DNA, or deoxyribonucleic acid, is a chain that is made up of smaller parts known as nucleotides. These nucleotides are then made up of three different parts of nucleotides: the sugar group, the phosphate group and a group of four different nitrogen bases, which are adenine (A), cytosine (C), guanine (G) and thymine (T).
Each nucleotide is known as a base, with a sugar group on one side, a phosphate group on the other, and one of the four nitrogen bases in the middle. In order for the bases to link up correctly, the sugar and phosphate groups must alternate sides. DNA is composed of two strands of nitrogen bases linked together to form a double helix. It can be thought of like a ladder, where the sugar and phosphate groups form the sides and each pair of bases forms the rungs.
The sugar and phosphate groups are merely the links between the various nitrogen bases, as it is the order of the four nitrogen bases that determines which genes are expressed. For example, ACCTG could be the DNA sequence that results in brown hair, while ACGTC could result in blond hair.
Human DNA consists of a pair of 23 chromosomes, which contain approximately 20,000 different genes. In turn, these genes are made up of over 3 billion of the nitrogen bases.