Question

You decided to run a PCR to amplify a piece of DNA double helix that is 1,000 base pairs in length. How many rounds of PCR would it take (assuming an infinite amount of nucleotides at your disposal) for the total length of DNA obtained to exceed 10,000 km.

Answer 1

The distance between two bases in a DNA is 0.34 nm.

So, the length of 1000 bases long DNA will be (0.34 * 1000) = 340 nm.

So, to obtain DNA of total length 10000 km, you need (10000 km/ 340nm) copies of DNA.

= 2.94 * 1013 copies of DNA.

Now, we know that in PCR, the DNA amplify in a exponential manner. And the relation between the total number of copies after PCR or Nt = N0 * 2n where N0 is the initial copy number of DNA and 'n' is the number of PCR cycle.

Here we have started with a single copy of DNA.

So, 2n = 2.94 * 1013 ,

or, log (2n) = log (2.94 * 1013 ),

or, nlog2 = log (2.94 * 1013 ),

or, n = log (2.94 * 1013 )/log2,

or, n = 44.74.

So, you need to run around 45 cycles of PCR.