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Friday, February 10, 2012

How E. coli grows so fast

Even for bacteria, Escherichia coli (E. coli) can multiply at a prodigious rate.  In fact, they can divide faster than they can replicate their DNA.  Although this seems impossible, E. coli have a neat trick for accomplishing this feat.

First, let’s look at how cells normally divide.  In order to duplicate itself, a single-celled organism like E. coli first makes a copy of its DNA, then divides in two.  Each resultant daughter cell receives one copy of the DNA.  You can see an animation of DNA replication below:

It takes about forty minutes to make one complete copy of the E. coli genome.  Therefore, each generation of E. coli should last just under an hour.  Instead, E. coli can divide (under ideal conditions) in as little as 20 minutes.  How is this possible? 

It turns out that E. coli can begin a new round of DNA replication before the previous round is complete. When the cell splits in two, each daughter cell receives a strand of DNA that is already in the process of being copied.  That is, the daughter cell receives DNA that is halfway prepared for the eventual granddaughter cells.

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Replication pattern of rapidly growing E. coli wild-type cells. Cells (yellow) with chromosomes (blue lines) and origins (black squares) are drawn schematically to show the number of replication forks and origins at different stages of the cell cycle. In this example, initiation of replication occurs at four origins at the same time as cell division (bottom). A young cell therefore contains four origins and six replication forks (upper left). As replication proceeds, the oldest pair of forks reach the terminus and the two sister chromosomes segregate. The cell then contains four origins and four replication forks (upper right). Initiation then occurs again at 4 origins and generates 8 new forks giving a total of 12 forks, as cell division approaches (bottom). 

As you can imagine, this requires tight control.  The bacterium can’t divide until it has at least two complete and separate genomes, regardless of how much extra replication is going on.  An international team of scientists led by Matthew Grant from the University of Cambridge has found that specific factors govern just how this occurs.  This might give researchers a better handle on controlling bacterial cell growth.