Question
Describe the levels of DNA packaging in eukaryotes — from the DNA double helix to the metaphase chromosome. What is a nucleosome? How is chromatin organised into chromosomes?
Solution — Step by Step
Human DNA in a single cell, if stretched out, would be about 2 metres long. This must fit inside a nucleus that is only 5-10 micrometres in diameter. The solution: an incredibly efficient multi-level packaging system that compresses DNA by about 10,000 times.
DNA wraps around a core of 8 histone proteins (an octamer: 2 copies each of H2A, H2B, H3, H4). About 146 base pairs of DNA wind 1.65 turns around each histone octamer. This DNA-histone complex is a nucleosome.
Between consecutive nucleosomes, a stretch of linker DNA (about 54 bp) connects them, often associated with histone H1. The “beads-on-a-string” appearance under an electron microscope is this nucleosome arrangement.
- Beads on a string (11 nm fibre) — nucleosomes connected by linker DNA
- 30 nm chromatin fibre — nucleosomes coil into a solenoid structure (6 nucleosomes per turn), stabilised by H1 histones
- Looped domains (300 nm) — the 30 nm fibre forms loops attached to a non-histone protein scaffold
- Condensed chromatin (700 nm) — loops coil further during cell division
- Metaphase chromosome (1400 nm) — maximum compaction, visible under light microscope
flowchart TD
A[DNA double helix 2 nm] --> B[Nucleosome: beads on a string 11 nm]
B --> C[30 nm chromatin fibre - solenoid]
C --> D[Looped domains 300 nm]
D --> E[Condensed chromatin 700 nm]
E --> F[Metaphase chromosome 1400 nm]
B --> G[8 histone proteins + 146 bp DNA]Why This Works
The packaging is dynamic — it can be loosened for gene expression (euchromatin, loosely packed, transcriptionally active) or tightened for storage and cell division (heterochromatin, densely packed, transcriptionally inactive). This allows cells to selectively access genes while keeping the rest safely compacted.
Alternative Method
To calculate the packaging ratio: total DNA length per cell = ~2 m = 2 x 10⁹ nm. Nucleus diameter = ~10 μm = 10⁴ nm. Packaging ratio = 2 x 10⁹ / 10⁴ = ~200,000 fold compression needed. Nucleosome level alone provides ~7x compaction; the full hierarchy achieves the rest.
Common Mistake
Students say “DNA wraps around histones during cell division.” DNA is ALWAYS wrapped around histones (as nucleosomes) — this is the default state. During cell division, the additional levels of compaction (30 nm → looped → metaphase) increase. The nucleosome structure is permanent, not just for mitosis.
Euchromatin = loosely packed, light-staining, gene-rich, transcriptionally active. Heterochromatin = tightly packed, dark-staining, gene-poor, transcriptionally inactive. These terms are frequently tested in NEET alongside DNA packaging.