In figure 9.4, we are shown the idea of what happens to the extension of a DNA strand as the force increases. The DNA used was 10416 bps in length, with one side anchored to a glass slide, the other to a bead which was pulled by optical tweezers.
A) What happens initially as we pull the bead is that the molecule remains very nearly a random coil the ends have mean-square separation of Lseg(N)^(1/2)
B) After this initial part of the curve, we see that the relative extension quickly curves off and plateaus around 20pN force. This is the point where the DNA is virtually straight, since z approximately equals L.
C) Along this minutely increasing slope, the DNA starts to become stretched out to be longer than the actual total contour length. This is known as "intrinsic stretching"
D) "Overstretching Transition" - around 65pN force, a sharp transition occurs as the molecule jumps to 1.6 times the length of its relaxed state.
E) Molecule continues to be stretched until the molecule breaks (like when you decide to stretch that rubber band to launch paper too far)