Practice question

Histone N-terminal tails protrude beyond DNA gyres mediating essential internucleosomal interactions crucial for folding beyond beads-on-string. Unmodified tails, particularly H4 residues 16 to 24 highly basic, bind acidic patch formed by H2A glutamates 56,61,64 and H2B helices on adjacent nucleosome, drawing nucleosomes together during 30 nanometer fiber formation as shown by Dorigo tetranucleosome arrays. Acetylation at H4 K16 abolishes affinity, decompacting fiber. Tail removal prevents fiber condensation in vitro. Roles include recruiting modifying and remodeling enzymes, but major structural function involves chromatin compaction not replication initiation or termination.