A-LevelBiologyThe mitotic cell cycle9700/22 · May/June 2024 · Q55 Marks

During interphase of the cell cycle, individual chromosomes cannot be seen within the nucleus. The genetic material is termed chromatin during this stage.

(a)Changes occur to chromatin during mitosis so that chromosomes become visible. State what happens to chromatin so that individual chromosomes can be seen during mitosis.[1]
(b)[Figure 5.1] is a transmission electron micrograph of two human chromosomes at metaphase of mitosis. Describe the structure of chromosomes at metaphase, such as the two chromosomes shown in Fig. 5.1.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[1] coiling / supercoiling / condensation / becomes more compact/AW; e.g. becomes shorter and fatter
[1] each has two, sister / (genetically) identical, chromatids ; R homologous
[1] (chromatids) joined by a centromere ;
[1] each chromatid has a (single) DNA molecule ; A 2 chromatids and 2 DNA molecules
[1] DNA associated with, histone proteins / histones / basic proteins;
[1] telomeres / repeating non-coding sequences, at ends of, chromatids / chromosomes ;
[1] AVP; e.g. chromosomes, highly / most, condensed

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