What is everything we need to know about PCR?
You need to know the exact steps of how it works, and apply it to the situation given.
The steps in paragraph form are:
DNA fragments would be heated up to around 90-95°C to denature them into single-stranded fragments. The DNA is then cooled to 55°C and free DNA nucleotides and DNA primers are added to allow synthesis of the strand. This is then heated up to 72°C and taq polymerase is added, allowing the new strands to be synthesised as double-stranded segments. This process is then repeated millions of times, allowing millions of copies to be produced from the original DNA strand.
It appears a lot on exams so it's useful to know its application.