The best time to obtain a clear image of chromosomes during a mitotic cell cycle is during the metaphase stage. Fig. 6.1 is a scanning electron micrograph of a group of human chromosomes at metaphase. [Figure 6.1]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [1] correct working; e.g. 14 000 000 ÷ 1400 (A–B = 13 to 15 mm)
2. [1] (x) 9286 / 9642 / 10 000 / 10 357 / 10 714 ;
3. [1] incorrect conversions used in working or incorrect measured value of A–B can be awarded the answer mark as ecf – the working must show use of dividing by 1400 (or incorrect conversions of 1400)
4. [1] any four from
5. [1] two chromatids drawn with, non-metacentric centromere / chromosome arms unequal length ;
6. [1] (sister) chromatid ;
7. [1] centromere ; allow as a constriction
8. [1] telomere ; must be labelled at / towards the end
9. [1] AVP; e.g. four telomeres labelled or telomere sectioned off at each chromatid end and one labelled
10. [1] DNA and histones
11. [1] gene / allele must be shown as a section
12. [1] R homologous chromosomes
13. [1] any one from
14. [1] most condensed state ; AW I chromosomes, are condensing / condense
15. [1] chromosomes (all) in one, plane / focus ;
16. [1] A idea of all at the equator (so clearly visible)
17. [1] I line up. horizontally / vertically
18. [1] anaphase ;