How PCR polymerase chain reaction detect very low amount of DNA
Answers
PCR is basically mimicking what the cell does in nature during the DNA replication:
as you can see the PCR it is made of cycles: after the first dentarutation step needed for the the DNA to unwind and to be single stranded, so that the primers can anneal, the step of annealing>extension>dentaturation are cycled for 30 times (it can be also increased in case your amplicons are very rare, but this will also affect the precision because you may also detect unspecific bands ). As you can see from the picture, at the end of 30 cycles the amount of your amplicon will be 10^9 copies. This makes possible to detect even very low amount, of course the detection will depend not only on the amount of DNA (for example 25 ng of DNA are enough to get a nice gel band ) but also on your primers and on the PCR setting. If the primers are binding very loosely because were badly designed or because in the PCR setting the temperature for the annealing its higher than the melting you will get less or not at all. The annealing temperature its the one at wich the primer can still bind the DNA while the melting its the one at which the primers detach from the DNA. If the temperature its not set properly, your primer won´t be able to bind the DNA. However, if it is too low, the primers will loose specificity, cause they may be able to bind in another pocision of the genome and you will get aspecific bands
How PCR polymerase chain reaction detect very low amount of DNA
Answer =>
PCR helps in amplifying the low amount of DNA which makes multiple copies that help in the diagnosis of low amount of DNA