D. why are both the gene of interest and the plasmid cut with the same restriction enzyme? e. what is the role of dna ligase in this process? 11. why is the dna sample to be separated by gel electrophoresis always loaded at the cathode or negative end of the power source? 12. explain why shorter dna molecules travel farther down the gel than larger molecules. 13. the polymerase chain reaction (pcr) is a nobel prize–winning idea that is used by scientists to amplify dna, particularly when the quantity of dna is very small or contaminated. explain the three initial steps that occur in cycle 1 of pcr.