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Question
In her practical exams, Asha was provided with a sample of scraped cells from the human buccal cavity. She, very carefully, extracted the genomic DNA, amplified the desired DNA and ran gel electrophoresis. However, she was utterly disappointed as she could not find the DNA bands herself. However, she scored full marks in her practical. What can be the possible reason as to which Asha was not able to see the bands that the teacher could find out?
In her practical exams, Asha was provided with a sample of scraped cells from the human buccal cavity. She, very carefully, extracted the genomic DNA, amplified the desired DNA and ran gel electrophoresis. However, she was utterly disappointed as she could not find the DNA bands herself. However, she scored full marks in her practical. What can be the possible reason as to which Asha was not able to see the bands that the teacher could find out?
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Solution
The correct option is B She was observing the gel by exposing it to incandescent bulb
In gel electrophoresis, the negatively charged DNA molecules which are separated according to their sizes are observed under ultraviolet radiation after ethidium bromide staining. Ethidium bromide is the stain that acts as an intercalating agent in nucleic acids. It gets itself inserted between the spaces present between the double helix of the DNA. This absorbs wavelength corresponding to ultraviolet radiation (UV absorbance maxima is 300 and 360nm) and emits another wavelength (emission maxima is at 590nm) that is visible to us. This can be seen as orange colored bands.
Fig: DNA bands in gel stained with ethidium bromide visualised under UV radiation
The bands on the gel were observed by the teacher under UV light. Hence the experiment must have been conducted appropriately by Asha, justifying the full marks.
Asha might not have observed the gel under ultraviolet radiation. She must have observed it by exposing it to an incandescent bulb. Incandescent light is characterised by a very low ultraviolet fraction. So, ethidium bromide molecules exhibited negligible fluorescence. Hence, the bands were not observed by her.
Since the gel was perfect in all aspects, hence, option a, c and d might not have occurred.
In gel electrophoresis, the negatively charged DNA molecules which are separated according to their sizes are observed under ultraviolet radiation after ethidium bromide staining. Ethidium bromide is the stain that acts as an intercalating agent in nucleic acids. It gets itself inserted between the spaces present between the double helix of the DNA. This absorbs wavelength corresponding to ultraviolet radiation (UV absorbance maxima is 300 and 360nm) and emits another wavelength (emission maxima is at 590nm) that is visible to us. This can be seen as orange colored bands.
Fig: DNA bands in gel stained with ethidium bromide visualised under UV radiationThe bands on the gel were observed by the teacher under UV light. Hence the experiment must have been conducted appropriately by Asha, justifying the full marks.
Asha might not have observed the gel under ultraviolet radiation. She must have observed it by exposing it to an incandescent bulb. Incandescent light is characterised by a very low ultraviolet fraction. So, ethidium bromide molecules exhibited negligible fluorescence. Hence, the bands were not observed by her.
Since the gel was perfect in all aspects, hence, option a, c and d might not have occurred.
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