DNA is the inherited genetic material that controls gene expression through protein synthesis. This is a fancy way of saying that DNA is a chemical form of a genetic blueprint. The genetic instructions contained in DNA are housed in genes.

Genes are individual segments of a DNA molecule. Genes are further broken down into smaller, linear building blocks called nucleotides. Nucleotides are aptly named for the nitrogenous base they contain. Much of biology hinges on the chemistry of DNA.

A nucleotide monomer can be chemically broken down into three parts. The first part is a five-carbon sugar, also called a pentose sugar. The second part is a central phosphate atom bonded to a ring of four oxygen atoms, also called a phosphate group. The third part is one nitrogen-containing base from a possible four: Thymine, cytosine, adenine, or guanine.

These four nitrogenous bases, also represented as A, T, C, or G, are one of two chemical structures: Pyrimidines or purines. Pyrimidines are chemically a single-ringed structure; however, purines form a double-ring. Thymine and cytosine are classified as pyrimidines, whereas guanine and adenine are purines.

The structure of DNA is dependent upon the unique chemical interaction between a purine and a pyrimidine. This precise pairing of purines to pyrimidines essentially represents the mortar that bonds two strands of DNA. The result is a structure called a double helix.

Double helix is the name given to a DNA molecule because of its twisted shape. A DNA molecule is chemically two strands of DNA held together by hydrogen bonding. One strand of DNA might look like a straight ladder cut down the middle, splitting it into a left and right side.

Each strand is a chemical complement of the other. The nucleotides that pair together represent the rungs of the ladder. So, when the halves of the ladder bond, a single ladder is formed.

For genes to properly function, the nucleotides must pair correctly. They do this by adhering to a strict set of base-pairing rules that were discovered by pioneers in the science of genetics, James Watson and Francis Crick. Base-pairing rules dictate that a purine always pairs with a pyrimidine. In other words, adenine always pairs with thymine and cytosine always pairs with guanine.

Nucleotide bonding between two DNA strands occurs due to weak, but important, hydrogen bonds. The chemical structure of DNA appears twisted due to this and other complex bonding between the various elements in the DNA molecule itself. The twisted ladder shape of DNA is what gives the molecule its unique name - the double helix.