Single replication machines at work:
The coordination of daughter strand synthesis
By Karl Duderstradt (van Oijen lab, University of Groningen)
One of the most important events a cell has to undergo before it will divide is DNA replication. In this event, the amount of genomic DNA is doubled by taking the two strands apart and synthesize complementary strands to them. This process is schematically shown in Figure 1:
Figure 1: Figure 5-19 in Molecular Biology of the Cell (©Garland Science)
The collection of molecules involved in the replication machinery called the replisome. It is not yet entirely known how the replication machinery is assembled and what coordinates the function of it, but Karl Duderstadt works on these problems. He does this by single-molecule assays, in which single molecules are labeled with a fluorescent dye or protein, and can be followed. The power of single molecule assays is that instead of looking at what a lot of particles do together and identify averages, you can look at individual rates. This is similar to the difference between looking down from a skyscraper down to traffic and standing beside the road looking at individual cars. Every car will do very different things in both cases, but when you look down from the skyscraper you only see the average of many.
Karl uses E. Coli in his research. To be more specific, he looked at the bacteriophage T7 replisome, and was especially interested in how the enzymatic events on leading and lagging strands were coordinated. To better understand his findings, take a look at Figure 2 which shows the replication slightly more detailed. As can be seen there, the replication machinery uses two kinds of loops to synthesize the lagging-strand. These loops are called the priming loop and the trombone or replication loop. About 1% of the lagging strand DNA is synthesized in the replication loop, which are also five times less likely than the priming loops.
Figure 2: Priming loop and Trombone loop. Found at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2896039/figure/F4/
Kasper Spoelstra – firstname.lastname@example.org