A DNA molecule is composed of two long strands, each of which is made of building blocks called nucleotides bonded together.

Every nucleotide is made of three components: a phosphate group, a deoxyribose sugar, and a nitrogen-containing base. Nucleotides are all identical except for their base. DNA has four different bases, known as adenine, thymine, cytosine, and guanine. Bases are often referred to by the letters A, T, C, and G.

On the left is a gray box labeled "Nucleotide." In it, a dark gray circle labeled "Phosphate" is connected by a black line to a blue hexagon labeled "Sugar." Another black line connects this sugar to a dark gray rectangle labeled "Base (A, T, C or G)." The grey box points to a smaller box showing the position of a nucleotide in a large double-stranded molecule of DNA. The double-stranded molecule is not twisted. It has two grey strips on either side labeled sugar-phosphate backbone. Sticking out from the strips are yellow bands labeled "A" connected by two dashed lines to purple bands labeled "T" and red bands labeled "C" connected by three dashed lines to turquoise bands labeled "G." There are eight pairs of these bands between the two strips. "Base pairing" is written above the double-stranded molecule. To the right is a twisted molecule of DNA labeled "Double helix."

The Structure of DNA

The two strands in DNA are held together by hydrogen bonding between the bases. DNA bases always pair in the same way: A with T and C with G.

The double-stranded molecule is twisted into a double helix shape resembling a twisted ladder. Opposite strands run antiparallel to each other, meaning that they run in opposite directions. This ensures that the strands fit tightly together.