USA Biology Olympiad (USABO) Practice Exam

What would be the result if radioactive thymine was added to an actively dividing culture of E. coli bacteria?

1. Only the original cell would be radioactive

2. Only one daughter cell would be radioactive

3. DNA in both daughter cells would be radioactive

4. DNA would not be affected by radioactive thymine

The correct answer is: DNA in both daughter cells would be radioactive

When radioactive thymine is introduced to an actively dividing culture of E. coli bacteria, it becomes incorporated into the DNA during DNA replication. E. coli, like all living organisms, utilizes thymine, one of the four nucleotides that comprise DNA. During cell division, specifically during the S phase of the cell cycle, the DNA is replicated so that each daughter cell receives a complete copy of the genetic material. Since thymine is a key component of DNA, the introduction of radioactive thymine means that during replication, E. coli will synthesize new DNA strands incorporating the radioactive version of thymine in place of the non-radioactive form. Consequently, both daughter cells, which arise from the division of the parent cell, will contain DNA that has incorporated the radioactive thymine. Therefore, when the E. coli completes its cell division, both daughter cells will have DNA that is radioactive, as they each receive one copy of the parent’s DNA that includes the radioactive thymine. This demonstrates how the genetic material is passed on during cellular division and highlights the significance of nucleotide incorporation in replication processes.