Draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA.

Explain transcription.

Distinguish between continuous and discrete variation, using examples.

a. ribose drawn as pentagon and labelled sugar/ribose;
b. base drawn with correct link to (C₁ of) ribose and labelled base/nitrogenous base;
c. phosphate drawn with correct link to (C₅ of) ribose and labelled P/phosphate;
d. two (or more) ribonucleotides drawn with correct link (C₃ to C₅)

a. synthesis of RNA/mRNA / transcription of DNA to RNA;
b. RNA nucleotides linked together to form a strand/chain;
c. RNA strand assembled on DNA template/antisense strand / copy made of sense strand;
d. RNA polymerase carries out transcription/links RNA nucleotides;
e. uncoiling/separation of DNA strands;
f.  5’ end of nucleotides linked to 3’ end of (growing RNA) strand;
g. complementary base pairing (is the basis of copying the base sequence);
h. uracil instead of thymine in RNA;
i. starts at/RNA polymerase binds to a promoter;
j. regulated by transcription factors/DNA binding proteins/nucleosomes;

Annotated diagrams can be used.

Many candidates were able to draw the structure of an RNA nucleotide and link it correctly by a 5’ to 3’ bond to another nucleotide. The commonest error was to show two strands of nucleotides linked by base pairing, indicating confusion between RNA and DNA.

Answers were mixed. Strong candidates had no difficulty in describing transcription in detail but weaker ones tended to get confused with replication and/or translation. Helicase was often stated as the enzyme that uncoils and splits the double helix, rather than RNA polymerase. Marks were not awarded merely for stating that transcription is 5’ to 3’ unless it was clear that the candidate understood that the 5’ terminal of a free nucleotide is linked to the 3’ terminal of the chain of nucleotides already linked up. Candidates are expected to show understanding in their answers, rather than just state memorized phrases.

Differences between continuous and discrete variation were not well known and the average performance for this question was the lowest for any part of Section B. A general fault was to describe the two types of variation separately and in consequence forget to include both sides of a distinction. For example, some candidates stated that the environment can cause continuous variation, but most did not then also state that discrete variation is generally unaffected by environment. A mark was awarded for examples of the two types of variation, but in some cases an inappropriate example was chosen, such as hair colour for discrete variation or eye colour for continuous variation. The latter example was best avoided entirely as there are aspects of both continuous and discrete variation in the pigmentation of the iris.