Genome Packing: DNA to Chromosome

Compaction Level

Beads-on-a-string (10 nm)

Level 1.00

Nucleosome linker DNA 50 bp

Live Readouts

Compaction ratio

Fiber diameter

10 nm

Packed length of a 200,000 bp segment (extended = 68.0 µm)

11.3 µm

Repeat length (core 147 bp + linker)

197 bp

How it works

The ~6.4 billion base pairs of a diploid human genome, laid end to end, form roughly 2 metres of double helix -- yet it fits inside a nucleus only a few micrometres across. That requires folding the DNA through several real, well-characterised hierarchical levels, each one wound up inside the last:

Level 0  B-DNA double helix        2 nm       1x    (bare DNA)
Level 1  Beads-on-a-string         10 nm      ~6x   (nucleosides: DNA + histone octamer)
Level 2  30 nm chromatin fiber     30 nm      ~40x  (solenoid/zigzag coiling of nucleosomes)
Level 3  Looped domains           ~300 nm    ~1,000x (loops anchored to a protein scaffold)
Level 4  Metaphase chromatid      ~700-1400nm ~10,000x (loops coiled into a chromosome arm)

Each nucleosome wraps 147 bp of DNA twice around a histone octamer; the adjustable linker DNA between nucleosomes (here 20-80 bp, biologically typical) sets the nucleosome repeat length shown in the readout and how tightly beads pack at levels 0-2. The compaction ratio is the extended contour length divided by the packaged length at that level -- it is what lets a several-centimetre chromosome's worth of DNA condense into a rod a few micrometres long just before cell division.

The geometry drawn here is a schematic, illustrative model of this well-established packing hierarchy (Alberts et al., Molecular Biology of the Cell) -- exact fiber path (solenoid vs. zigzag, loop size) is still an active research question, but the compaction levels and approximate diameters/ratios are the standard textbook values.

DNA / linker Nucleosome (histone core)

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