During transcription, base pairing occurs between nucleotides. Fig. 3.1 is a diagram to show complementary base pairing between a DNA nucleotide and an RNA nucleotide. Only the base pair is shown in molecular detail. [Figure 3.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

1. [1] do not allow 'base' as AW for 'nucleotide' any one from: idea that phosphodiester bond forms, between nucleotides on the same strand / between adjacent nucleotides / during polynucleotide formation / to form a sugar-phosphate backbone; AW look for understanding that more than one nucleotide needs to be present on the same strand idea that phosphates, shown are only bound to one (pentose) sugars / each phosphate needs to be bound to two (pentose) sugars;
2. [4] base / nucleotide, on left identified as guanine and, base / nucleotide, on right identified as cytosine ; A guanine and cytosine as question has DNA-RNA I cytosine and guanine unless further qualified dotted line as hydrogen bond / annotation related to dotted lines as hydrogen bonds / there are three hydrogen bonds between the bases; A H-bond circle is phosphate; pentagon/pentose, on left is deoxyribose ; also AVP if 'pentose' double ring/ base on left, identified as purine ; A guanine is a purine if mp1 gained single ring / base on right, identified as pyrimidine ; A cytosine is a pyrimidine if mp1 gained AVP; e.g. label to any solid line bond as covalent bond phospho-ester bond, labelled / described pentose sugar, label to / described for, deoxyribose / ribose box drawn around one completed nucleotide and labelled 5' and 3' shown annotated ref. to antiparallel nature of base-pair