DNA's building blocks are called nucleotides. Nucleotides are made up of a five-carbon sugar, a nitrogenous base and one phosphate. The nitrogenous base can be adenine, cytosine, guanine and thymine, which are often shown on DNA models as the letters A, C, G and T.
The bases are broken down into two groups, the purines and the pyrimidine. The purines group is made up of the adenine and guanine. This group has a double ring structure, which makes them very easy to tell from the other group. The pyrimidine group is made up of the thymine and cytosine bases. The sugars and the phosphate make up the backbone of the double helix DNA, leaving the bases to essentially create steps of the DNA ladder. The way the bases fit together inside the double helix is very specific. An adenine may only pair with a thymine and a guanine only with a cytosine. This specific pairing style keeps the width of each pair the same. The pairs are held together by weak hydrogen bonds, which is easily broken when the DNA is unzipped for translation. Once it is unzipped, the DNA can be reproduced and the information held in the base pairs shared via mRNA, beginning the process of DNA replication.