What is the purpose of gel electrophoresis in molecular biology?

• A. To sequence DNA.
• B. To separate DNA fragments by size for analysis or identification.
• C. To amplify DNA.
• D. To transcribe RNA.

Answer: (B)

The main concept is separating DNA fragments by size using a gel and an electric field. DNA carries a negative charge, so when placed in a gel and subjected to current, all fragments migrate toward the positive end. The gel's porous matrix acts like a sieve: smaller fragments move through the pores more quickly than larger ones, so fragments end up at different distances from the start. This creates a band pattern that reflects fragment lengths, allowing you to estimate sizes, compare fragments produced by restriction digestion or PCR, and identify samples by matching to a DNA ladder. This is about analyzing and sizing fragments, not about determining their sequence, amplifying DNA, or transcribing RNA.