| | Re: Extremely desperate noob question: How do these PCR work?
A thermocycler does exactly what it says on the tin: it cycles thermo i.e. temperature. PCR requires three temperatures to be cycled through several times (usually 25-40 in my case). It goes 94 degrees for, say, 30 seconds, then 45-65 degrees for again, say, 30 seconds, and then 68-72 degrees for whatever length of time required depending on the length of DNA. So that's how the machine works. It doesn't replicate the DNA. The polymerase enzyme does it with the help of the primers to get it started and the nucleotides as building blocks. In order to see if it worked you need to run the sample on a gel and visualise the DNA, or send it off to be sequenced.
To check if a fruit has been genetically modified (assuming you did the genetic modification yourself and know exactly what mutation/insertion you're looking for) you would get the DNA out of the fruit, amplify the region you're interested in with PCR and then send the result off for sequencing.
I hope this is of use to you.