Question

Solenoid is a structure of
A) Nucleosomal organisation with 10nm thickness
B) Condensed chromatin fibre with 30nm diameter
C) Highly condensed form of chromatid with 300nm thickness
D) Well organised chromatid with 700nm thickness
E) Well organised chromosome with 1400nm thickness

Answer 1

Hint: If DNA of a human is uncoiled it covers almost 67 billion miles long that means it can wrap the earth four times. So, such a long DNA is compact inside the human chromosomes with the help of a protein and to understand this compact structure solenoid structure is introduced.

Complete Answer:
- The 10nm fibres of nucleosomes get coiled and produce 30 nm wide helix that consist of five to six nucleosomes per helix. Due to the successive turns the helix came closer to each other so that the center to center distance was 10nm. This 30nm structure was termed as solenoid.
- We can compare the structure with the winding of a cable on a spool. In the coiling of this structure it forms a compact form due to presence of histone protein, this order of folding was not present and the irregular clumping of nucleosomes are observed.
- The histone molecules aggregate by cross linking of polymers and this maintains the formation of solenoid. It is observed that solenoid has to fold or coil while condensation of chromatin.
- Now, the DNA packaging is carried so that the 166bp of DNA is wrapped around the core of the nucleosome.
- The DNA that connects the two adjacent nucleosomes is termed as linker DNA. It seems like beads on string, arranged so that it covers 10nm of thickness that again gets condensed and coiled upon itself to form the solenoids. Therefore, the solenoid structure of condensed chromatin fibers have 30nm diameter.

Thus, the correct option is ‘B’. Condensed chromatin fibre with 30nm diameter.

Note: The histone proteins that involve in the formation of compact structure of DNA are of four different types amongst them the H1 does not form the nucleosome instead it sits above the structure keeping in place the DNA that wrapped around the nucleosome.