Question 22N.2.HL.TZ0.4
| Date | November 2022 | Marks available | [Maximum mark: 8] | Reference code | 22N.2.HL.TZ0.4 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Explain, Identify, Outline, State | Question number | 4 | Adapted from | N/A |
The diagram shows a nucleosome from the nucleus of a eukaryotic cell.
[Source: Weissman Lab at UCSF. UCSF Team Views Genome as it Turns On and Off Inside Cells. [diagram online]
Available at https://www.ucsf.edu/news/2011/01/98118/ucsf-team-views-genome-it-turns-and-inside-cells
[Accessed 1 December 2022].]
Identify the protein labelled in the diagram.
[1]
histone;
Nearly all candidates know that the proteins in nucleosomes are histones.
Outline how nucleosomes affect the transcription of DNA.
[1]
- (nucleosomes can) promote AND inhibit transcription of genes/expression of genes;
- (nucleosomes can) prevent transcription by (tight) condensation/supercoiling/packing of DNA;
- (nucleosomes can) allow/prevent binding of RNA polymerase/transcription factors;
- tagging/acetylation/methylation of nucleosomes/histones can promote/inhibit transcription;
- movement of histones/nucleosomes (along DNA) can affect which genes are transcribed;
‘Affects transcription’ is in the question — no mark.
Many candidates knew something of the role of nucleosomes in regulating DNA transcription. A wide range of answers was accepted for the single mark.
The image shows the regulation of the gene responsible for producing lactase.
[Source: Lac Operon, n.d. [diagram online] T A RAJU. Available at:
https://commons.wikimedia.org/wiki/File:Lac_Operon.svg [Accessed 29 October 2021].]
Identify X, the enzyme which copies a DNA sequence.
[1]
RNA polymerase;
About half of candidates knew that X must be RNA polymerase. Some candidates thought that it was DNA polymerase, perhaps because the question referred to copying a DNA sequence. This answer was not accepted as the diagram did not show replication.
Identify Y, non-coding DNA at the start of a gene.
[1]
promoter;
Fewer candidates knew that Y must be the promoter. This could be deduced from the diagram which showed that Y is the site to which RNA polymerase first binds. Other incorrect suggestions were primer, intron, start codon and telomere.
Explain the role of lactose in the expression of the gene for lactase production.
[3]
- lactose binds to repressor protein;
- repressor protein (with lactose bound) cannot block/bind to the promoter/Y;
- RNA polymerase/X binds to the promoter/transcribes the gene;
- lactase produced (if lactose present)/lactase production inhibited if lactose absent;
Accept the converse of mpa to mpd with lactose absent.
Many candidates realised from the diagram that lactose binds to the repressor. Fewer could explain convincingly that this removes the repressor from the promoter, so RNA polymerase can bind and transcribe the gene for lactase. Many candidates earned a mark by giving the last idea on the mark scheme, which is that presence of lactose results in lactase production. It should be emphasized that the lac operon does not need to be taught to future students — the expectation here was that answers would be based on general understanding of transcription and on the information provided in the question. These was no expectation that candidates would already be familiar with the lac operon.
State one reason that identical twins may show different methylation patterns as they grow older.
[1]
(different) environment/illness/disease/diet;
Not mutation
Most candidates knew that differences in environmental factors can cause differences in methylation pattern.