| Date | May 2012 | Marks available | 1 | Reference code | 12M.2.HL.TZ1.2 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Label | Question number | 2 | Adapted from | N/A |
Question
The diagram below shows the process of transcription.
DNA replication involves a number of enzymes including DNA polymerase. Identify one other enzyme involved in DNA replication.
Explain the role of Okazaki fragments in DNA replication.
Label the sense and antisense strands.
Draw an arrow on the diagram to show where the next nucleotide will be added to the growing mRNA strand.
Markscheme
helicase / RNA primase / (DNA) ligase
DNA fragments/sections (formed) on the lagging strand;
because replication must be in the 5' –3' direction;
replication starts repeatedly and moves away from replication fork;
both strands clearly labelled
Check carefully whether the correct strand has been labelled if the labels are shown in helical parts of the DNA.
Reject if the sense strand label points to the mRNA.
a clearly drawn arrow pointing at the free 3' end of the mRNA strand or to the first free nucleotide on the antisense strand to the left of the mRNA or to a nucleotide added by the candidate to the left hand end of the mRNA
Examiners report
All but the weakest candidates were able to name an enzyme involved in DNA replication.
This question discriminated very well with the best candidates writing authoritatively about Okazaki fragments, but weaker candidates struggling. Some teachers felt that the word role was inappropriate here, but any answer explaining that Okazaki fragments are formed on the lagging strand because nucleotides can only be added in a 5' to 3' direction would have scored both marks. A common error was to refer to the lagging strand as the antisense strand. This is not correct -on a DNA molecule the lagging strand is the antisense strand for some genes and the sense strand for others.
About half of the candidates knew that the transcribed strand is the antisense strand, with the others either getting the strands the wrong way round or thinking that the mRNA was either the sense or the antisense strand.
When asked in part (ii) to show where the next nucleotide will be added to the mRNA strand the weakest candidates labelled various places other than an end of the mRNA; of the other candidates, more than half labelled the right hand end, whereas the left hand was the 3' terminal so that is where the 5' end of a nucleotide would be added.
