Nucleotides localize to nucleic acid synthesis sites

Steps:

• Labeled nucleotides serve as building blocks for DNA and RNA.
• DNA synthesis and replication occur in the nucleus (structure 2).
• RNA transcription happens in the nucleus, with processing in the nucleolus and translation at ribosomes (structure 3).
• Radioactivity thus accumulates where nucleic acids are actively synthesized and assembled.

Why C is correct:

• Structures 2 (nucleus) and 3 (ribosomes/rough ER) are primary sites for DNA/RNA incorporation, per molecular biology principles.

Why the others are wrong:

• A: Structure 1 (e.g., cytoplasm) lacks direct nucleic acid synthesis machinery.
• B: Structure 4 (e.g., mitochondria) uses nucleotides minimally, not concentrating label.
• D: Omits nucleus (2), missing key DNA site.

Final answer: C