What is a chromosome?

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.