The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer, a large and complex molecule. It is made up of two strands forming a twisted ladder structure called a double helix. DNA is contained in structures called chromosomes.
A gene is the unit of heredity, and may be copied and passed on to the next generation.
The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.
The cell's nucleus contains chromosomes. These are long threads of DNA, which are made up of many genes. Humans have around 25,000 genes across their 23 pairs of chromosomes.
The genome of an organism is the entire genetic material of that organism. The sequence of letters in the human genome has been determined, but research is still ongoing. Understanding the human genome has great importance for medicine.