Explain why DNA must be replicated before mitosis and the role of helicase in DNA replication.

Explain how the base sequence of DNA is conserved during replication.

Describe the events that occur during mitosis.

two genetically identical nuclei/daughter cells formed during mitosis (so hereditary information in DNA can be passed on);
two copies of each chromosome/DNA molecule/chromatid needed;
helicase unwinds the DNA/double helix;
to allow the strands to be separated;
helicase separates the two (complementary) strands of DNA;
by breaking hydrogen bonds between bases;

DNA replication is semi-conservative;
DNA is split into two single/template strands;
nucleotides are assembled on/attached to each single/template strand;
by complementary base pairing;
adenine with thymine and cytosine with guanine / A with T and C with G;
strand newly formed on each template strand is identical to other template strand;
DNA polymerase used;

Marks may be awarded for any of the above points if clearly presented in a well-annotated diagram.

sequence of stages is prophase → metaphase → anaphase → telophase;
chromosomes condense/supercoil/become shorter and fatter in prophase;
spindle microtubules grow (from poles to equator) in prophase/metaphase;
nuclear membrane breaks down in prophase/metaphase;
spindle microtubules attach to the centromeres/chromosomes in metaphase;
chromosomes line up at equator in metaphase;
centromeres divide / (paired) chromatids separate / chromosomes separate into two chromatids in metaphase/anaphase;
(sister) chromatids/chromosomes pulled to opposite poles in anaphase;
spindle microtubules disappear in telophase;
nuclear membrane reforms around chromosomes/chromatids in telophase;
chromosomes/chromatids decondense in telophase;

Practically everybody knew the role of helicase in DNA replication. Extremely few could clearly explain the need for mitosis. 

The question was often confused with other details of DNA replication, transcription and even translation. Though DNA replication was correctly described as semiconservative, further expansion of that term became muddled.  Most knew A-T and G-C base pairing but the idea of complementarity was not always included. Diagrams were drawn but lacked labels and annotations most of the time.  Occasionally, candidates mentioned that DNA polymerase was used

Of all the parts in Section B, this one (describe the events of mitosis) was answered best. Many candidates earned close to the maximum number of marks. A few candidates thought that interphase is a part of mitosis.