Imagine a string that is a ten-millionth of an inch across and a ten-thousandth of an inch long. Suppose you wanted to test its strength, measure its length, or pick it up and move it. How would you hold it? When the string is really a molecule of DNA, you need a pair of molecule-size tweezers.

The “optical tweezer” was invented around 1980. It turns out that a tiny bead attached to a strand of DNA can be attracted to the spot of a laser's light. Pick one end of the strand, shine the light, and you can hold the molecule in place. Fix the other end, move the light, and you can stretch the molecule.

In 1995, scientists were able to pull straight the normally crumpled DNA molecule and measure the amount of work it took. The force required was only about a millionth the weight of a drop of water. The researchers showed that DNA first stretches by being straightened; but once it is straight, the “string” itself can stretch. By looking at just one molecule, they were able to test a theory of how DNA acts as a mechanical object.

The entire genetic code for a human being (the human genome) has 4 billion “base pairs” of molecular data. The full set is stored in duplicate in almost every one of the body's roughly 1013 cells. Altogether, this amounts to about 20,000,000,000 miles of DNA per human body, enough to stretch around the earth a million times. At the normal rate of cell reproduction, each of us is making new DNA at a rate faster than 10,000 miles per hour.

Only a small part of this DNA is actually used in any single cell. The rest contains the code for making other types of cells in the rest of the body. This means that each cell must find just the right little bit of the DNA crowded into the small space of the cell nucleus. It becomes a big problem to hold all that DNA, pick out the right bit, and open it up to read its message. The physics of DNA stiffness, twisting, and sticking becomes a major factor in understanding how this genetic material works.

FIGURE 3.9 Using “optical tweezers” to stretch a strand of DNA. One end of the strand is attached to a stationary glass surface. A tiny bead attached to the other end is then trapped using laser light. Adjusting the laser trap moves the bead and stretches the DNA molecule. (Courtesy of Princeton University.)

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