In DNA agarose-gel electrophoresis, how do larger DNA fragments migrate and where do they typically appear in the gel photograph?

Difficulty: Easy

Correct Answer: Slowly, appearing nearer the top (near the wells)

Explanation:


Introduction / Context:
DNA agarose-gel electrophoresis separates fragments based on size as they migrate through a porous matrix under an electric field. This question checks understanding of how fragment length influences mobility and band position in a standard gel photo.


Given Data / Assumptions:

  • Agarose gel with wells at the top; DNA is loaded into wells.
  • Electric field drives negatively charged DNA toward the bottom (anode).
  • All fragments have similar charge density (phosphate backbone).


Concept / Approach:
Mobility in a sieving matrix is inversely related to fragment length. Larger fragments experience more frictional resistance and thus migrate more slowly than smaller fragments.


Step-by-Step Solution:
1) DNA is negatively charged and moves toward the anode at the bottom of the gel.2) Agarose forms pores; migration speed depends on ability to thread through pores.3) Larger fragments are hindered more, so they travel a shorter distance in a fixed time.4) Therefore, large bands remain closer to the wells at the top in the gel photograph.


Verification / Alternative check:
Standard DNA ladders on gels show high-molecular-weight bands near the wells and low-molecular-weight bands farther down, confirming the rule.


Why Other Options Are Wrong:
They confuse speed and position: fast fragments end up near the bottom; slow fragments remain near the top. Mixing these yields incorrect pairings.


Common Pitfalls:
Assuming charge differences dominate; for DNA, charge per base pair is approximately constant, so size-dependent sieving is the key factor.


Final Answer:
Slowly, appearing nearer the top (near the wells).

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