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3.1 The chemical structures of DNA and RNA

Figure 3.9 The double helix structure of DNA. The form of DNA shown is the B form, which is most prevalent in cells. (a) ‘Photo 51’, an X-ray diffraction image taken by Rosalind Franklin and Ray Gosling at King’s College London in May 1952, provided confirmation of the helical structure of DNA. The photograph (together with their own deductions) enabled James Watson and Francis Crick of the University of Cambridge to build the first correct model of the DNA molecule. A schematic structure based on the Watson–Crick model is shown in (b). The space-filling model in (c) illustrates the two grooves of different sizes in which the edges of the base pairs are exposed to the exterior of the molecule. Atom colours are: C, grey; N, blue; O, red; P, orange. Source : (a) King's College London Archives/Science Photo Library. (b) Adapted from Watson et al. (2014) Molecular Biology of the Gene , 7th edn. Pearson. (c) Shutterstock.

5. Complementary base pairing, with purine–pyrimidine pairs, means that the two strands are evenly spaced throughout the length of molecule. 6. The base pairs are tucked away inside the molecule, so that their hydrophobic faces are not exposed to the aqueous environment. However, the edges of the base pairs are accessible in the major and minor grooves, as shown in Figure 3.9b. We explain the sig nificance of this for gene regulation in Chapter 5. 7. The base pairs are arranged like the rungs of a ladder, but each successive base pair is slightly rotated compared to the one below it, giving the helical structure. 8. The helices are right-handed. Imagine walking up a strand as if it were a spiral staircase—you will continually turn clock wise. Alternatively, imagine a right-handed person driving in a standard screw—their wrist rotates in the same direction as the helix. 9. The dimensions of the DNA double helix vary slightly depend ing on its base sequence and whether it is in its crystallized form or in solution. Approximate dimensions are:

■ 2 nm diameter ■ 10–10.5 base pairs per 360° turn

■ A ‘pitch’ (vertical length of one 360° turn) of 3.4 nm and hence a distance from one base pair to the next of approximately 0.34 nm.

PAUSE AND THINK Nucleic acid sequences are, by convention, written in the 5 ′ to 3 ′ direction from left to right. (a) What would be the complementary sequence to a single strand of DNA that has the sequence 5 ′ GATCGGA 3 ′ ? (b) What would be the sequence of an RNA molecule that is complemen tary to the DNA sequence 5 ′ GATCGGA 3 ′ ?

(a) 5 ′ TCCGATC 3 (b) 5 ′ UCCGAUC 3

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