Question

1. Explain why, when PCR is used to amplify the same region of DNA from two different
people, the size of the DNA fragment(s) generated may be different?

2. What characteristic of the DNA molecule makes it possible to use electrophoresis to
separate DNA molecules by size? Explain why this characteristic is important for
electrophoresis and what part of the DNA molecule creates this characteristic.

3. You are performing PCR. After four cycles of PCR, how many double-stranded copies of
the region that you wish to amplify do you have for every double-stranded copy that
you began with? How many do you have after five cycles?

Answer 1

1. The same region of DNA from two different people may vary in size because of single nucleotide polymorphisms (SNPs) or insertion/deletions of some bases due to random mutation.

2. Occurrence of negative charges in DNA molecules makes it possible to separate DNA molecules by size. So negatively charged DNA moves towards positively charged electrode upon application of electric charges; more the size of DNA, lesser its speed through the gel.

Presence of phosphate groups (PO₃^-) creates this characteristic.

3. For a single double-stranded DNA, number of double-stranded copies produced after n cycles is 2ⁿ.

So after four cycles, number of copies expected is = 2⁴ = 16

After five cycles, number of copies expected is = 2⁵ = 32.