Practice question

Fundamental repeating unit revealed by micrococcal nuclease digestion is nucleosome core particle. High resolution crystallography shows 146 to 147 base pairs wrapped 1.65 left-handed superhelical turns around histone octamer disk. Octamer comprises two copies each of H2A, H2B, H3 and H4, totaling eight proteins organized as central H3-H4 tetramer flanked by two H2A-H2B dimers assembled via histone fold handshake motifs. This assembly induces negative supercoiling, reduces DNA length and provides stability through electrostatic contacts between arginine-rich histone surfaces and DNA phosphate backbone, controlling genome compaction.