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DNA sequencing exam pack
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Biology OCR A; Module-6, DNA sequencing questions and answers
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1 DNA profiling uses techniques to separate lengths of DNA to produce a profile that is unique to each individual.Explain why only selected sections of non-coding DNA are used when profiling a human.
[3]
2 Tissue traces from a crime scene often need to be identified. DNA from the tissue is ‘amplified’ by the
polymerase chain reaction (PCR) to get samples large enough for further analysis.
Modern PCR technique uses DNA polymerase from the bacterium Thermus aquaticus. Why is this enzyme
chosen?
[2]
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, 3(a) In order to sequence the whole genome of an organism it may be necessary to sequence billions of nucleotides.
The human genome is approximately 3.2 billion nucleotides long.
Sequencing DNA requires a series of steps.
Place the following steps in the correct sequence. The first and last ones have been done for you.
A place sections in order by matching overlapping regions
B cut DNA into sections of varying length
C sequence short sections of DNA
D amplify the DNA (create many copies)
E extract samples of DNA from cells
E __________ __________ __________ A
[2]
The development of high-throughput sequencing techniques has enabled whole genomes to be sequenced more
rapidly. Table 17.1 compares a number of DNA sequencing techniques.
Technique Rate of sequencing (Mb Maximum length of Typical number of errors
–1
day ) nucleotide chain per 100 000 nucleotides
sequenced
Sanger (chain termination 6 1000 5
technique)
Roche pyrosequencing 750 500 50
SOLiD 5000 50 500
Helicos 5000 32 1000
Table 17.1
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, (b) Roche pyrosequencing relies on building a chain of nucleotides against a template. It involves the following
steps:
Nucleotides are washed over the template in a specific order.
When the correct nucleotide is present it joins the new chain.
The addition of a nucleotide to the chain releases energy.
The energy is used to activate a protein called luciferin.
Light released by luciferin is detected.
If two identical nucleotides are added together then the intensity of the light emitted is doubled.
Fig. 17.1 shows a readout from a pyrosequencing machine.
Read off the sequence of bases in the length of DNA.
[1]
(c) The protein coded for in a gene is 200 amino acids in length. How many errors could be expected in the exons of
the sequenced gene when using the least accurate sequencing technique shown in Table 17.1.
Answer_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ [2]
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