Question

Number of nucleosomes found in a helical coil of 30 nm chromatin fibre is
A.6
B.10
C.12
D.15

Answer 1

In eukaryotes, nucleosomes are the basic structural unit of DNA packaging and the fundamental subunit of the chromatin fibre, which consists of DNA segments that wound or wrap around eight histone proteins.

Complete answer:
Option A- 6
Nucleosomes along with histones form a histone octamer. The position of each nucleosome has significance in determining the accessibility of the DNA to regulatory proteins. Nucleosomes give the chromatin appearance of “beads on a string” like. Each bead of nucleosome consists of 200 base pairs of DNA that wound 1.65 times around the octamer.

The length of a helical coil of 30 nm chromatin fibres forming a solenoid contains 6 nucleosomes per turn of a solenoid.

Option B- 10
This is an incorrect option.

Option C- 12
This is an incorrect option. 2 turns of solenoid consist of 12 nucleosomes as explained above.

Option D- 15
This is an incorrect option.

Thus, the correct option is option A- 6.

Note:
Each nucleosome is a disc-like particle of about 11 nm diameter and consists of two copies of each four nucleosome histones that are H2A, H2B, H3 and H4. Histones play an important role in packaging of DNA molecules in a particular manner. The core of the nucleosome is made up of H3 and H4. The DNA extends and runs from a nucleosome to another nucleosome which is separated by a region in between that contains linker DNA.