A back-to-back zig-zag model for higher order chromatin structure.

The evidence for the next level of chromatin structure above the 'zig-zag' is examined and a system of large, uncoiled, 'hair-pin' loops, folded until the zig-zags are back to back, is proposed as being more compatible with the evidence than coiling models. The loop model suggests a simple way for the base sequences on the DNA to control the higher order folding of chromatin, and is also strongly suggestive of a simple mechanism ensuring homologous alignment of nucleofilaments at synapse.