What is a chromosome?
3D rendering X chromosomes
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.
