What is the function of proteinase K in DNA extraction?

Contaminating proteins that may be present during DNA extraction need to be removed. Nucleases, which are enzymes that catalyze the breakdown of nucleic acids, may also be present and need to be removed. 

Proteinase K is a broad spectrum serine protease. It is used during DNA extraction to digest the contaminating proteins and degrade the nucleases that may be present, thus protecting the nucleic acids from contamination and nuclease attack.

In addition, proteinase K is stable over a wide range of pH, making it suitable for use in DNA extraction.