AAMC FL Practice Exam

What characteristic do suitable primers for PCR have?

1. High AT content with A or T base pairs at both ends

2. Low GC content with equal distribution of A, T, C, and G

3. High GC content with G or C base pairs at the 5' and 3' ends

4. No specific base pair requirement

The correct answer is: High GC content with G or C base pairs at the 5' and 3' ends

Suitable primers for PCR typically have a high GC content, and having G or C base pairs at both the 5' and 3' ends is particularly advantageous for several reasons. Firstly, primers with higher GC content generally form more stable hydrogen bonds compared to those with higher AT content. This increased stability is critical during the denaturation and annealing steps of the PCR cycle, where the primers must efficiently bind to the target DNA template at a high temperature. The presence of G or C at the ends of the primers enhances the binding affinity, promoting specificity and reducing the likelihood of non-specific amplification. In PCR, the melting temperature (Tm) is an essential factor for successful amplification, and higher GC content contributes to a higher Tm, which allows the primers to remain bound to the template DNA even under the elevated temperatures used during amplification. This specificity is crucial for the efficiency of the amplification process, ensuring that the desired product is generated rather than non-specific products. While the other options suggest varying requirements for primer composition, they do not incorporate these essential characteristics that support effective and specific binding of the primers during PCR. Hence, the focus on GC-rich primers with critical terminal base pair characteristics makes this choice an optimal requirement for successful PCR amplification.