CBSE 2026 · Region 3 · Set 1 · Q20 · 2 marks
(a)Given below is the schematic representation of the process of electrophoresis. Identify the alphabets representing the :(I)Anode end, and(II)Lightest/Smallest DNA in matrix. 
end (U) $\displaystyle 4$ $\displaystyle \square$ II I $\displaystyle 3$ $\displaystyle \square$ III $\displaystyle 2$ $\displaystyle \square$ I I $\displaystyle 1$ $\displaystyle \square$ I I
end(ii)What is Agarose gel and why is it used in this process ?After performing gel electrophoresis, a student observes that DNA fragments are not visible.(i)Suggest a reason for this and explain how fragments can be made visible.(ii)Also explain how a specific DNA fragment can be collected from the gel for further use.
(a)
Given below is the schematic representation of the process of electrophoresis. Identify the alphabets representing the :
(I)
Anode end, and
(II)
Lightest/Smallest DNA in matrix. end (U)
end
end (U) | $\displaystyle 4$ | $\displaystyle \square$ | II | I |
| $\displaystyle 3$ | $\displaystyle \square$ | III | |
| $\displaystyle 2$ | $\displaystyle \square$ | I | I |
| $\displaystyle 1$ | $\displaystyle \square$ | I | I |
(ii)
What is Agarose gel and why is it used in this process ?
After performing gel electrophoresis, a student observes that DNA fragments are not visible.
(i)
Suggest a reason for this and explain how fragments can be made visible.
(ii)
Also explain how a specific DNA fragment can be collected from the gel for further use.
Marking-scheme solution
(i)
(I)
Anode end is at S
(II)
Lightest/Smallest DNA is shown as R
(ii)
Agarose gel is made from a natural polymer extracted from sea weeds.
Agarose gel provides sieving effect due to which DNA fragments separate according to their size.
(i)
DNA fragments might not be stained or exposed to UV rays,
They can be seen only after staining with ethidium bromide followed by exposure to UV radiation when DNA bands appear orange in colour.
(ii)
The desired DNA fragment is cut from the gel and extracted by elution.
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CBSE Class 12 Biology past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.