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, made from many small monomers It is made up of two strands forming a twisted ladder structure called a double helix. It carries the genetic code, which determines the characteristics of a living organism.

Except for identical twins, each person's DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, which can form a 'bar code' that is different from one person to the next.


The cell's nucleus contains chromosomes. These are long threads of DNA, which are made up of many genes.


A gene is a small section of DNA in a chromosome. Each gene codes for a particular sequence of amino acids in order to make a specific protein. A gene is the unit of heredity, and may be copied and passed on to the next generation.

Diagram showing the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes

DNA base pairs

The basic units (monomers) of DNA are nucleotides. These nucleotides consist of a deoxyribose sugar, phosphate and base. The nucleotides are identical except for the base, which can be an adenine (A), thymine (T), guanine (G) or cytosine (C).

Bases are complementary. This means they always pair in the same way: A with T, T with A, C with G and G with C.

A nucleotide consisting of a deoxyribose sugar, phosphate and base

These basic units are linked together to form strands by strong covalent bonds between the deoxyribose sugar of one nucleotide and the phosphate of the next nucleotide. These strong bonds form a sugar-phosphate backbone.