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DNA fragments during gel electrophoresis, separate (resolve) according to their size due to sieving effect provided by agarose gel.

1. Loading buffer is used in gel and SDS-PAGE electrophoresis in order to increase the density of the sample. DNA loading buffers contains a coloured dye and a density agent. The density agent serves to enhance the density of the DNA sample allowing the DNA to sink into the bottom of the well. The dye adds visibility to the DNA sample and also serves as a tracking dye allowing the user to monitor the DNA migration during electrophoresis.
2. DNA fragments separate due to their charge.
3. DNA fragments separate due to their shape.
4. DNA fragments separate due to their sequence.

Correct answer: Loading buffer is used in gel and SDS-PAGE electrophoresis in order to increase the density of the sample. DNA loading buffers contains a coloured dye and a density agent. The density agent serves to enhance the density of the DNA sample allowing the DNA to sink into the bottom of the well. The dye adds visibility to the DNA sample and also serves as a tracking dye allowing the user to monitor the DNA migration during electrophoresis.

Solution

DNA fragments are resolved in agarose mainly by size because the gel acts as a molecular sieve. The loading buffer’s role is practical: it adds density so the sample sinks into the well and includes dye to visualize and track the run.

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