To separate genomic DNA fragments by size, which laboratory method is most useful?

Electrophoresis is the ideal method for separating genomic DNA fragments by size due to its principle of using an electric field to move charged molecules through a gel medium. In this process, DNA fragments are placed in a gel matrix and subjected to an electric current. Because DNA is negatively charged, it migrates towards the positive electrode. Shorter DNA fragments can move through the gel more easily and quickly than longer ones, allowing for the separation based on size. The result is distinct bands representing different sizes of DNA fragments, which can then be visualized.

In contrast, polymerase chain reaction (PCR) is a technique primarily used to amplify specific DNA sequences rather than separate them. While it is crucial in the process of preparing samples, it does not sort DNA by size. Western blotting focuses on proteins, utilizing specific antibodies to identify them, making it irrelevant for DNA fragment separation. Microarray analysis is used for simultaneously measuring the expression levels of multiple genes or for genotyping, but again, it does not pertain to the physical separation of genomic DNA based on fragment size. Therefore, electrophoresis stands out as the most suitable method for this purpose.