A-LevelBiologyNucleic acids and protein synthesis9700/23 · May/June 2017 · Q512 Marks

Describe the process of DNA replication.

(a)Describe the process of DNA replication.[5]
(b)State the name of the part of the chromosome that prevents the loss of genes during DNA replication.[1]
(c)(i)During DNA replication, the use of an incorrect base in the newly synthesised strand can lead to a mutation. A transversion event is where a pyrimidine is used in the newly synthesised strand instead of a purine, or the other way round. Name the two possible bases that could be used instead of cytosine in a transversion event.[1]
(c)(ii)A transition event is where an incorrect purine is used or an incorrect pyrimidine is used. Suggest why transversion events are less likely to occur than transition events.[2]
(d)Outline how mutations can cause healthy cells to become tumour cells.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[max 5] 1 DNA (double helix) unwinds; A uncoils I unzips R DNA strand unwinds 2 hydrogen bonds break between, base pairs / bases / strands ; 3 both strands used as templates; 4 catalysed by / AW, DNA polymerase ; 5 ref. to (free) activated nucleotides / AW ; 6 complementary (DNA) nucleotides added ; R RNA nucleotides A described in terms of complementary base pairing 7 step-by-step / sequentially / AW ; 8 idea that process continues, along whole DNA molecule ; 9 replication bubbles / described or ref. to Okazaki fragments ; 10 replication is semi-conservative / each newly formed molecule contains one original and one newly synthesised strand 11 AVP; e.g. ref. to repair / proofreading ref. to, helicase / ligase in correct context
[1] telomere(s);
[1] adenine and guanine ;
[max 2] idea that purines and pyrimidines are different sizes / two rings and one ring; purine normally bonds with pyrimidine (to maintain DNA double strand width); idea that two purines or two pyrimidines will distort the double helix width (in a transversion event); ora AVP; e.g. (transversion event) more likely to be detected by the repair mechanism ora
[max 3] idea that mutation occurs for a gene controlling cell division ; detail; e.g. proto-oncogene to oncogene tumour suppressor gene switched off ; ref. to disruption of cell cycle / shortened interphase ; (results in) uncontrolled cell division; I uncontrolled growth other detail of result of mutation; e.g. divide indefinitely / no programmed cell death do not respond to signals to stop dividing loss of, specialisation / function