Dear Ayush 

Chelex 100 is an ion exchange resin that is added as a 5% solution (wt/vol).

Chelex is composed of styrene divinylbenzene copolymers containing paired iminodiacetate ions that act as chelating groups in binding polyvalent metal ions such as magnesium (Mg2+).

By removing the Mg2+ from the reaction, nucleases are inactivated and the DNA is protected.