5.1. Replication and division of nuclei and cells
A subsection of Biology, 9700, through 5. The mitotic cell cycle
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A diagram of a chromosome from a dividing cell is shown in . A dividing cell is at risk of losing genetic material each time DNA replication occurs. On , add a label line and the letter G to show the location on the chromosome of an area that helps to prevent the loss of genes. The chromosome shown in consists of one long DNA molecule associated with histone proteins. Name one stage of mitosis in which a chromosome would have the same general structure as the chromosome shown in . Name the stage in the mitotic cell cycle during which the cytoplasm and the cell divide to produce two genetically identical daughter cells. The control of the cell cycle can be affected by extracellular chemical messengers that bind to proteins and glycoproteins in the cell surface membrane. The overall mechanism is known as cell signalling. State the term used to describe the proteins and glycoproteins that function in this way.
9700_m17_qp_22
THEORY
2017
Paper 2, Variant 2
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In a dividing cell, DNA replication occurs before mitosis. Steps in DNA replication are outlined in . Complete by filling in the gaps using the most appropriate terms. 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). 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 . DNA , the monomers of DNA, are activated with two additional phosphates and are free in the nucleus for the synthesis of the new strands. The bases of the DNA monomers form hydrogen bonds with the bases on each separated parental strand of DNA, according to the rules of . 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. 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 . is a photomicrograph of root tip cells at different stages in the cell cycle. A cell in interphase is labelled. cell in interphase J K L Name the stage of mitosis shown in each of cells J, K and L in . Write your answer in the space next to each letter on . Explain how it is possible to deduce that the labelled cell in interphase shown in is in late, rather than early, interphase. Describe the stage of mitosis shown in cell J.
9700_m18_qp_22
THEORY
2018
Paper 2, Variant 2
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