Choose the correct answer from the options given below:

1. C – B – E – A – D
2. E – D – C – B – A
3. A – B – C – D – E
4. E – B – C – D – A

   C – B – E – A – D is the correct sequence of DNA packing levels from double helix to metaphase chromosome.

   Question Breakdown

   This question tests the hierarchical organization of eukaryotic DNA packaging, achieving ~10,000-fold compaction to fit 2 meters of DNA into a 10 µm nucleus. The sequence progresses from naked DNA through structural intermediates to the final mitotic chromosome.

   Correct Sequence Explanation

   DNA packaging follows these precise levels:

   • C (Double helix DNA): B-form Watson-Crick double helix (2 nm diameter) is the starting point.

   • B (Nucleosome): DNA wraps ~1.65 turns around histone octamers (H2A, H2B, H3, H4), forming “beads-on-a-string” (11 nm fiber).

   • E (30 nm fiber): Nucleosomes coil into solenoid or zigzag 30 nm chromatin fiber via H1 linker histones.

   • A (Radial loops attached with protein scaffold): 30 nm fibers form ~300 nm loops anchored to scaffold/matrix attachment regions (SARs/MARs) by non-histone proteins (topoisomerase II, SMC).

   • D (Metaphase chromosome): Loops further condense into 700 nm-1400 nm metaphase chromosomes via histone phosphorylation and scaffold contraction.

   Why Other Options Are Wrong

   • E – D – C – B – A: Starts with 30 nm fiber before DNA/nucleosomes; impossible reversal.

   • A – B – C – D – E: Loops (A) before basic nucleosomes (B); foundational levels first.

   • E – B – C – D – A: 30 nm fiber (E) before nucleosomes (B); higher-order precedes basic packaging.

   The DNA to chromosome packing levels sequence achieves dramatic compaction essential for mitosis and gene regulation. This guide details the different levels of packing from DNA to chromosome—C (double helix DNA) → B (nucleosome) → E (30 nm fiber) → A (radial loops) → D (metaphase chromosome)—crucial for NEET/CSIR-NET Life Sciences preparation.

   DNA Packaging Hierarchy Overview

   ~2 meters of human DNA compacts 10,000-fold through 5 structural levels, regulated by histone modifications and non-histone chromosomal proteins (NHPs). Each level multiplies compaction geometrically.​

   Step-by-Step Packing Sequence (C-B-E-A-D)

   1. C. Double helix DNA: 2 nm B-DNA, naked nucleic acid.

   2. B. Nucleosome: 146 bp DNA × 1.65 turns around H3₂H4₂H2A₂H2B₂ octamer + H1; 7-fold compaction.

   3. E. 30 nm fiber: 6 nucleosomes/turn solenoid (H1-mediated); 40-fold total compaction.

   4. A. Radial loops: 30-90 kbp loops on protein scaffold (topo II, condensins); 300 nm fiber.

   5. D. Metaphase chromosome: 700-1400 nm, 10,000-fold compaction for mitosis.​

   Level	Structure	Compaction	Key Proteins
   C	Double helix	1×	None
   B	Nucleosome (11 nm)	7×	Core histones + H1
   E	30 nm fiber	40×	H1 linker histone
   A	Radial loops (300 nm)	1,000×	Scaffold (topo II, SMC)
   D	Metaphase chromosome	10,000×	Condensins, H1-P

   This chromosome packing sequence enables mitotic segregation while maintaining DNA accessibility for transcription in interphase chromatin.​