(a) As part of a study of the mitotic cell cycle, a student made stained sections of a root tip of onion, Allium cepa, and observed them with a light microscope. The student made drawings of six of the cells, A to F, using the high power of the microscope, as shown in Fig. 6.1. [Figure 6.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

1. [(a)(i)] [A] C E D B F ;
2. [(a)(ii)] I early and late except where indicated
3. [(a)(ii)] DNA replication: interphase / S phase ;
4. [(a)(ii)] division of centromeres: anaphase ; R late anaphase
5. [(a)(ii)] condensation of chromatin: prophase ;
6. [(a)(ii)] contraction of spindle fibres: anaphase ;
7. [(a)(ii)] organisation of chromosomes at the equator: metaphase
8. [(b) I antibodies have receptors]
9. [(b) any three from]
10. [1] clones / AW, of B / T cells consist of very few cells ; AW = types of / specific / groups of
11. [2] (mitosis occurs) during clonal expansion ;
12. [3] many, B cells / plasma cells, to make antibodies ;
13. [4] many, T-helper / Th, cells to secrete, cytokines / lymphokines / interleukins ;
14. [5] many, T-killer cells / T-cytotoxic cells, to kill, infected cells / cancer cells ;
15. [6] memory cells are produced for, secondary response / faster response (when antigen encountered again) ; A during a secondary infection (implying of the same pathogen)
16. [7] need to make genetically identical cells ;