Which method specifically assesses DNA fragment sizes and copy number, not just presence of a sequence?

• A. PCR-based detection
• B. Sanger sequencing
• C. Microarray analysis
• D. Southern blot

Answer: (D)

Measuring both how big DNA fragments are and how many copies of a sequence exist requires a method that shows fragment sizes directly and also reflects copy number. Southern blotting does this: DNA is cut with restriction enzymes, separated by size on a gel, transferred to a membrane, and probed for the sequence of interest. The band positions reveal fragment sizes, while the number and intensity of bands indicate how many copies of that sequence are present.

PCR-based detection focuses on whether a sequence can be amplified, not on the actual fragment lengths in the genome. Sanger sequencing provides the exact nucleotide sequence of a defined DNA fragment but doesn’t reveal how many genomic copies exist or the sizes of other fragments in the genome. Microarray analysis can detect copy number variations across many loci, but it infers copy number from hybridization signals rather than giving precise fragment sizes of genomic DNA.