NEET2026BiologyChapterActual
During a recombinant DNA experiment, a student performs agarose gel electrophoresis to separate a mixture of DNA fragments. After the run is complete, the student directly exposes the gel to UV radiation but fails to observe any DNA bands, even though the tracking dye indicates proper migration. Which of the following is the most likely reason for this observation?
Options
- AThe DNA fragments migrated towards the cathode because they are positively charged.
- BThe student forgot to stain the gel with ethidium bromide before UV exposure.
- CThe DNA fragments were not eluted from the agarose gel piece prior to visualization.
- DThe sieving effect of the agarose matrix completely blocked the entry of DNA fragments.
Correct answer
B. The student forgot to stain the gel with ethidium bromide before UV exposure.
Step-by-step solution
DNA fragments are colourless and cannot be visualised directly under UV radiation. To observe the separated DNA fragments, the agarose gel must be stained with a fluorescent dye such as ethidium bromide (EtBr). Ethidium bromide intercalates between the base pairs of DNA and emits bright orange fluorescence when exposed to UV light. Since the student directly exposed the gel to UV radiation without staining it, no DNA bands were visible. DNA is negatively charged and migrates towards the anode, not the cathode. Elut