What is a chromosome?

Chromosomes are bundles of tightly coiled DNA, located, in pairs, within the nucleus of almost every cell in our body. Humans have 23 pairs of chromosomes.

Illustration showing how DNA is packaged into a chromosome.
Image credit: Genome Research Limited


  • In plant and animal cells, DNA is tightly packaged into thread-like structures called chromosomes. This is in contrast to bacteria where DNA floats freely around the cell.
  • A single length of DNA is wrapped many times around lots of proteins called histones, to form structures called nucleosomes.
  • These nucleosomes then coil up tightly to create chromatin loops.
  • The chromatin loops are then wrapped around each other to make a full chromosome.
  • Each chromosome has two short arms (p arms), two longer arms (q arms), and a centromere holding it all together at the centre.
  • Humans have 23 pairs of chromosomes (46 in total): one set comes from your mother and one set comes from your father.
  • Of these 23 pairs, one pair are sex chromosomes so differ depending on whether you are male or female (XX for female or XY for male).
  • The other 22 pairs are autosomes (non-sex chromosomes) and look the same for both males and females.
  • The DNA making up each of our chromosomes contains thousands of genes.
  • At the ends of each of our chromosomes are sections of DNA called telomeres. Telomeres protect the ends of the chromosomes during DNA replication by forming a cap, much like the plastic tip on a shoelace.

This page was last updated on 2014/11/17