CAIE A-Level Biology 6.1.4 DNA Double-Helix Structure

Practise labelling antiparallel DNA strands, phosphodiester-linked backbones and complementary base pairs, including two A–T and three C–G hydrogen bonds.

Exam points

label the antiparallel 5′ to 3′ and 3′ to 5′ sugar–phosphate backbones
identify phosphodiester bonds joining adjacent nucleotides within each strand
show A–T with two and C–G with three hydrogen bonds across equal-width base pairs

[Maximum number: 2]

DNA and RNA are polynucleotides.

Scientists have synthesised four synthetic bases, Z, P, S and B . The base pairings of the synthetic bases are shown in Fig. 6.1.

Fig. 6.1

State and explain which DNA base pair is most similar to the synthetic base pairs in Fig. 6.1.

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6.1.4—DNA double helix structure question 1