Answer to Question #58262 in Cell Biology for Sanjukta Ghosh

Question

I'm in need of an explanation for zig-zag or two start model of chromatin packaging. Can you provide some help.

Thank you.

Answer 1

"Zig-zag" or "two start" model of chromatin packaging are identical names for spatial structure of 30-nanometer chromatin fiber. According to it, in chromatin DNA is joined with histones (proteins that form nucleosomal core - H2A, H2B, H3, H4 and additional H1). Nucleosomes are connected with short linker regions of approximately 50 base pairs. If linkers just continue nucleosomal DNA straightly towards the same direction (without histone H1), then nucleosomes should be located in a zigzag manner (back and forth). This is how decondensed nucleosome fiber (without H1 and in conditions of the low ionic strength of solution) looks like under the microscope. In presence of H1 polynucleosome fiber condenses due to formation of the "stems" (structures of two neighbouring linker regions connected with each other by positively charged H1-tail) in the outlet of nucleosomes. This pulls nucleosomes closer and promotes their crosslinking by the tails of core histones, stabilizing the whole condensed fiber.
Therefore, superstructure of chromatin is a condensed polynucleosome fiber (30 nm), which forms a three-dimensional zigzag comprising two stacks of nucleosomes that are connected by almost straight linkers directed to the centre of the fiber. H1-histones are located inside the fiber and stabilize it.