A graduate student wishing to study DNA replication patterns in eukaryotic cells is attempting to study the locations of the origins of replication by adding fluorescently labeled dTTP during S-phase. While he is performing the experiment, he is also listening to an Illini game. With the game on the line, he becomes distracted and accidently adds flourescently labeled ATP by mistake. When the graduate student comes back to analyze his samples, he is disappointed to find that rather than a crisp fluorescent signal in the origins of replication found in the nucleus, the entire cell is lit up under the fluorescent microscope. Assuming he followed all the correct steps aside from adding the wrong fluorescent nucleotide, which one of the following most accurately explains his observations?

ATP, unlike dTTP, is synthesized in the cell. Using fluorescently labeled ATP is not useful because the ratio of existing (unlabeled) ATP to added (labeled) ATP is too low, and specific fluorescence is difficult to detect.

Adenine is a purine, while thymine is a pyrimidine. Because purines cannot be properly fluorescently labeled, they cause a diffuse signal throughout the cell.

ATP is incorporated into RNA, which is found in the nucleus and the cytoplasm. Additionally, ATP is used as an energy currency throughout the cell. Given this, labeled ATP is not useful when you want to visualize nuclear processes.

Adenine, unlike thymine, cannot penetrate the nuclear envelope and thus will cause the entire cell to light up instead of just the nucleus.

dTTP is only incorporated into RNA molecules, not DNA. Because RNA is transcribed in the nucleus, initiating at the origin of replication, dTTP should be used to help visualize this process. ATP, however, is incorporated throughout DNA and because DNA occupies a large area throughout the cell, this accounts for the diffuse fluorescence pattern.

Please explain >_< I need to understand dis!