Biology · Biotechnology: Principles and Processes · NEET
DNA carries a negative charge because of the phosphate groups in its sugar-phosphate backbone. Opposite charges attract, so the negatively charged DNA is pulled towards the positive electrode, which is the anode. This is why DNA samples are always loaded in the wells at the cathode (negative) end so they travel across the gel to the anode.
Smaller fragments move FARTHER. The agarose gel is a network of tiny pores that acts as a molecular sieve. Small fragments slip through the pores easily and travel a long distance. Large fragments get held back by the mesh and stay near the wells. So on the finished gel, the smallest fragments are nearest the anode and the largest are nearest the wells.
Agarose forms a gel with a mesh of small holes, like a sieve. When DNA is pushed through by the electric field, this mesh resists larger fragments more than smaller ones. This size-based resistance is what separates fragments of different lengths into distinct positions (bands) on the gel.
Yes. A higher agarose concentration makes a tighter mesh with smaller pores, which slows DNA down and better separates small fragments. A lower concentration makes larger pores, better for separating big fragments. In NEET, remember gel concentration DOES affect movement — a statement saying it has no effect is INCORRECT (asked in NEET 2019).
The separated fragments appear as bands. They are stained with ethidium bromide and viewed under UV light as bright orange bands. A specific band can then be cut out and the DNA recovered from the gel piece — this recovery step is called elution. The purified DNA is then used to make recombinant DNA.
What is the criterion for DNA fragments movement on agarose gel during gel electrophoresis?
Statement I: The DNA fragments extracted from gel electrophoresis can be used in construction of recombinant DNA. Statement II: Smaller size DNA fragments are observed near anode while larger fragments are found near the wells in an agarose gel. Choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
DNA moves toward the anode (positive electrode) because DNA is negatively charged due to its phosphate backbone, and opposite charges attract.
The agarose gel acts as a molecular sieve. Smaller fragments pass through the pores with less resistance, so they move a greater distance than larger fragments in the same time.
It separates DNA fragments on the basis of their SIZE (length in base pairs), through the sieving effect of the agarose gel.
The largest fragments remain near the wells at the cathode end because the gel mesh slows them down the most.
The direction of movement is decided by charge (all DNA goes to the anode), but the actual separation into bands is by size. Do not confuse the two — this is a common NEET trap.
