The spool-and-thread problem
Add up the DNA in all 46 human chromosomes and you get roughly two metres of double helix — packed into a nucleus only about six-thousandths of a millimetre across. That is like fitting forty kilometres of fine thread into a tennis ball. The cell manages it with a clever, layered system of folding, and the first layer is the most important to understand.
The packaged material — DNA plus its proteins — is called chromatin. Its core building block is the nucleosome: a short length of DNA wound a little under two turns around a tiny spool made of histone proteins. Picture beads on a string, where the string is DNA and each bead is a histone spool.
From beads to a chromosome
- The bare double helix is the thinnest level — just the two strands twisted together.
- DNA wraps around histone spools to make nucleosomes: the “beads on a string.”
- The bead string coils into a thicker fibre, like a telephone cord.
- That fibre loops and folds on itself, again and again.
- At cell division it condenses fully into the compact rod we call a chromosome.
Packing is not only about saving space — it is also a control switch. Where chromatin is wound tightly, the machinery that reads genes cannot reach the DNA, so those genes stay quiet. Where it is loosened, genes become available to switch on. This link between packaging and gene expression is a recurring theme in genetics, and you will meet it again when you study how cells decide which genes to use.