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Question
A plasmid DNA and a linear DNA (both are of the same size) have one site for a restriction endonuclease. When cut and separated on agarose gel electrophoresis, plasmid shows one DNA band while linear DNA shows two fragments. Explain.
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Solution
Restriction digestion of a circular plasmid DNA :
Plasmid DNA is a circular molecule. On being cleaved by a restriction endonuclease at a single site, it becomes a single linear molecule.
Therefore, it shows a single band in gel electrophoresis.
Restriction digestion of a linear DNA :
Linear DNA has two ends. When it is digested by a restriction enzyme at one site, it results in the production of two linear DNA fragments. Therefore, two bands are observed on the agarose gel.
Diagrammatic representation :
Plasmid DNA is a circular molecule. On being cleaved by a restriction endonuclease at a single site, it becomes a single linear molecule.
Therefore, it shows a single band in gel electrophoresis.
Restriction digestion of a linear DNA :
Linear DNA has two ends. When it is digested by a restriction enzyme at one site, it results in the production of two linear DNA fragments. Therefore, two bands are observed on the agarose gel.
Diagrammatic representation :
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