The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people.

The DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases--adenine (A), cytosine (C), guanine (G), or thymine (T).

Bases are the part of DNA that stores information and gives DNA the ability to encode phenotype, a person’s visible traits. Adenine and guanine are purine bases. These are structures composed of a 5-sided and 6-sided ring. Cytosine and thymine are pyrimidines which are structures composed of a single six-sided ring.

DNA is a long polymer made from repeating units called nucleotides, each of which is usually symbolized by a single letter: either A, T, C, or G. Chargaff's rules state that DNA from any species of any organism should have a 1:1 protein stoichiometry ratio (base pair rule) of purine and pyrimidine bases (i.e., A+T=G+C) and, more specifically, that the amount of guanine should be equal to ...

In DNA, these bases are cytosine (C), thymine (T), adenine (A) and guanine (G). Note: These are called "bases" because that is exactly what they are in chemical terms. They have lone pairs on nitrogens and so can act as electron pair donors (or accept hydrogen ions, if you prefer the simpler definition).

DNA structure, showing the nucleotide bases cytosine (C), thymine (T), adenine (A), and guanine (G) linked to a backbone of alternating phosphate (P) and deoxyribose sugar (S) groups. Two sugar-phosphate chains are paired through hydrogen bonds between A and T and between G and C, thus forming the twin-stranded double helix of the DNA molecule.