Question 20N.3.hl.TZ0.b(ii)
| Date | November 2020 | Marks available | [Maximum mark: 2] | Reference code | 20N.3.hl.TZ0.b(ii) |
| Level | hl | Paper | 3 | Time zone | TZ0 |
| Command term | Draw | Question number | b(ii) | Adapted from | N/A |
The diverse functions of biological molecules depend on their structure and shape.
Draw the nitrogenous base that is paired with guanine in DNA, showing the hydrogen bonds between the bases. Use section 34 of the data booklet.
[2]
cytosine drawn ✔
appropriate representation of three hydrogen bonds AND between correct atoms ✔
Structure of cytosine must be given for M1.
Ignore missing hydrogens on carbon atoms in cytosine.
Dashed lines (horizontal or vertical) OR dots can be used to represent hydrogen
bonds.
Only award M2 if M1 correct.
This question proved to be very challenging and although many were able to identify and draw cytosine correctly, few were able to score M2 which required the representation of the multiple hydrogen bonds between guanine and cytosine. It was very disappointing that such a high percentage of candidates did not understand the nature of a hydrogen bond and equally tried to represent a hydrogen bond using a full line notation (typical of a covalent bond), instead of the more customary dashed lines (either horizontal or vertical) or dots type representation, used to represent hydrogen bonds.