User:Eran Hodis/Sandbox3

Introduction

The nucleosome core particle contains two copies of each histone protein (H2A, H2B, H3 and H4) and 146 basepairs (bp) of superhelical DNA wrapped around this histone octamer. It represents the first order of DNA packaging in the nucleus and as such is the principal structure that determines DNA accessibility.

The Histone Octamer

Two copies of each histone protein, H2A, H2B, H3, and H4, are assembled into an octameric disc. Although each monomer has an N-terminal tail that projects from the histone core, the structure at left shows only a single tail from one H3 monomer.

All four histone proteins share a highly similar structural motif, the histone fold, comprising three alpha helices connected by two loops (shown here for H2A):

alpha1-loop1-alpha2-loop2-alpha3