MACHINES IN MOTION

As you stand there in the quiet, you find yourself wondering if the nucleus of a cell is really as still as a museum. Then you notice another display. Above a glass case containing a length of model DNA is a sign that reads: “Push Button for Demonstration.” You push the button, and a narrator explains: “DNA has at least two very important jobs. The first is called replication. DNA has to be copied so that every new cell will have a complete copy of the same genetic information. Please watch this simulation.”

Through a door at one end of the display comes a complex-looking machine. It is actually a cluster of robots closely linked together. The machine goes to the DNA, attaches itself, and begins to move along the DNA as a train might follow a track. It moves a little too fast for you to see exactly what it is doing, but you can easily see that behind it, there are now two complete DNA ropes instead of one.

The narrator explains: “This is a greatly simplified version of what goes on when DNA is replicated. A group of molecular machines called enzymes travel along the DNA, first splitting it in two, then using each strand as a template to make a new, complementary strand. We cannot show you all the parts involved​—such as the tiny device that runs ahead of the replication machine and snips one side of the DNA so that it can twirl around freely instead of getting wound up too tight. Nor can we show you how the DNA is ‘proofread’ several times. Errors are detected and corrected to an amazing degree of accuracy.”​—See the diagram on pages 16 and 17.

The narrator continues: “What we can show you clearly is the speed. You noticed this robot moving at a pretty good clip, didn’t you? Well, the actual enzyme machinery moves along the DNA ‘track’ at a rate of about 100 rungs, or base pairs, every second.²³ If the ‘track’ were the size of a railroad track, this ‘engine’ would be barreling along at the rate of over 50 miles (80 km) per hour. In bacteria, these little replication machines can move ten times faster than that! In the human cell, armies of hundreds of these replication machines go to work at different spots along the DNA ‘track.’ They copy the entire genome in just eight hours.”²⁴ (See the box “A Molecule That Can Be Read and Copied,” on page 20.)