CfE Higher Sht1_3A
HBiolSheet 1_3A
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1. Free DNA nucleotides are needed for DNA replication. Name one other substance that is needed for DNA replication.
2. The arrangement of DNA in bacteria, yeast (fungi) and eukaryotic cells differs. In your jotter construct a table to compare the organisation of DNA in prokaryotic and eukaryotic cells. Your table should include information about genetic material outside the nucleus, refer to the manner in which DNA is packaged, the role and type of proteins involved in packaging and the shape of any chromosomes.
Use a separate column for bacteria, fungi and animal/ plant cells.
3. During DNA profiling, the polymerase chain reaction (PCR), can be used to amplify a sample of DNA.
The diagram below shows how substances X, Y and Z are involved in the PCR.
It also gives the temperature treatments T1, T2 and T3 at various stages.
a) Name substances X, Y and Z.
b) The statements below describe changes in temperature during the PCR reaction. Answer the questions which follow.
A – heated to 90–95 °C
B – heated to 75 °C
C – cooled to 55–60 °C
D – cooled to 4 °C
i) Which of the changes occurs at T1?
ii) Which of the changes occurs at T2?
iii) Which of the changes occurs at T3?
c) Suggest why DNA polymerase from human sources is not suitable for use in a PCR machine.
d) Explain why species of plants cannot be identified from woody (xylem) material using the PCR technique.
4. During the polymerase chain reaction (PCR), a solution of DNA molecules is heated to separate the double helix into single strands. This “melting” of DNA can be monitored by measuring how much UV light the solution absorbs. By the time the melting is complete tand all the double stranded DNA is unpaired, UV absorption almost doubles (Figure 1). The melting temperature (Tm) of the DNA is taken to be the one where half the base pairs have separated. Tm depends on the number of G-C base pairs in the sample (Figure 2).
a) What is the function of the polymerase chain reaction?
b) What is the value of Tm for the sample shown?
c) What percentage of A-T base pairs does the sample contain?
d) What evidence is there that G-C base pairs confer stability on the DNA double helix?
e) The G-C base pair is held together with three hydrogen bonds and an A-T base pair only contains two hydrogen bonds. Predict the effect of increasing the amount of A-T base pairs in a strand of DNA would have on Tm. Explain your answer.
