Adenine, guanine, cytosine and thymine are the four chemical bases found in DNA. In a given DNA molecule, adenine always pairs with thymine, and guanine always pairs with cytosine.
DNA molecules consist of long sequences of paired bases, which are each connected to a sugar and a phosphate molecule. DNA sequences vary from person to person, and this variation accounts for different individual traits. However, about 99 percent of the DNA found in a given individual has exactly the same sequence as everyone else on the planet.
Since DNA is capable of self-replication, both strands of a DNA molecule contain all of the information needed to make a full DNA copy with the same base sequence as the original molecule.