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PCR in Diagnosis

A researcher ...

Question

A researcher has identified the location of the gene in a eukaryotic cell that codes for protein P and has managed to excise it from the DNA. She has also generated DNA from the mRNA that codes for the protein. If she runs these two sequences through gel electrophoresis, what should she expect to see?
It must be noted that the wells where the DNA was loaded onto the gel are at the top of the gels shown below.

A

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B

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C

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D

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Solution

The correct option is C
Excised DNA would be the original sequence. The cDNA is synthesised from mRNA (after post-transcriptional change), hence does not contain introns like the unprocessed excised sequence.
Gel electrophoresis separates fragments based on size. Molecular size of excised DNA will be greater than cDNA. Hence, the former will move slower as compared to later one. Thus, option c is the most appropriate one.

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Q. A mixture of DNA fragments from the same source, after amplification was loaded into wells of two different gels, A and B. The magnified image of the agarose gel after gel electrophoresis has been given below with the possible marker values given aside.

Select the appropriate statement(s) among the following.
I. Gel A has greater agarose concentration than gel B
II. Gel A has lesser agarose concentration than gel B
III. The pore size of gel A is smaller than that of gel B
IV. The pore size of gel B is smaller than that of gel A

Q. Match the following component of the electrophoresis unit in column I with its use in column II.

Column I	Column II
1. Comb	a. Prevents DNA from floating out of wells and makes the position of the DNA easier to see.
2. Gel box	b. Houses gel and expose it to an electrical current generated by a power supply.
3. Micropipette	c. Contains DNA molecules of known size that may be used to gauge the location of fragments of interest in the gel.
4. Loading buffer	d. Creates wells on one end of the gel for loading of the DNA
5. DNA standard size	e. Used to transfer small volumes of liquid.

Q. A mixture of DNA fragments, such as those made by restriction enzyme digestion, can be resolved by gel electrophoresis. Loading DNA sample into wells at the top of the gel (see figure) will migrate through the gel in response to an electrical field.
What component of the DNA macromolecule is responsible for its migration toward the positive terminal?

Q. 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?

Q. Preetha was practising PCR hands-on in her laboratory. She successfully isolated the gene of interest (GOI), however the quantity was too low. She desired to amplify the sequence. She conducted a PCR after which she tried to check for amplification through gel electrophoresis. She observed that all the lanes of the gel contained two bands, A and B.

Given that B was comparatively closer to the positive terminal, predict the correct alternative.

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