Some Questions about PCR - ... based von Wikipedia article (Mar/12/2006 )
I found following picture at wikipedia (the world of "superficial knowledge" - in a positiv AND negativ meaning), and that posed some questions, because im don't feel certain in the field of molecular methods yet.
1) How are the Overlapping ends cut off?
2) Isn't it an idealistic depiction, that the template-DNA always rearranges? The probability that one of the smaller DNA pieces adheres is much hicgher, isn't it? [maybe thes just want to show how the amount of DNA develops - but i think its mesleadinng - like the lengs of the template DNA, thatsschnging and is at one time shorter than the PCR product]
I also have a question about the possible length of the product:
In the german article (which has a "featured status") it is said, that the maximum lentgh ist 1-4 kbp, the english version says (like the italion text) that the maximum length is 10-40 kbp.
I'm not sure what to believe.
Also the time mullins "re-invented" the PCR is differing. in the portugees-version they say it was 1980 - strange,....
thanks for your help
The drawing is accurate for the first round of PCR, but you're right -- it is a bit misleading (or confusing). You're also right in suspecting that the probability that one of the smaller DNA pieces (aka the primers) will adhere is much higher than the template-template arrangement -- it is this fact that makes PCR work so well.
So, after the first round of PCR, we have the situation depicted in part 4 of the figure. The next cycle remelts these structures, and the primers rebind to the separated strands. The DNA is extended to the end of the template (which is now defined as the 5' end of the opposite primer), and the polymerase falls off. After a cycle or two, the number of templates that are like those depicted in the right hand side of part 4 vastly out numbers anything like that on the left hand side of part 4.
There is no cleavage involved (though the way the drawing is rendered does suggest that there is), it's just that the products produced that end in the primers (as depicted below part 4 of the figure) outnumber the other situation by a huge amount.
Kary B. Mullis was awarded the 1993 Nobel prize for the PCR reaction (see here). PCR was invented in 1983 (see here), and the first publication was in 1985 (see here).
Since I have personally produced products far in excess of 4 kb using regular Taq, I'd ignore the statement in the German article.