Q.Given below is the diagram representing the observations made for separating DNA fragments by Gel electrophoresis technique. Observe the illustration and answer the questions that follow :
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Start your 14-day free trial to unlock the full solution →Gel electrophoresis separates DNA fragments by size because DNA is negatively charged and moves toward the positive electrode (A → B). The medium is agarose gel, and fragments are visualised using ethidium bromide staining under UV light.
The Concept: Why DNA Moves in an Electric Field
DNA is a highly negatively charged molecule due to its phosphate backbone. When placed in an electric field, it will migrate toward the positive electrode. In gel electrophoresis, the gel acts like a molecular sieve — smaller fragments slip through the pores easily and travel farther, while larger fragments get tangled and lag behind. This is why we see a ladder-like pattern of bands, with the smallest fragments closest to the positive end.
Now let’s break down each part of the question.
(a) Why are the DNA fragments seen to be moving in the direction A → B?
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Charge determines direction. DNA molecules carry a uniform negative charge along their sugar-phosphate backbone. In an electric field, opposite charges attract — so DNA moves toward the positive electrode.
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Identify the electrodes. In the diagram, direction A → B points toward the positive terminal (anode). The negative terminal (cathode) is at the starting end (near A). So fragments start near the negative end and migrate toward the positive end.
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Result. The movement from A to B is simply because DNA is negatively charged and the positive electrode is at B.
A common mistake is to think DNA moves toward the negative electrode because "DNA is negative." Remember: opposites attract — negative DNA goes to positive anode.
(b) Write the medium used on which DNA fragments separate.
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The medium is a gel. For DNA separation, the most common medium is agarose gel, a polysaccharide extracted from seaweed. It forms a porous matrix when cooled.
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Why agarose? Its pore size is ideal for separating DNA fragments ranging from about 100 bp to 20 kb. For smaller fragments (like in sequencing), polyacrylamide gel is used instead.
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Answer. The medium is agarose gel (or simply "gel" in context).
Agarose concentration controls pore size: higher % agarose = smaller pores = better separation of small fragments. For typical genomic DNA work, 0.8–1.2% agarose is standard.
(c) Mention how the separated DNA fragments can be visualised for further technical use. …
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