A-LevelBiologyThe mitotic cell cycle9700/22 · Oct/Nov 2023 · Q17 Marks

During interphase and mitosis of the cell cycle, the chromosomes within a cell go through a number of changes. Each chromosome is composed of DNA complexed with proteins.

(a)In interphase, individual chromosomes are too diffuse (long and thin) to be visible using a microscope. In this stage, the chromosomal material is known as chromatin. State the term used to describe the proteins that are complexed with DNA and form part of chromatin.[1]
(b)When viewed through a microscope, a chromosome is most clearly visible during the metaphase stage of mitosis. Complete Fig. 1.1 to produce a labelled diagram of the metaphase stage of mitosis in an animal cell with two chromosomes.[3]
(c)Outline the changes that occur to the structure and behaviour of a chromosome: from the start of the S phase to the end of interphase during prophase of mitosis.[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 2023). The mark or mark type is shown in brackets.

[1] histones; A basic / histone, proteins
[3] max 2 if the spindle /centrioles extend outside the cell
[3] 1 two chromosomes, each with two chromatids, drawn approximately along central equator area; R if drawn as a, bivalent /homologous pair (close together or further apart) A if equator is drawn across wider diameter
[3] 2 plus one other drawn feature ; e.g. spindle diamond/oval shape, minimum three spindle fibres as shown or spindle fibre from each pole connecting to centromere area of one chromosome or centromere drawn holding sister chromatids together or pair of centrioles at each pole
[3] 3 any three labels ; I label to chromosome e.g. (sister/identical) chromatid/s centromere spindle / spindle fibres spindle equator/metaphase plate A equator if spindle/spindle fibre label given centriole/s A centrosome(s) / pole(s) annotations may contain more than one acceptable label label lines to touch structure
[3] assume sequential interphase to prophase if not named
[3] any three from:
[3] interphase max 2 (chromosomal) DNA replicates ; A DNA synthesis I chromatin R if stated as in, G1/G2
[3] (each new molecule of) DNA, complexes / associates / AW, with, histones / proteins;
[3] sister/identical, chromatids form, qualified ; e.g. after (DNA) replication /during / end of, S phase must be clear they are formed before prophase
[3] prophase max 2 chromatids / chromosome, condense; R if also in prophase A become(s) shorter and fatter A coiling / supercoiling, in context of, DNA / chromosomes / chromatids R chromosomes condense to form chromatin
[3] becomes visible / appears, as two (sister / identical) chromatids ; I composed of two chromatids
[3] becomes attached to the spindle fibres ;
[3] (spindle) microtubules attach to centromere / kinetochore formation;