the sugar ribose.
  • DNA contains the base thymine; mRNA contains uracil in place of thymine (uracil still pairs with adenine).
  • A DNA molecule is normally double-stranded (two strands held together by base pairing); an mRNA molecule is normally single-stranded.

(Any two of these, clearly stated, would gain full credit. The sugar difference and the thymine/uracil difference are the two most fundamental.)

Part (c): Antiparallel strands

The two strands of the DNA double helix are not simply two identical strands lying side by side, they run in opposite directions. Each strand has a direction defined by its sugar-phosphate backbone, conventionally described as running from its 5’ end to its 3’ end. In the double helix, if one strand runs 5’ to 3’ from top to bottom, the strand paired with it runs 3’ to 5’ from top to bottom (i.e. 5’ to 3’ from bottom to top). This opposite orientation of the two backbones is what “antiparallel” describes, and it is essential for the complementary bases on the two strands to align correctly for hydrogen bonding.

Final answers

  • (a) A nucleotide = phosphate group + deoxyribose (pentose sugar) + a nitrogenous base, covalently bonded.
  • (b) Any two of: deoxyribose vs ribose; thymine vs uracil; double-stranded vs (typically) single-stranded.
  • (c) Antiparallel = the two strands run in opposite directions, one 5’ to 3’ and the other 3’ to 5’.