Read the following statements and choose the set of correct statements :

(a) Euchromatin is loosely packed chromatin

(b) Heterochromatin is transcriptionally active

(c) Histone octamer is wrapped by negatively charged DNA in the nucleosome

(d) Histones are rich in lysine and arginine

(e) A typical nucleosome contains 400 bp of DNA helix

Choose the correct answer from the options given below:

Concept: 

• A nucleosome is a section of DNA that is wrapped around a core of proteins (histones).

• Nucleosomes are known as the fundamental subunit of chromatin.

• On the extension of chromatin under a microscope, the structure resembles beads on a string. 

​

Explanation: 

Statement a: 

Euchromatin is the loosely packed chromatin region. These are active during transcription.

Statement b:     

• Heterochromatin is transcriptionally inactive. 
• The tight packing makes them inaccessible to the protein involved in gene expression.
• They don't undergo crossing over.

Statement c:

• A nucleosome consists of DNA wrapped around a set of eight proteins called histones, which are known as histone octamer.
• Histone octamer is composed of two copies of each of the histone proteins H₂A, H₂B, H₃, and H₄.

• The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called a nucleosome.

Statement d: 

• Histones are rich in basic amino acid residues lysine and arginine.

Statement e: 

• A typical nucleosome contains 200 bp of DNA helix.
• Therefore option 3 is correct as it has the correct set of statements, i.e., statements a, c and d.

So, the correct answer is option 3.