I've been told that the maximum number of cycles in PCR is between 20 and 30.
Is this true, and what are the reasons for this limitation?
I would draw the line beyond 35, but thats a bit cosmetic. The reasons are manyfold:
If you need a higher sensitivity with more cycles, you can use the technique called "nested PCR". There you do a first round with a primer pair specific for the region of interest and then do a second round with primers which are located slightly to the inside of the amplified DNA. This is done to avoid the amplification of unwanted contaminations. Since you do some 50-70 rounds of PCR amplification in total, this method is extremely sensitive (also to contaminations). See the image from the Wikipedia article for details:
How many cycles of PCR before dNTPs run out?
Assume a 25 μl reaction.
Assume 200 μM dNTPs.
200 μM dNTPs = 200 pmol μl -1
so in 25 μl reaction, there are 5000 pmol of dNTPs
= 5000 x 10-12 x 6 x 1023 molecules
= 3 x 1015 molecules dNTP
Assume that we start with 1 molecule of a 1000 bp template, 50% GC
1 kb = 2000 nucleotides
So , how many of these molecules can we construct using 3 x 1015 molecules dNTP?
= 3 x 1015/2000
= 1.5 x 1012 DNA molecules
How many cycles of PCR to produce this many from a single template molecule?
2n = 1.5x 1012
nlog2 = log(1.5) + log(1012)
nlog2 = 12.18
n = 12.18/ log2 = 41 cycles
Of course this in an absolute upper limit. The estimate assumes that you start with one template molecule of 1 kb, and that dNTPs aren't being hydrolysed, or otherwise degraded.