In gel electrophoresis, why do DNA fragments separate by size?

In gel electrophoresis, why do DNA fragments separate by size?

• A. Smaller fragments move faster through the gel
• B. Larger fragments move faster through the gel
• C. Fragment charge determines migration
• D. pH of gel determines migration independently of size

Answer: (A)

DNA fragments separate by size in a gel because the gel acts as a porous matrix that functions like a sieve. When an electric field is applied, all fragments move toward the positive electrode thanks to their negative phosphate backbone, but they don’t travel at the same speed. Smaller pieces slip through the gel pores more easily and encounter less resistance, so they migrate faster. Larger pieces have more trouble moving through the network of pores, facing greater friction and getting delayed, which creates distinct bands by size. While DNA’s charge is important for movement, in typical gels the charge-to-length is fairly uniform, so size-related resistance through the gel is the dominant factor. The idea that migration is determined solely by charge or independently of size (via pH alone) isn’t accurate in this context.

DNA fragments separate by size in a gel because the gel acts as a porous matrix that functions like a sieve. When an electric field is applied, all fragments move toward the positive electrode thanks to their negative phosphate backbone, but they don’t travel at the same speed. Smaller pieces slip through the gel pores more easily and encounter less resistance, so they migrate faster. Larger pieces have more trouble moving through the network of pores, facing greater friction and getting delayed, which creates distinct bands by size. While DNA’s charge is important for movement, in typical gels the charge-to-length is fairly uniform, so size-related resistance through the gel is the dominant factor. The idea that migration is determined solely by charge or independently of size (via pH alone) isn’t accurate in this context.