Question 21M.2.SL.TZ2.a
| Date | May 2021 | Marks available | [Maximum mark: 5] | Reference code | 21M.2.SL.TZ2.a |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Describe | Question number | a | Adapted from | N/A |
Describe the structure of the DNA molecule.
[5]
a. two stranded/double helix ✔
b. antiparallel / strands running in opposite directions
OR
one strand organized 5’ to 3’ and the other 3’ to 5’ ✔
c. sugar-phosphate backbone ✔
d. each strand formed by chains of nucleotides ✔
e. each nucleotide is formed by a phosphate, a deoxyribose and a base / annotated diagram of a nucleotide clearly indicated as a nucleotide ✔
f. the bases are adenine, guanine, cytosine and thymine ✔
g. strands held together by hydrogen bonds (between complementary base pairs)
OR
A pairs with T and C pairs with G ✔
Both helix and two strands needed for mp a. Double helix is sufficient for the mark.
Points can be awarded to annotated diagrams.
For mp c, the explicit label sugar phosphate backbone is required.
To award mp d from a diagram, at least three pairs of nucleotides should be shown.
For mp e, the diagram would need to be labelled as a nucleotide.
For mp e, expect deoxyribose not just sugar.
The written names of the bases are required for mp f.
Do not penalize twice for mp f and g for using letters.
If they only ever use the symbols A,T,C and G they are ineligible for mp f. If however, they say A pairs with T and C pairs with G, then they would get mp g. If they wrote adenine pairs with thymine and cytosine pairs with guanine, then they would obtain both mp f and mp g.
A surprising number of students failed to discuss nucleotides explicitly.
