Question

suppose you are provided with an actively dividing culture of escherichia coli bacteria to which radioactive thymine has been added. what would happen if a cell replicates once in the presence of this radioactive base?

one of the daughter cells, but not the other, would have radioactive dna.
neither of the two daughter cells would be radioactive.
all four bases of the dna would be radioactive.
dna in both daughter cells would be radioactive.

Explanation:

DNA replication in prokaryotes (like Escherichia coli) is semi - conservative. This means that during replication, each new DNA molecule consists of one old (parental) strand and one new (daughter) strand. When radioactive thymine is present, the new strands being synthesized will incorporate this radioactive thymine. After one round of replication, each daughter cell will receive a DNA molecule that has one strand with radioactive thymine (the newly synthesized strand) and one non - radioactive parental strand. So, the DNA in both daughter cells will be radioactive.

• For the option "One of the daughter cells, but not the other, would have radioactive DNA": This is incorrect because semi - conservative replication ensures that both daughter cells get DNA with a new (radioactive) strand.
• For the option "Neither of the two daughter cells would be radioactive": Since new DNA strands are made using the radioactive thymine, the daughter cells' DNA will have radioactive components, so this is wrong.
• For the option "All four bases of the DNA would be radioactive": Radioactive thymine only affects the thymine in the DNA. The other bases (adenine, guanine, cytosine) are not made with radioactive thymine in this scenario, so this is incorrect.

Answer:

D. DNA in both daughter cells would be radioactive.