In a dividing cell, DNA replication occurs before mitosis. Steps in DNA replication are outlined in Fig. 6.1. [Figure 6.1]. Fig. 6.2 is a photomicrograph of root tip cells at different stages in the cell cycle. A cell in interphase is labelled. [Figure 6.2]

1. (a)Complete Fig. 6.1 by filling in the gaps using the most appropriate terms. 1 Helicase enzyme allows the DNA double helix to unwind and the hydrogen bonds between the two strands to break, exposing the four bases, (A), (T), (C) and (G). 2 An enzyme molecule attaches to each of the two separated parental strands. The two enzyme molecules move in opposite directions, each catalysing the formation of a new strand of DNA. This enzyme is known as . 3 DNA , the monomers of DNA, are activated with two additional phosphates and are free in the nucleus for the synthesis of the new strands. 4 The bases of the DNA monomers form hydrogen bonds with the bases on each separated parental strand of DNA, according to the rules of . 5 One DNA strand is synthesised continuously and the other is synthesised in sections known as Okazaki fragments. The fragments are joined by an enzyme, , which catalyses the formation of phosphodiester bonds. 6 The result of replication is two DNA molecules, each one containing an original parental strand and a newly synthesised strand. This type of replication is described as .[6]
2. (b)(i)Name the stage of mitosis shown in each of cells J, K and L in Fig. 6.2. Write your answer in the space next to each letter on Fig. 6.2.[3]
3. (b)(ii)Explain how it is possible to deduce that the labelled cell in interphase shown in Fig. 6.2 is in late, rather than early, interphase.[1]
4. (b)(iii)Describe the stage of mitosis shown in cell J.[2]

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