Why do the two possible PCR products differ in size by 300 base pairs?

• A. The PCR reaction temperature causes one product to denature and reanneal, increasing its size by 300 base pairs.
• B. A mutation in the template DNA causes one product to be 300 base pairs longer.
• C. The DNA polymerase used in the reaction adds 300 extra base pairs to one product.
• D. One of the primers binds to a site 300 base pairs upstream or downstream compared to the other, resulting in a size difference.

Answer: (D)

Verified step by step guidance

1. Understand the concept of PCR (Polymerase Chain Reaction): PCR is a technique used to amplify specific DNA sequences. It requires primers, which are short DNA sequences that bind to the target DNA and define the region to be amplified.
2. Recognize the role of primers in determining the size of the PCR product: The primers dictate the start and end points of the amplified DNA fragment. If one primer binds to a site upstream or downstream compared to the other, the amplified region will differ in size.
3. Analyze the scenario described in the problem: The size difference of 300 base pairs is due to one primer binding to a site that is 300 base pairs upstream or downstream relative to the binding site of the other primer.
4. Rule out incorrect explanations: The temperature of the PCR reaction does not cause a size difference, mutations in the template DNA do not explain the consistent 300 base pair difference, and DNA polymerase does not randomly add extra base pairs to the product.
5. Conclude that the correct explanation is based on primer binding locations: The difference in size arises because one primer binds to a site that is 300 base pairs away from the binding site of the other primer, resulting in a PCR product that is either longer or shorter by 300 base pairs.