In methods for analyzing DNA, gel electrophoresis separates DNA fragments primarily according to their:

• A. Size (length in base pairs), with smaller fragments migrating farther through the gel
• B. DNA strandedness, with double-stranded DNA migrating farther than single-stranded DNA of the same length
• C. Base composition (the percent of $GC$ content), with higher $GC$ fragments migrating farther
• D. Degree of methylation, with more methylated fragments migrating farther

Answer: (A)

Verified step by step guidance

1. Understand that gel electrophoresis is a technique used to separate DNA fragments by applying an electric field to a gel matrix.
2. Recognize that DNA molecules are negatively charged due to their phosphate backbone, so they move towards the positive electrode during electrophoresis.
3. Know that the gel acts like a sieve, allowing smaller DNA fragments to move more easily and thus migrate farther than larger fragments.
4. Note that factors like DNA strandedness, base composition (GC content), or methylation do not primarily determine migration distance in standard gel electrophoresis.
5. Conclude that the primary factor affecting DNA migration distance in gel electrophoresis is the size (length in base pairs) of the DNA fragments, with smaller fragments traveling farther.