general question about PCR

A.F. Simpson AFS7 at
Wed Nov 17 13:39:32 EST 1999

jennnn1970 at wrote:

> I understand the process of collecting tail DNA, making of primers, the
> thermalcycles and such. I just don't understand how you'd get a bunch
> of fragments to run on a gel at the end of the PCR?

As other people have said, the easiest way to understand it to draw it
out.  I didn't really understand PCR until I'd filled a few pages of a
large notebook with the first few cycles of a reaction.  (I also have a
rather large piece of ASCII drawing which illustrates it.)

The basic answer is that the amount of full length template remains the
same (the amount originally added), the amount of long transcripts
increses linearly (because it can only come from the full length
template) whilst the amount of product between the primers increases
exponentially (because it can be produced from all the products of each
round of amplification, thereby doubling the pool of correct product
with each round).

There _is_ product of different sizes produced during PCR.  However,
because the amount of product of the desired size increses
exponentially, it completely overwhelms the other oddments produced.

An analogy is the old story of a general who, after a great victory, was
asked for a reward.  He said that he would be happy with grain - one
grain placed on the first square of a chessboard, two on the second,
four on the third, eight on the fourth, sixteen on the fifth,
etc.  If was quickly discovered that this amounted to rather more grain
than was in the entire kingdom.

Compare this to the amount of rice you would have if you placed a single
grain on every square - that's the difference between exponential and
linear growth.

> Jen :)


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