MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A researcher is attempting to amplify a specific gene from a DNA sample using Polymerase Chain Reaction (PCR). They design primers that are 20 nucleotides long. If the target gene is 1000 base pairs (bp) long, and the annealing temperature for the primers is approximately 55°C, what would be the approximate expected product size after successful PCR amplification?
- A20 bp
- B1000 bp
- C1040 bp
- D2000 bp
Show answer & explanationAnswer & explanation
Correct answer: B. 1000 bp
PCR amplifies a specific region of DNA defined by the forward and reverse primers. The 'target gene' length of 1000 bp refers to the region between the primer binding sites. Successful PCR amplification will produce copies of this target region, so the product size will be the length of the target gene itself.
Why the other options are wrong
- A. 20 bp is the length of a single primer, not the amplified product.
- C. 1040 bp would imply the addition of primer lengths to the target, which isn't how PCR product size is typically stated for the *gene* length, though the actual amplicon includes the primers. However, 'target gene' specifically refers to the sequence being copied, which is 1000 bp.
- D. 2000 bp would be double the target gene length, which is incorrect for a single amplification product.
PCR Product Size
The length of the DNA fragment amplified by Polymerase Chain Reaction (PCR), which corresponds to the region of interest between the forward and reverse primer binding sites.
- Determined by primer placement.
- Equals length of target sequence.
- Measured in base pairs (bp).
- Used for gene identification and quantification.
Memory trick: Product is 'P'rimed, 'P'art of the gene, 'P'recisely!