A-LevelBiologyThe mitotic cell cycle9700/23 · Oct/Nov 2022 · Q411 Marks

The early development of an animal involves divisions of the zygote and daughter cells by mitosis to form an embryo consisting of genetically identical cells. Fig. 4.1 shows several cells at various stages of the cell cycle in an embryo of whitefish, Coregonus artedi. [Figure 4.1]

(a)(i)Name the stage of mitosis in cell A and in cell B, shown in Fig. 4.1. A .............................................................. B ..............................................................[2]
(a)(ii)Fig. 4.1 shows microtubules in the cells that are dividing. Describe the role of microtubules in mitosis.[3]
(a)(iii)State what happens in cell C, shown in Fig. 4.1, until two new cells are formed.[3]
(b)The cells produced in the developing whitefish are genetically identical. Identify and explain two events that occur during the cell cycle that lead to daughter cells being genetically identical.[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 2022). The mark or mark type is shown in brackets.

[1] A - metaphase ;
[1] B - anaphase ;
[1] form spindle, apparatus / fibres ;
[1] attached to, centromere / kinetochore ;
[1] move / AW, chromosomes / sister chromatids, to, (spindle) equator / metaphase plate ;
[1] disassembly / shortening, of microtubules / spindle fibres ;
[1] centromeres divide ;
[1] chromatids move to (opposite), poles ; AW
[1] daughter nuclei receive, one chromatid of each chromosome ;
[1] AVP ; e.g. pole to pole spindle fibres lengthen to extend the cell
[1] nuclear envelope (re)forms around each group of chromosomes ;
[1] uncoiling / decondensing / AW, of chromosomes ;
[1] nucleolus / nucleoli (in each nucleus), reform(s) / reappears / visible ;
[1] organelles distributed between two halves of cell ;
[1] cytokinesis / division of cytoplasm ;
[1] cleavage furrow forms ;
[1] AVP ; e.g. ref to ring of microfilaments / contractile ring, draws membrane closer together / AW
[1] semi-conservative replication of DNA ;
[1] idea that each new chromatid is identical to old one / sister chromatids are identical ;
[1] alignment of chromosomes on the, (spindle) equator / metaphase plate ;
[1] chromatids separate so each cell receives a (sister / identical) chromatid of each chromosome ;
[1] centromere(s) split / shortening of spindle fibres ;
[1] sister / identical, chromatids move to (opposite), poles ; AW
[1] checkpoints occur within cell cycle ;
[1] prevents cells entering next stage if there are errors ;
[1] (so that) errors in replication are repaired (by DNA polymerase) ;
[1] to avoid mutations in daughter cells ;