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Biology · 2026 · 3 marks
Biotechnology : Principles and ProcessesProcesses of Recombinant DNA Technology3 marksAnalyseshort answer
CBSE 2026 · Region 1 · Set 1 · Q28
Observe the given picture carefully. A mixture of DNA with fragments ranging from 100 base pairs to 1800 base pairs were separated by electrophoresis…
Observe the given picture carefully. A mixture of DNA with fragments ranging from $\displaystyle 100$ base pairs to $\displaystyle 1800$ base pairs were separated by electrophoresis on agarose gel with the following arrangement:
(a)
What result will be obtained in staining with ethidium bromide? Explain with reasons.
(b)
The above setup was modified as shown below and a band with $\displaystyle 100$ base pairs was obtained at X.
What changes were made to the previous design to get a band at 'X'. Why did the band appear at 'X' ?
Marking-scheme solution
(a)
• No bands will be seen in agarose gel.
• DNA fragments being negatively charged will not move towards negative end or cathode / DNA being negatively charged will remain positioned at anode or positive end.
(b)
• Position of positive terminal or anode and negative terminal or cathode has been interchanged.
• Band at X being the smallest fragment of DNA move farthest towards the anode.
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CBSE Class 12 Biology past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.