A-LevelBiologyThe mitotic cell cycle9700/22 · Feb/Mar 2021 · Q510 Marks

Each meristem cell in a leaf bud is able to grow and divide by mitosis to produce two daughter cells that are genetically identical to each other and to the original dividing cell.

(a)(i)Fig. 5.1 lists the stages in the mitotic cell cycle of a meristem cell. Outline and explain the events occurring during S phase, metaphase and anaphase of the mitotic cell cycle that are important in the production of genetically identical daughter cells. [Figure 5.1][4]
(a)(ii)Immediately after cytokinesis, daughter cells are not identical even though they are genetically identical. Suggest a reason why daughter cells are not identical immediately after cytokinesis.[1]
(b)(i)Some of the cells resulting from mitotic division in the young leaf form elongated cells that develop into xylem vessel elements. The xylem vessel elements are joined end to end to form xylem vessels. Suggest the structural changes that occur when elongated cells develop into xylem vessel elements and explain how these changes help xylem vessels to perform their function in transport.[4]
(b)(ii)Fig. 5.2 is a plan diagram of a transverse section through a dicotyledonous leaf. Add a label line and the letter X to Fig. 5.2 to identify the location of xylem tissue. [Figure 5.2][1]

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Mark scheme

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

[4] 5(a)(i) if one phase not attempted, max 3 any four from: S phase: 1 DNA replication; explanation: 2 produces two genetically identical, DNA molecules (one for each daughter cell ; A (sister) chromatids for DNA molecules A description of semi-conservative replication metaphase: 3 chromosomes / pairs of sister chromatids align / orientate at (spindle) equator; A metaphase plate for equator; explanation: 4 chromosomes orientated so that sister chromatids will be shared out to daughter cells / AW ; anaphase: 5 centromere splits / spindle fibres shorten / AW ; explanation: 6 so that daughter chromosomes / sister chromatids / identical chromatids, move to opposite poles ; 7 AVP; e.g. ref. to checkpoints and ref. to error, detection / repair ref. to kinetochore / centromere, and attachment to spindle fibres at, metaphase / anaphase
[1] 5(a)(ii) unequal sharing out of, cytoplasm / organelles / named organelles, (at cytokinesis) ; A uneven sizes
[4] 5(b)(i) max 3 structural changes: 1 ref. to function of xylem ; e.g. transport of water and (dissolved) mineral ions transport of water from roots to other parts of the plant in context of leaf xylem receipt of water from roots 2 end walls broken down ; 3 (so) tubes form (to transport water long distances); 4 (so), uninterrupted / unhindered / continuous, flow ; 5 ref lignification ; 6 lignin strengthens vessels ; 7 ref. to cell death ; 8 cells, become hollow / lose contents / lose cytoplasm or organelles / named organelles, digested / broken down ; 9 greater quantity of water can flow (per unit, time / length) ; 10 AVP; e.g. tonoplast/vacuole, breakdown / rupture, to release hydrolytic enzymes ref. to xylem patterning during growth ref, to pits
[1] 5(b)(ii) label line to xylem, labelled X ;