Question

Which of the following statements about DNA packaging in eukaryotes are correct?
i. The negatively charged DNA helix wraps around a positively charged histone octamer to form a nucleosome.
ii. The histone octamer consists of eight molecules, two of each of the four types: H2A, H2B, H3, and H4 .
iii. One nucleosome contains approximately 200 base pairs (bp) of DNA helix.
iv. The chromatin fiber is further organized into a solenoid structure of 30 nm diameter, needing additional NHC (Non-Histone Chromosomal) proteins for higher-level packaging.

• A. i, ii, and iii only
• B. ii, iii, and iv only
• C. i, iii, and iv only
• D. All i, ii, iii, and iv

Hint:

Key Exam Tip:
Remember the hierarchical organization of eukaryotic DNA: DNA wraps around histones to form nucleosomes (10 nm fiber), which then condense into a 30 nm fiber (solenoid), and further into looped domains and supercoiled structures that form mitotic chromosomes.

Answer 1

The Correct Option is D

Statement i: DNA is a polyanionic molecule due to the phosphate groups in its sugar-phosphate backbone, giving it a net negative charge. Histones, on the other hand, are positively charged due to their high content of basic amino acids like lysine and arginine. This electrostatic attraction is crucial for the stable association of DNA with histones to form nucleosomes, the fundamental repeating units of chromatin. This statement is correct.
Statement ii: The histone octamer, also known as the histone core, is the central structural component of the nucleosome. It is composed of eight histone proteins: two copies each of histones H2A, H2B, H3, and H4. This precise stoichiometry is essential for the proper formation of the nucleosome core. This statement is correct.
Statement iii: A nucleosome core particle consists of the histone octamer around which approximately 147 base pairs of DNA are wrapped. However, the nucleosome unit, including the linker DNA that connects adjacent nucleosomes, typically involves about 166 to 230 base pairs, with an average of around 200 base pairs often cited for the repeating unit. Thus, this statement is considered correct in the context of the repeating nucleosome unit.
Statement iv: The chromatin fiber, which is initially about 10 nm in diameter (the "beads-on-a-string" structure), undergoes further compaction. One model of this higher-order folding is the 30 nm fiber, often described as a solenoid structure. This coiling is facilitated by histone H1, which binds to the DNA where it enters and leaves the nucleosome, and potentially by other Non-Histone Chromosomal (NHC) proteins. These NHCs, along with H1, contribute to the further packaging of chromatin into loops and ultimately the highly condensed structures observed in chromosomes during cell division. This statement is correct.
Since all four statements are accurate descriptions of DNA packaging in eukaryotes, option (D) is the correct answer. Final Answer: \(\boxed{D}\)