Gel electrophoresis principle: which DNA fragments migrate fastest through an agarose or polyacrylamide gel? Select the most accurate statement about fragment mobility.

Introduction / Context:
Nucleic acid gel electrophoresis separates DNA or RNA by size as molecules migrate through a porous matrix under an electric field. Mobility depends primarily on length when conformation and buffer are controlled.

Given Data / Assumptions:

• Agarose or polyacrylamide gels act as sieves with pore sizes dependent on gel concentration.
• DNA carries a near-uniform charge density due to the phosphate backbone.
• Comparison is among linear fragments under standard conditions.

Concept / Approach:
Because DNA’s charge density is roughly constant, the electrophoretic force per unit length is similar across sizes. Frictional resistance increases with length, so shorter fragments experience less hindrance and travel farther in a given time. Gel concentration tunes the resolution range (agarose for kb range; PAGE for small fragments).

Step-by-Step Solution:
Assume linear fragments in the same buffer and gel matrix.Recognize size-dependent sieving: smaller fragments navigate pores more easily.Conclude that small fragments migrate faster and farther than large ones in a fixed time.

Verification / Alternative check:
DNA ladders demonstrate that fragments of 100–500 bp run closer to the dye front than 3–10 kb fragments at the same voltage, corroborating the size–mobility relationship.

Why Other Options Are Wrong:
Large fragments move more slowly. An intact genome is enormous and migrates poorly without prior shearing. A–T content can slightly affect curvature or melting but is not the primary determinant of mobility under standard conditions.

Common Pitfalls:
Comparing supercoiled vs linear forms (conformation matters), and using inappropriate gel percentage for the size range.

Final Answer:
Small fragments.