How can PCR detect disorders in early stages ?

By amplification of DNA.

PCR was invented by Kary Mullis in 1983. PCR requires a DNA primer designed for the DNA template and DNA polymerase, preferably a thermostable DNA polymerase. In the polymerase chain reaction, the reaction cycle is repeated many times to get billions of copies.

Polymerase chain reaction or PCR is a reaction that is utilised to amplify a gene or fragment of DNA of interest. It is done in vitro using a primer. This technique is used in labs to make billions of copies of the desired gene for research, diagnostic and therapeutic purposes.