What problem can occur if a DNA sample is vortexed harshly?

Correct answer: DNA can shear

The direct answer is D: DNA can shear. Shearing means that long DNA molecules break into shorter pieces because of strong mechanical force. Genomic DNA is especially vulnerable because its molecules are very long. The reasoning is: a long DNA molecule is placed in a liquid; harsh vortexing creates strong mixing forces; those forces can pull or break the long molecules; the sample then contains fragments of different sizes instead of intact high-molecular-weight DNA. Option D is correct because vigorous vortexing can damage long genomic DNA and reduce its quality for applications needing intact molecules. Option A is wrong because mechanical mixing does not make DNA longer; at most it can break it into shorter pieces. Option B is wrong because DNA does not produce light merely from vortexing. Option C is wrong because salt does not disappear; mixing may distribute it, but it does not remove it. Small circular plasmids may tolerate some mixing better than long genomic DNA, but “harsh” handling can still be undesirable. Gentle pipetting and slow mixing are safer when intact DNA is required. Memory cue: “long DNA plus strong force equals possible shearing.”