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What is Severe destruction of DNA can be tested by: and explain in detail?
PCR is an in vitro method for amplifying a selected DNA sequence that does not rely on the biologic (in vivo) cloning method. 
PCR permits the synthesis of millions of copies of a specific nucleotide sequence in a few hours. 
It can amplify the sequence, even when the targeted sequence makes up less than one part in a million of the total initial sample. 
The method can be used to amplify DNA sequences from any source, including viral, bacterial, plant, or animal.

Key concept:
Polymerase chain reaction can even find cases with severe destruction of DNA. 
Ref : Lippincott’s illustrated reviews for biochemistry , 7th edition.