What is a chromosome?

Chromosomes are tightly coiled bundles of DNA found in the nucleus of almost every cell in our body.

  • In plant and animal cells, DNA is tightly packaged into thread-like structures called chromosomes.
  • Humans have 23 pairs of chromosomes.
  • Other living things have a different number of chromosomes.

What is a chromosome?

 

  • A chromosome is a tightly wound bundle of DNA. It’s the way DNA is packaged in animal and plant cells – contrasting with bacteria where DNA floats freely around the cell.
  • The DNA making up each of our chromosomes contains thousands of genes.

 

How many chromosomes do we have?

 

  • Humans have 23 pairs of chromosomes (46 in total): one set comes from each biological parent
  • One pair are the sex chromosomes, called the X and Y chromosomes. People with XX are usually assigned female at birth while people with XY are usually assigned male at birth. Though rare, other combinations of sex chromosomes are possible.
  • The other 22 pairs are autosomes (non-sex chromosomes).

 

Human chromosome karyotype. Image credit: Doc. RNDr. Josef Reischig / via Wikimedia Commons

What is the structure of a chromosome?

 

  • 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.
  • 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 – a bit like the plastic tip on a shoelace.

 

Illustration showing how DNA is packaged into a chromosome. Image credit: Laura Olivares Boldú / Wellcome Connecting Science.

Some organisms have fewer chromsomes than the 23 pairs humans have - like the Koala with only 16 chromosomes in 8 pairs. Others have lots more than we do, such as several species of butterfly, such as the Atlas Blue, whose chromosomes number in the hundreds.

One full set of chromosomes from an organism is called their genome. Find out more on our next 'Introduction to Genomics' page.