Architecture of the Chinese hamster metaphase chromosome

The development of procedures for the isolation of unfixed metaphase chromosomes has made feasible a direct analysis of their morphology. Wholemount stereo electron microscopy was used to examine intact and partially disrupted chromosomes produced by physical shearing and extraction with salt and urea solutions. A model of chromosome architecture was developed to accommodate evidence from studies using both light and electron microscopy. In the proposed model the chromatid (anaphase chromosome) consists of two half-chromatids; each half-chromatid contains two deoxyribonucleoprotein ribbons wound into a single fiber (termed the core), with many loops of chromatin (termed epichromatin) attached along its length. The core ribbons are each about 50 Å thick by 4000 Å wide and are composed of many parallel deoxyribonucleoprotein strands. The epichromatin loops appear to be 250 Å supercoiled fibers containing about 75 per cent of the chromosomal DNA. The epichromatin can be selectively removed from the core fibers by extraction with 2.0 M NaCl or 6.0 M urea solutions.