Physics (Internal)

"Studying the dramatic looping of DNA in sperm using single molecule assays"

This event is part of the Biophysics Seminars. 12:30PM.

DNA in sperm cells is completely different from DNA in other cells. In sperm cells, protamine proteins dramatically condense DNA to almost crystalline packing levels by looping the DNA into a series of toroids. One question of interest is how protamine is able to fold the DNA into such a condensed state. Answering this question would be important in biomaterials research where one would like to engineer DNA nanostructures to condense on cue. To investigate this question, we studied the looping of DNA by protamine using two single molecule assays. In one assay, we use atomic force microscopy (AFM) to directly image loops. In the other assay, a tethered particle motion (TPM) assay, we use the motion of a particle attached to the DNA as a way to measure the real-time looping of the DNA. Although DNA looping in sperm condenses the DNA in several minutes to 1/20th of its original size, we find that looping is surprisingly not a one-step process. Here I will discuss the pathway and associated physics for DNA looping in sperm as well as the implications for engineering DNA biomaterials.