State the name of each phase shown, recording whether each phase has taken place at an early or intermediate stage of mitosis.

Phase A:  . . . . . . . . . . . . . . . . . . . . .occurs at an. . . . . . . . . . . . . . . . . . . . . stage
Phase B:  . . . . . . . . . . . . . . . . . . . . .occurs at an. . . . . . . . . . . . . . . . . . . . . stage
 

Outline the events occurring in phase A.

State what results when there is an uncontrolled division of cells in living organisms.

DNA in chromosomes undergoes replication before mitosis. Outline how complementary base pairing is important in this process.

phase A: anaphase (occurs at an) intermediate (stage); (both needed)
phase B: prophase (occurs at an) early (stage); (both needed)

centromeres split/break;
(sister) chromatids/chromosomes separate;
dragged/pulled/movement to separate poles;
by shortening of spindle microtubules;
Do not allow events other than those in anaphase

tumours / cancer

conservation of the base sequence of DNA;
adenine pairs with thymine and cytosine pairs with guanine;  (do not accept initials only)
both (daughter) cells/DNA strands produced have identical genetic information;

Many correctly identified Phase A in 3(a)(i) but often missed Phase B.

Part 3(a)(ii) was usually well answered. Unfortunately, some candidates referred to homologous chromosomes when they meant sister chromatids; homologous chromosomes separate in Anaphase I of meiosis. Few mentioned centromeres splitting.

Part 3(b) was very successfully answered. No credit was given for “mutation.”

Some candidates just wrote that “an exact copy of DNA is made” in 3(c) which is ambiguous and gained no credit;  it was not clear that they knew that replication is a semi-conservative process where each of the new DNA molecules has a parent strand (conserved) and a new strand made by complementary base pairing.  Also, full names, rather than just letters were required for the nitrogenous bases and both pairs were required.