Correct option is B
Key Concepts:
- PCR Amplification: PCR amplifies the 150 bp DNA fragment using primers and dNTPs (deoxynucleotide triphosphates). The labeled components will determine which parts of the DNA are radioactive and thus visible on the autoradiograph.
- Autoradiography: Detects radioactive emissions. Only DNA strands or nucleotides that are labeled with 32P will be visible.
- Agarose Gel: Separates DNA fragments by size. Since all reactions amplify the same 150 bp fragment, the bands should appear at the same position (same molecular weight) unless the labeling affects visibility.
- Labeling Types:
- 5’ 32P-labeled primers (Reaction i): The primers are labeled at their 5’ ends with 32P. In PCR, these primers are incorporated into the newly synthesized DNA strands. Thus, every amplified DNA strand (150 bp) will have a labeled 5’ end and will be visible on the autoradiograph.
- α-32P-labeled dCTP (Reaction ii): The α-32P label is on the dCTP nucleotide, which is incorporated into the DNA during synthesis wherever a cytosine (C) is added. Since the DNA is GC-rich, dCTP will be incorporated frequently, labeling the entire DNA strand. All amplified 150 bp fragments will be radioactive and visible.
- γ-32P-labeled dATP (Reaction iii): The γ-32P label is typically used to label the 5’ end of DNA (e.g., in kinase reactions), but dATP is a nucleotide, not a primer. In PCR, dATP is incorporated into the DNA strand during synthesis, but the γ-phosphate of dATP is released as pyrophosphate during DNA synthesis (not incorporated into the DNA). Therefore, the amplified DNA will not be labeled with 32P in this reaction, and no band will be visible on the autoradiograph.
Analysis of Options:
- Lane A should have a band (5’ 32P-labeled primers).
- Lane B should have a band (α-32P-labeled dCTP).
- Lane C should have no band (γ-32P-labeled dATP does not label the DNA).
