​A student used four 20-mer oligos to amplify DNA (using a regular Taq DNA polymerase) from a wild type (WT), a homozygous mutant having a deletion of the gene (del), and from the heterozygous mutant (het), as shown in the figure below.​

​Agarose gel electrophoresis profiles, using all four primers simultaneously, on each template are shown below. Which one of the options given below represents the correct profile?​

Correct option is B

Solution:

Step-by-step reasoning:

1. Primer and Product Sizes:

   From the diagram:

   • Primer FW and Primer FD amplify a 500 bp fragment on the WT allele (outside the deletion region).

   • Primer D and Primer R amplify a 300 bp fragment inside the deletion region.

   • The deletion removes 15 kb in the mutant allele.

2. Expected PCR Products:

   • Wild Type (WT):
     Both fragments (500 bp and 300 bp) will amplify because the sequence is intact.
     So, two bands: 500 bp and 300 bp.

   • Homozygous Deletion (del):
     Since the deletion removes the region that Primer D and Primer R would bind to, the 300 bp fragment will NOT be amplified.
     But the 500 bp fragment (amplified by Primer FW and Primer FD, outside the deletion) will be present.
     So, one band at 500 bp.

   • Heterozygous (het):
     Both alleles present.
     So, it will have the 500 bp band (from both alleles) and 300 bp band (only from WT allele).
     Thus, two bands: 500 bp and 300 bp.

3. Comparing with gel profiles:

   • WT lane should show two bands (500 and 300 bp).

   • Het lane should show two bands (500 and 300 bp).

   • Del lane should show one band (500 bp).

Correct Gel Profile Option:

Looking at the options, option 2 fits perfectly:

• WT: Two bands (1000 bp and 750 bp bands as per the gel; here they represent 500 and 300 bp).

• Het: Both bands appear.

• Del: Single band present (500 bp).

(Note: The gel ladder sizes in the image are scaled but the pattern matches option 2.)