Question

Given below are two statements:
Statement (I): In a standard PCR amplification with a co-dominant marker, a pair of primers (Forward and Reverse) is used.
Statement (II): In a standard PCR, only the segment of DNA whose 3'-ends are complementary to the two primers used only gets amplified.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Remember that DNA polymerase can only synthesize DNA in the 5' to 3' direction.
Because it requires a free 3'-OH group to initiate synthesis, the 3' ends of the forward and reverse primers must point toward each other to amplify the region between them.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Polymerase Chain Reaction (PCR) is an in vitro technique used to amplify a specific target segment of DNA.
The reaction relies on thermal cycling and is mediated by a thermostable DNA polymerase, which extends synthetic oligonucleotide primers designed to flank the target region.

Step 2: Detailed Explanation:
Let us analyze both statements:
1. Analysis of Statement (I): Co-dominant markers (such as SSRs or microsatellites) distinguish between homozygotes and heterozygotes based on differences in PCR product size.
Amplification of these loci requires two sequence-specific oligonucleotide primers: a Forward primer and a Reverse primer.
The forward primer anneals to one strand of the double-stranded DNA template, and the reverse primer anneals to the complementary strand. Therefore, Statement (I) is correct.
2. Analysis of Statement (II): During the annealing step of PCR, the primers hybridize to their complementary sequences on the template DNA.
DNA polymerase (such as Taq polymerase) can only synthesize new DNA by adding nucleotides to the free 3'-hydroxyl (3'-OH) group of an annealed primer.
Consequently, the primers must anneal such that their 3' ends point toward each other, flanking the target region.
Only the specific region of DNA located between the two primer binding sites gets exponentially amplified.
Therefore, Statement (II) is correct because the amplified segment is defined by the primer binding sites, which are complementary to the 3' ends of the primers.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are correct, which corresponds to option (A).