Why is it important to protect DNA from excessive mechanical force?

Correct answer: To prevent DNA shearing

The direct answer is D: protecting DNA from excessive mechanical force helps prevent DNA shearing. Long DNA molecules can be broken by vigorous pipetting, vortexing, shaking, or repeated rough handling. The reasoning is: genomic DNA has very long molecular chains; mechanical force places stress on those chains; stressed chains may snap into smaller fragments; therefore gentle pipetting and mixing help retain high-molecular-weight DNA. Option D is correct because preventing fragmentation is the main reason for gentle handling during isolation. Option A is wrong because gentle handling does not increase RNA; RNA quantity is controlled by different biological and purification factors. Option B is wrong because DNA handling force does not make protein; protein production requires gene expression, translation, and appropriate cellular machinery. Option C is wrong because removing salt requires washing or purification, not simply avoiding mechanical force. The question says “excessive” force: ordinary careful handling is not automatically harmful, but unnecessary vigorous treatment is risky. Memory cue: “handle long DNA like a delicate thread—pulling or shaking can break it.”