Probing Mitotic Chromosome Mechanics Using Optical Tweezers

During mitosis, cells compact their DNA into rodlike shapes, four orders of magnitude shorter than the DNA backbone contour length. We describe an experimental protocol to isolate and study these intricate mitotic chromosomes using optical tweezers. We touch upon the technical details of the required optical tweezers and microfluidics setup, including advanced force calibration procedures to accurately measure the high forces the chromosomes withstand. The procedure used to isolate mitotic chromosomes, including biotinylation of the telomeric ends to facilitate trapping them in optical tweezers, is described in detail. Finally, we provide a protocol for carrying out optical tweezers experiments on the isolated mitotic chromosomes.