A-LevelBiologyThe mitotic cell cycle9700/23 · Oct/Nov 2020 · Q310 Marks

Fig. 3.1 shows the nematode worm, Caenorhabditis elegans. [Figure 3.1] Stem cells of C. elegans have been studied. Fig. 3.2 shows the change in mass of DNA per nucleus in a stem cell during one cell cycle. 1 pg = 1 x 10⁻¹²g [Figure 3.2]

(a)(i)State the phases of interphase shown by A and B in Fig. 3.2. A B[2]
(a)(ii)State the stage of mitosis shown by D in Fig. 3.2.[1]
(a)(iii)Outline what happens in a cell in preparation for cytokinesis.[2]
(a)(iv)A stem cell of a female C. elegans has 12 chromosomes. Complete Table 3.1 to show the number of nuclei within the stem cell in stages A, B and D of the cell cycle shown in Fig. 3.2 and the number of chromosomes in each nucleus during these stages. Table 3.1 stage of the cell cycle A B D number of nuclei within the stem cell number of chromosomes in each nucleus[2]
(b)Young nematodes have stem cells throughout the body. Fig. 3.3 summarises three cell cycles of one of these stem cells. [Figure 3.3] With reference to Fig. 3.3, outline the role of stem cells in animals.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[2] A – G1 / gap 1 ;
B – S / synthesis ; A DNA replication | S1 / S2 / replication unqualified
[1] telophase ;
[2] any two from: increase in cytoplasm / increase in number of (named) organelle(s), during interphase ; A synthesis of organelles I cell growth
(re)formation of nuclear envelope ; nuclear envelope, forms around each group of chromosomes ; A nuclear membrane as an ECF if used for mp2
(movement of) organelles to be shared between two daughter cells ;
spindle, disassembles / breaks down / degrades / disappears / AW;
cleavage furrow forms ; A cytoplasm / cell membrane, pinches in / constricts / infolds
AVP ; e.g. (actin) microfilaments / microtubules, form contractile ring / AW, around equator of cell / beneath cell surface membrane A cytoskeleton for microtubules / microfilaments
[2] one mark per row
stages of cell cycle: A, B, D
number of nuclei within the stem cell: 1, 1, 2;
number of chromosomes ineach nucleus: 12, 12, 12;
[3] 1 (stem cells) continue to divide / divide repeatedly (by mitosis);
A at least one daughter cell, retains ability to divide / is a stem cell / A 'repeated mitosis' / A 'to produce more stem cells'
any two from mps 2-5: to produce cells 2 for growth ;
3 for repair of tissues ; R repair of cells
4 for replacement of, worn out / old / dead, cells ; A damaged cells if repair of tissues not given
5 to differentiate for formation of, tissues / organs ; A specialised cells for forming, tissue / organs
and: 6 AVP ; e.g. ref. to potency of stem cells (multipotent or pluripotent)