Why is DNA loaded into the negative end of the gel electrophoresis box?

Why is DNA loaded into the negative end of the gel electrophoresis box?

• A. DNA is positively charged.
• B. The gel matrix repels DNA to speed migration.
• C. It helps to heat the sample.
• D. DNA has a negative charge due to phosphate groups.

Answer: (C)

DNA moves in gel electrophoresis because of its negative charge. The phosphate groups in the DNA backbone give the molecule a net negative charge in solution, so when an electric field is applied, DNA fragments are pulled toward the positive electrode. Loading the sample near the negative end places the DNA in position to be drawn through the gel by this field, allowing separation by size as smaller fragments travel faster through the gel’s pores. The other ideas aren’t correct: the gel doesn’t repel DNA, heating isn’t what drives migration, and DNA isn’t positively charged.

DNA moves in gel electrophoresis because of its negative charge. The phosphate groups in the DNA backbone give the molecule a net negative charge in solution, so when an electric field is applied, DNA fragments are pulled toward the positive electrode. Loading the sample near the negative end places the DNA in position to be drawn through the gel by this field, allowing separation by size as smaller fragments travel faster through the gel’s pores. The other ideas aren’t correct: the gel doesn’t repel DNA, heating isn’t what drives migration, and DNA isn’t positively charged.