The double helix of DNA is one of the iconic images of the 20thcentury, well-known in both popular culture and in the scientific community. Its twisting spiral is a reflection of both the composition and the function of its molecules.

The DNA molecule
DNA is made up of sub-units called nucleotides. Each nucleotide is composed of a deoxyribose sugar, a phosphate group and a nitrogen-containing base. The nucleotides are strung together into long DNA molecules. A single strand of DNA may be made of millions of linked nucleotides.

DNA molecules most commonly occur in pairs, two strands that wrap tightly together to form a double helix. The DNA double helix looks something like a twisted ladder. The sides of the ladder are composed of the sugar and phosphate molecules. These are covalently bonded together, forming a strong bond that acts as the backbone of DNA. Each rung of the ladder is made up of two nitrogenous bases, one projecting out from each DNA strand. These are joined by weak hydrogen bonds, which means they are easily broken apart and then put back together.

Every nucleotide has an identical sugar and phosphate backbone, attached to one of four different bases. There are two pyrimidine bases, thymine and cytosine, and two purine bases, adenine and guanine. These are usually abbreviated as their first letters:

To maintain the even shape of the double helix there is a specific pairing pattern. A purine has two rings and so will only pair with a pyrimidine, which has only one ring.

Because the base pairs are so specific, the two strands of DNA in a double helix are perfect and specific complements of one another.

Chromosomes and Chromatin
DNA can exist in a condensed form called a chromosome, or in a non-condensed, loosely-coiled form called chromatin. As chromatin, it is more accessible to be copied for replication and transcription. As a chromosome, the DNA is better suited for movement and manipulation when cells divide.

DNA Functions
DNA’s double helix habit is critical to its function. This structure allows the DNA molecule to:

The processes that allow the DNA to be used as a molecule of inheritance are transcription, translation and replication. Transcription rewrites DNA into a type of RNA that carries the DNA’s message from the nucleus into the cytoplasm, where it can be turned into proteins via translation. Replication takes place prior to cell division to make copies of the DNA that will be divided into the new cells.