Describe how initiation of DNA replication in E. coli is regulated Explain how E. coli can grow in the lab with a generation time of 20 minutes when it takes 40 minutes to replicate the chromosome?

Answer 1

The process by which a cell doubles its DNA content, or genome, is called "DNA replication". The minimum time for DNA replication in E. coli is indeed 40 min as was reported repeatedly in scientific papers. The E. coli genome consists of a single circular double strand of DNA. It is replicated by splitting the double strand into single strands which function as templates for producing new counterparts of both single strands; it looks a bit like two interwoven rings in this stage. This mechanism of replication is called "replication fork". A replication fork always starts at the same location ("origin of replication" or Ori) and travels along all of the DNA in one direction until it reaches its origin again. Then, all DNA was replicated and the cell is ready for cell division.

If the genome was doubled with a single replication fork, the replication time could NOT exceed the time for cell division. BUT, as soon as the replication fork traveled a bit from the origin upstream, another replication fork can be initiated. Before the first fork reaches its origin a second, third, fourth etc. fork could be active. One cell may therefore contain one complete copy of its genome plus several partial copies in various stages of completion. This allows doubling times ("generation times") of less than the replication time, because the cell can divide as soon as the first copy is complete. In this case, both daughter cells get a single complete copy plus several partial copies of the genome. The partial copies can then be completed in less than the time required for a complete replication cycle.

But, of course, this can work only for a limited number of generations and therefore relatively short periods of time, eg the "exponential growth phase" of E. coli in liquid culture. In fact, the doubling time of 20 minutes for E. coli is a textbook figure. It is valid only for optimum conditions, i.e. when temperature, nutrient concentrations and cell density (number of E. coli cells in a volume of liquid medium) are all in optimum range and when no growth-suppressing substances are present (many bacteria produce such substances if their cell density becomes too high) . Even slight deviations from these conditions may easily lead to generation times of more than 30 minutes instead of 20 min. Then, the replication time increases along with the generation time, but slower. In effect, at some point, the generation time exceeds the replication time. In their natural environments bacteria usually grow much slower than in the lab, because there usually is some growth limiting factor (most often a kind of nutrient) and then the generation time is indeed greater than the replication time.

Bacteria usually inhabit rapidly changing environments. The ability to divide faster than the genome needs to be replicated can be regarded as an adaptation to such environments, because it can be of great advantage to be able to perform cell divisions in short time when the conditions become favorable.