Magnesium chloride is used as a source of magnesium ions in various molecular biology applications like PCR reactions and for the preparation of competent cells for transformation. It is an essential co-factor in many enzymes, including DNase, some restriction enzymes, and Ribonuclease H.DNase/RNase not detected. Magnesium chloride is used as a source of magnesium ions in various molecular biology applications like PCR reactions and for the preparation of competent cells for transformation. It is an essential co-factor in many enzymes, including DNase, some restriction enzymes, and Ribonuclease H.DNase/RNase not detected.