SAJC_2008_H2-Bio-TYS-ANS

2008 ‘A’ Level H2 Biology Mark Scheme (updated 10 Jan 2015) 1 2008 ‘A’ Level H2 Biology Mark Scheme PAPER 1 (MCQ) 1 A 21 D 2 B 22 A 3 C 23 C 4 A 24 B 5 B 25 C 6 A 26 B 7 B 27 D 8 A 28 D 9 C 29 A 10 D 30 A 11 B 31 C 12 A 32 B 13 C 33 C 14 B 34 A 15 B 35 D 16 D 36 A 17 D 37 B 18 C 38 B 19 C 39 D 20 C 40 C

2008 ‘A’ Level H2 Biology Mark Scheme (updated 10 Jan 2015) 2 PAPER 2 (CORE) QUESTION 1 (a) (i) 1 Metaphase 2 Anaphase (ii) 1 centromeres divide after 15 min R centromeres split 2 chromatids separate to become chromosomes which are then pulled apart by spindle fibres to opposite poles of the cell REJECT vague references to moving of chromatids without reference to spindle fibres REJECT vague references to ends of the cells rather than poles of the cell (iii) 1 poles move closer together and then move further apart 2 ref supporting data 3 role of pole-to-pole fibres (polar microtubules/polar spindle fibres) in elongating the centre of the spindle so that poles are pushed further apart REJECT poor reference to curve C moving (b) 1 repetitive DNA 2 involved in chromatids adhesion followed by division during anaphase 3 ref to kinetochore formation on centromeres, being proteins which bind onto centromeres to anchor spindle fibres so that they could be pulled to the poles 4 involved in chromatid alignment and separation (c) 1 homologous chromosomes form bivalents in meiosis but not mitosis 2 crossing over occurs in meiosis but not in mitosis 3 homologous pairs line up at the equator in meiosis but in mitosis chromosomes line up singly 4 homologous chromosomes separate in anaphase 1 of meiosis but chromatids separate in anaphase of mitosis 5 meiosis results in the haploid number of chromosomes but mitosis maintains the diploid number. 6 there are 2 divisions in meiosis but only one in mitosis. Examiner’s comments: Note that each mark requires a difference to be stated between the behaviour of chromosomes in meiosis and mitosis.

2008 ‘A’ Level H2 Biology Mark Scheme (updated 10 Jan 2015) 3 QUESTION 2 (a) 1 (base) substitution REJECT missense mutation, point mutation (b) 1 tertiary structure of the molecule changes; 2 ref different R groups of hydrophobic valine (substituting hydrophilic glutamic acid) involved in protein folding; 3 effect on HbS occurs at low oxygen concentration, hydrophobic regions on different molecules stick together, solubility of deoxygenated HbS decreases 4 HbS will polymerize into fibres, ability to carry oxygen decreases. (c) 1 Affected red blood cells (REJECT HbS) will adopt a sickle shape; 2 ref tendency for red blood cells to stick/clump together; 3 sickle shaped red blood cells clog capillaries, preventing other cells from moving through capillaries; obstruction of blood flow to organs, organ damage occurs 4 shorter lifespan of red blood cells resulting