IB past paper biology

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Hey, I’m not familiar with this concept
The answer is D
Can anyone explain?

This is a southern blot, Its probably in Ari’s biology book in the Biotechnology chapter. I need to review this subject myself so I can’t give you a full answer, but basically this is it:
DNA has a negative charge due to the phosphate groups in each nucleotide. therefore, if you put DNA fragments in an electrophoresis device witch contains a thick gel substance, and a positive charge at the other end of the plate. Then all DNA molecules will be attracted and move towards the positive end, but the smaller molecules will go through the gel more quickly. its a way to differentiate and recognize molecules…

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The figure depicts about the gel electrophoresis, where negatively charged DNA fragments are pulled from one end of a groove to the other end by the positive electrodes, therefore, the distance each fragment reach is directly proportional to how strong the charge is and inversely proportional to the size of the fragment.

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