Microscopy of DNA in dilute polymer solutions

The mechanism of separation of DNA in polymer solutions is not well underst ood, In this paper we use epifluorescence videomicroscopy to investigate th e dynamic behavior of DNA electrophoresing through dilute polymer solutions . DNA collides with polymer obstacles, which cause the conformation of DNA to change from the globular, random coil conformation it takes in free solu tion. There are two main types of DNA-polymer collisions: U-shape collision s and brief collisions. In U-shape collisions, the DNA collides with a poly mer obstacle, extends into a U-shape, and then slides around the polymer ob stacle like a pulley. There are occasionally multiple entanglement points, causing the DNA to take more complex conformations, such as W-shapes. In th e brief collision process, the DNA collides with a polymer obstacle and beg ins to extend but then collapses back into its globular conformation before a full U-shape is formed. The frequency of these interactions increases as the DNA size increases, and it also increases when the polymer size or con centration increases. These data support the transient entanglement couplin g mechanism of separation of DNA, which states that entanglements between D NA and polymer molecules result in the separation of DNA.