pcr

Having more GC pairs in a primer means a greater Tm. Likewise, I read that having more GC are more prone to errors in PCR. However, the answer to this question, without explanation, says having more GC pairs is more suitable and when it flanks the 5' and 3' ends. Thoughts? Thank you!

Okay. This may be a really stupid question and I may be thinking to hard but for PCR, if you have a broken or impure sample of a DNA, doesn't that mean that it is not the whole DNA? As in, it is only a SEGMENT of a DNA? So say if one performs a PCR and takes that broken/impure sample of DNA...