The genome of virus SV40 is a circular double?stranded DNA molecule 5 kb long (i
ID: 30614 • Letter: T
Question
The genome of virus SV40 is a circular double?stranded DNA molecule 5 kb long
(i.e., it contains 10,000 bases).
a. A 1 kb region is amplified by PCR for four cycles. (Assume that specific
amplification of the target region occurs.) What fraction of the total DNA
does the target sequence constitute? Show your work.
b. After many generations of growth in medium containing 14N, the medium
was replaced with new medium containing 15N. After five generations, what
is the ratio of light:heavy DNA strands? Show your work
Explanation / Answer
a. Most PCR methods typically amplify DNA fragments of between 0.1 and 10 kilo base pairs .
Total DNA is double stranded and 5kb long; 5kb means 5000bp; means 10000 bases. Fraction of total DNA in target DNA= 1/5; it can also be said that 2000 bases or 1000base pairs are present in the target DNA.
b. After growing for many generation in N14, all DNA strands will become light; hardly any light DNA strand will be left.
After replacing the medium with N15, ratio of heavy: light strand will be 1:1(in first generation,if 2 strands were there, after PCR 4 strands will be formed, of which 2 will be heavy and 2 will be light.)
In 2nd generation, 8 strands will be formed, of which 6 will be heavy and 2 will be light.
In 3rd generation, 16 strands will be formed, of which 14 will be heavy and 2 will be light.
In 4th generation, 32 strands will be formed, of which 30 will be heavy and 2 will be light.
In 5th generation, 64 strands will be formed, of which 62 will be heavy and 2 will be light.
Thus, ratio of light to heavy strands after 5th generation will be 1:31
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