After analysis of the results, you decided that there were two problems with Dr. Gen E. Tics Unidirectional and Single-Fork Hypotheses:

  1. The second audioradiograph taken of the E. coli replicating chromosome did not agree with the Unidirectional Hypothesis.
  2. The rate of bacterial growth did not agree with the Single-Fork Hypothesis.

1. Disproving The Unidirectional Hypothesis

If the doctor was right, the audioradiograph should show one replication fork proceding in one direction away from the origin. You sketched a diagram of what this would look like:

The second audioradiograph did not look like this. It showed two replication forks moving in opposite directions away from the origin instead of just one.

You conclude that replication of the E. coli chromosome is bidirectional, instead of unidirectional as the doctor had predicted.

2. Disproving The Single-Fork Hypothesis

If E. coli replicates 1000 nucleotides per second and the chromosome is 4 million nucleotides long, it would take 2000 seconds or 33 minutes for the two replication forks to travel around the chromosome before meeting at the bottom. If the single-fork hypothesis was correct, E. coli could only double its population every 33 minutes.

Yet, E. coli is capable of doubling its population in 20 minutes, indicating the E. coli is growing faster than the two replication forks can complete the circular chromosome. You conclude that E. coli can initiate multiple replication forks at the origin so that a new pair of forks begin before the previous pair is finished. In this way, the overall rate of DNA replication matches the rate of cell division.

You write down your conclusions and send them to Dr. Gene E. Tics so that he might understand his mistakes. (Click on the Outcome button in the left margin)