State the type of bonds that

(i) connect base pairs in a DNA molecule.

(ii) link DNA nucleotides into a single strand.

Distinguish between DNA and RNA nucleotides by giving two differences in the chemical structure of the molecules.

Explain the role of transfer RNA (tRNA) in the process of translation.

(i) hydrogen

(ii) covalent / phosphodiester linkage

DNA has deoxyribose, RNA has ribose;

DNA has base T/thymine, RNA has base U/uracil;

Do not accept double or single strands as chemical structure.

tRNA attaches to (specific) amino acid;

tRNA (with amino acid) moves to the ribosome;

anticodon of tRNA binds with codon of mRNA;

(i) Most were correct.

(ii) Less success was seen here. A number of students mistakenly wrote phosphate bonds or peptide bonds. “Covalent bonds” was commonly stated for the mark and, sometimes, even the more sophisticated answer of phosphodiester linkage was given.

Occasionally, candidates reversed hydrogen and covalent for parts (i) and (ii) and lost both marks.

Easy marks for many candidates. However, some misread the questions and described differences in the physical/molecular structure of DNA and RNA molecules. Rather than restricting answers to nucleotide differences, double and single strands were described resulting in no credit.

This question was worth only two marks but explaining the role of tRNA during translation could easily have been worth more. A few candidates wrote stellar answers that far exceeded the two mark maximum. The marking points most often awarded were that tRNA attaches to an amino acid and that tRNA has an anticodon complementary to the mRNA codon. Many candidates provided inaccurate information.