Question
How is the enormously long DNA molecule packaged to fit inside the tiny nucleus, and what is the hierarchy from nucleosome to chromosome?
Solution — Step by Step
A single human cell contains about 2 metres of DNA, but the nucleus is only about 6 micrometres in diameter. That is like fitting 20 km of thread into a tennis ball. This requires multiple levels of coiling and folding — called DNA packaging or chromatin organization.
DNA wraps around protein complexes called histones.
- Each nucleosome core has 8 histone molecules (an octamer): 2 each of H2A, H2B, H3, and H4
- 146 base pairs of DNA wrap around this octamer (1.65 turns)
- Between nucleosomes, a stretch of linker DNA (~60 bp) connects them, with histone H1 sitting at the entry/exit point
Under an electron microscope, this looks like “beads on a string.” This gives about 7-fold compaction of DNA.
The beads-on-a-string further coils into a thicker fiber called the 30 nm chromatin fiber (solenoid model). Six nucleosomes per turn, held together by H1 histones and internucleosomal interactions.
This gives about 40-fold compaction.
The 30 nm fiber forms loops of 30-90 kb, anchored to a non-histone protein scaffold (nuclear matrix). Each loop is an independent functional domain.
This gives about 680-fold compaction.
During cell division, the looped chromatin further condenses into the maximally compact metaphase chromosome — visible under a light microscope.
Total compaction: approximately 10,000-fold.
Summary of hierarchy:
| Level | Structure | Diameter | Compaction |
|---|---|---|---|
| DNA double helix | Watson-Crick | 2 nm | 1x |
| Nucleosome (beads on string) | DNA + histone octamer | 11 nm | ~7x |
| Solenoid | 30 nm fiber | 30 nm | ~40x |
| Looped domains | Loops on scaffold | 300 nm | ~680x |
| Metaphase chromosome | Maximum condensation | ~1400 nm | ~10,000x |
flowchart TD
A["DNA double helix: 2 nm"] --> B["Nucleosome: 11 nm, beads on a string"]
B --> C["30 nm fiber: solenoid"]
C --> D["Looped domains: 300 nm"]
D --> E["Metaphase chromosome: 1400 nm"]
B --> F["8 histones: 2 each of H2A, H2B, H3, H4"]
B --> G["H1 histone: linker, stabilizes entry/exit"]
A --> H["2 meters of DNA in each human cell"]
E --> I["10,000-fold compaction achieved"]Why This Works
The packaging hierarchy is not just about fitting DNA inside the nucleus — it also regulates gene expression. Euchromatin (loosely packed) allows genes to be transcribed, while heterochromatin (tightly packed) silences genes. The cell controls which regions are accessible by modifying histones (acetylation loosens, methylation tightens) and by remodelling chromatin structure.
This means DNA packaging is both a structural solution and a regulatory mechanism — two problems solved by one system.
Alternative Method
For NEET, remember the packaging order with the mnemonic: “DNA Never Stops Loading Chromosomes” — DNA, Nucleosome, Solenoid, Looped domains, Chromosome. The diameter increases at each level: 2, 11, 30, 300, 1400 nm.
Common Mistake
Students often say “there are 8 histone proteins in a nucleosome including H1.” Wrong — the nucleosome core has an octamer of 8 histones (H2A, H2B, H3, H4 — two of each), and H1 is NOT part of the octamer. H1 sits outside the core, at the linker DNA entry/exit point, and helps stabilize the higher-order 30 nm structure. NEET 2022 specifically asked “which histone is not part of the nucleosome core” — the answer is H1.