| Date | May 2014 | Marks available | 4 | Reference code | 14M.3.hl.TZ2.8 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Explain | Question number | 8 | Adapted from | N/A |
Question
Nucleic acids, which are polynucleotides present in cells, transmit essential genetic information.
Explain the double helical structure of DNA, including the importance of hydrogen bonding.
Markscheme
DNA made up of two polynucleotides that join to form a spiral/double helix;
two strands of nucleic acid interaction through hydrogen bonding network between
bases / polynucleotides linked by hydrogen bonds between bases;
alternating phosphate and sugar groups form the backbones with bases on the sugar;
thymine/T bonds to adenine/A and cytosine/C bonds to guanine/G;
double helix stabilized by dipole-dipole and vdW interactions between base pairs;
Examiners report
In Q8 the best understanding was that of the complementary base pairs and the hydrogen bonding between them; the rest was not well explained and very few understood the importance of the hydrogen bonding. Candidates were either well prepared for DNA profiling – or showed very little understanding. The order of steps seemed to be incertain. It is accepted that “describe” followed by “explain the importance” would have been a more effective wording of the question.
