Q52.Arrange the following in the correct sequence of DNA packaging:
A. DNA Scaffold
B. Nucleosome formation
C. Chromatosome formation
D. Histone Dimerization
E. 30 nm fiber
Choose the correct answer from the options given below:
(1) D, A, C, E, B
(2) D, C, B, E, A
(3) D, B, C, E, A
(4) D, B, E, C, A
The correct sequence of DNA packaging is D → B → C → E → A, corresponding to option (3).
DNA packaging compacts the long DNA molecule into the nucleus through hierarchical steps involving histones and higher-order structures. This ensures genomic DNA fits efficiently while regulating access for processes like transcription.
Correct Sequence (Option 3)
D. Histone Dimerization: Core histones (H2A-H2B and H3-H4) first form dimers, which assemble into an octamer core. This is the initial step before DNA interaction.
B. Nucleosome formation: DNA (~147 bp) wraps ~1.65-2 turns around the histone octamer, creating the “beads-on-a-string” structure (11 nm fiber).
C. Chromatosome formation: Linker histone H1 binds the entry/exit DNA of the nucleosome, stabilizing ~167 bp DNA per unit and sealing the nucleosome for further compaction.
E. 30 nm fiber: Chromatosomes coil into a solenoid or zigzag helix (~6 nucleosomes/turn), forming the 30 nm chromatin fiber via H1-mediated interactions.
A. DNA Scaffold: The 30 nm fiber forms radial loops (~300 nm) anchored to a protein scaffold (e.g., topoisomerase II, condensins), leading to metaphase chromosomes (~700-1400 nm).
Why Other Options Are Incorrect
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Option (1) D, A, C, E, B: Places scaffold (A) too early, before basic nucleosome (B) assembly; illogical as scaffold organizes pre-formed fibers.
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Option (2) D, C, B, E, A: Chromatosome (C) before nucleosome (B) reverses core formation; nucleosomes precede H1 addition.
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Option (4) D, B, E, C, A: 30 nm fiber (E) before chromatosome (C) skips H1 stabilization needed for solenoid formation.
The DNA packaging sequence is crucial for compacting 2 meters of human DNA into a microscopic nucleus, enabling cell division and gene regulation. Starting with histone dimerization (D), where H2A-H2B and H3-H4 pairs form an octamer, DNA wraps around it during nucleosome formation (B) to create the 11 nm fiber. Next, chromatosome formation (C) adds H1 histone, followed by coiling into the 30 nm fiber (E), and finally attachment to the DNA scaffold (A) for chromosome condensation.
This hierarchy reduces DNA length ~10,000-fold. For competitive exams, remember: D-B-C-E-A (option 3) as the precise order.
Key Levels Table
