2025 ASCP Molecular Biology MB Technologist Practice Exam – Complete Prep

Magnesium ions, commonly provided in the form of magnesium chloride (MgCl₂), play a critical role as a cofactor in PCR reactions primarily for the polymerase enzyme. DNA polymerases, which are essential for the synthesis of new DNA strands during PCR, require magnesium to function effectively.

The presence of magnesium promotes the correct binding of the nucleotides (dNTPs) to the active site of the polymerase, aiding in the catalytic process of DNA synthesis. It stabilizes the negatively charged phosphate groups of the dNTPs, facilitating the formation of phosphodiester bonds between nucleotides thereby enabling the elongation of the newly synthesized DNA strand.

While the buffer is important for maintaining the pH and ionic environment of the reaction, and the dNTPs and primers are essential components of the PCR process, neither of these directly requires magnesium as a cofactor for their functionality. Thus, the role of magnesium as a cofactor specifically pertains to the polymerase enzyme's activity in aiding DNA synthesis, making it the correct association in this context.