4.2.1.P2

One interesting use of DNA is as springs to pull on protein molecules in order to probe their structure.  Two unwound strings of DNA are attached to a polystyrene bead at one of its ends and to a protein molecule at the other.  The polystyrene beads are then pulled apart using a laser tweezers.  As the force on the DNA molecules are increased, they stretch, exerting an increasing force on the protein.  At some point, the force on the protein is strong enough such that the protein unfolds and the distance between the beads jumps.  The curve of force versus distance will therefore tell something about the structure of the protein.

Recall that DNA molecules are not perfect “Hooke’s law springs” and that the force to stretch them a given distance might not be directly proportional to the stretch.  Also note that it may not be possible to get identical lengths of DNA attached to the two sides of the protein.

A. Given the facts in the paragraph above, draw a system schema for the objects above in the box at the right identifying the character of the interactions.

B. Draw free-body diagrams showing the forces on the left DNA molecule, the protein molecule, and the right DNA molecule at an instant when everything is tense and at rest. Label each force in your diagrams being careful to specify in your force labels the type of force, what object is causing the force, and what object is feeling the force.

C. If any of the forces that you have put in your three diagrams are equal, identify which are equal and why you think they are equal.

Joe Redish 12/29/11