The following statements were made about the structure of the 30-nm chromatin fiber:

A. In the solenoid model, the linker DNA connects the consecutive core particles.
B. In the zig-zag model, alternating nucleosomes become interacting neighbors.
C. In the solenoid model, 12 nucleosomes are organized into two separate stacks, whereas 8 nucleosomes per turn make a single stack in the zig-zag model.
D. H1 histone is essentially required as per the zig-zag model, but not as per the solenoid model.
E.Chromatin fibers prepared with H4 histones that lack their tails could fold into higher-order fibers.

Which one of the following options represents the combination of all correct statements?

Correct option is C

The 30-nm chromatin fiber is a higher-order structure of chromatin organization. There are two main models:

1. Solenoid Model:

   • Nucleosomes are tightly coiled into a helical structure, with linker DNA bending to bring nucleosomes into closer contact.
   • Requires H1 histone to stabilize the structure.
   • Forms a compact, stacked arrangement.

2. Zig-Zag Model:

   • Nucleosomes are connected by longer linker DNA, forming a zig-zag pattern instead of a tightly coiled helix.
   • Requires less involvement of H1 histone.
   • Alternating nucleosomes become direct interacting neighbors.

Evaluating the Statements

1. Statement A: "In the solenoid model, the linker DNA connects the consecutive core particles."

   • Correct.
   • The solenoid model describes nucleosomes being stacked in a helical array, where the linker DNA connects adjacent nucleosomes in a compact, coiled manner.

2. Statement B: "In the zig-zag model, alternating nucleosomes become interacting neighbors."

   • Correct.
   • In the zig-zag model, nucleosomes do not coil tightly but form a structure where alternating nucleosomes interact, leading to a more open chromatin arrangement.

3. Statement C: "In the solenoid model, 12 nucleosomes are organized into two separate stacks, whereas 8 nucleosomes per turn make a single stack in the zig-zag model."

   • Incorrect.
   • While the solenoid model does involve stacking, the numbers given are not universally accepted. The zig-zag model forms a zig-zag array, not a single stack.

4. Statement D: "H1 histone is essentially required as per the zig-zag model, but not as per the solenoid model."

   • Incorrect.
   • H1 histone is required more for the solenoid model because it stabilizes the tight coiling.
   • In the zig-zag model, H1 histone is less crucial, as nucleosome interactions are largely mediated by linker DNA.

5. Statement E: "Chromatin fibers prepared with H4 histones that lack their tails could fold into higher-order fibers."

   The statement is incorrect because H4 histone tails are essential for chromatin compaction. These tails facilitate nucleosome interactions, stabilizing higher-order chromatin structures. Without them, chromatin remains less compact and fails to fold into higher-order fibers.