Geneticists have long been intrigued as to how several meters of DNA are packed into the eukaryotic nucleus. The discovery of nucleosomes has provided significant insights into this question. The cover illustration depicts the X-ray crystal structure of the nucleosome core particle at a resolution of 2.8 A. The nucleosome is an integral part of the structure of eukaryotic chromatin. The double-helical ribbon represents 146 base pairs of DNA surrounding four pairs of histone proteins. The nucleosome core particle is repeated over and over in the chromatin fiber and is the principal packaging unit of DNA in the nucleus. (Pearson)