Mass is proportional to sequence length.
Let one base pair have mass M.
Thus,
[tex]3.2*10^9 bp[/tex] will have mass [tex](3.2*10^9) * M[/tex]
And the 3.2kb target will have a mass of
For the total amount of amplified target sequence to constitute 50% (by mass) of the total DNA in the reaction mixture, it has to be amplified to such an extent that the total mass of the amplicon matter is also [tex](3.2 * 109) * M[/tex]
PCR amplification happens geometrically, doubling the DNA target with each cycle.
Thus, if the number of cycles in N,
Then,
[tex](3.2 * 10^9) x M = 3.2 x 1000 x M x (2)N\\\\or 10^6 = (2)^N\\\\or 6 = N log2\\\\or N = \frac{6}{log2} = 20[/tex]
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