The structure of a DNA molecule is the famous double helix, which was one of the greatest scientific discoveries of modern times. The double helical structure of DNA eluded generations of scientists since the discovery of the basic principles of genetics in the 1800s. It wasn't until 1953 that two scientists, by the names of James Watson and Francis Crick, described the double helix in a landmark paper published in 1953. Today many of us are familiar with DNA's spiral staircase or twisted ladder shape, consisting of sugar-phosphate sides and base pair rungs.

What are the building blocks of DNA?

The building blocks of nucleic acids are called nucleotides. On a molecular level, every piece of DNA is a long chain of nucleotides joined together end-to-end, like links in a chain. A type of covalent bond, known as a phosphodiester bond, forms between the 5' phosphate group of one nucleotide and the 3'-hydroxyl (3'-OH) group of the adjacent nucleotide.

What is the structure of a nucleotide?

Every nucleotide is made of three basic components: a phosphate group, a five-carbon sugar molecule, and a nitrogenous base. The phosphate group is a cluster of specific atoms named for the fact that it contains phosphorus. Phosphate groups are identical across all nucleotides. The sugar molecule is exactly what it sounds like—a type of sugar, just like the ones that we eat in our diets. Nucleotides may contain one of several different types of sugar molecules. The sugar found in the nucleotides of DNA is called deoxyribose, hence the full name of DNA (deoxyribonucleic acid). The nitrogenous base is a molecule named for the fact that it contains nitrogen. It is the only part of a DNA nucleotide that varies from building block to building block.

How does the structure of DNA store information?

Nitrogenous bases pair together in a very specific way: Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). The order of the A, T, G, and C nucleotides that make up your DNA is like a four-letter language that spells out instructions for how the cells in your body should operate. Your cells read the language or code given by the sequence of nucleotides in your DNA and carry out the instructions it contains. These instructions are responsible for all of your biological functions.

Over 99.9% of human DNA is identical from person to person, meaning that it contains the same sequence of A, T, G, and C nucleotides and therefore the same instructions.

In less than 0.1% of human DNA, the order of the nucleotides and the instructions that they provide can vary. For example, one person's DNA may spell out instructions for lower melanin, resulting in green or blue eyes, while another person's DNA may spell out instructions for higher melanin, resulting in brown eyes.

Thus understanding the structure of DNA is an important part of understanding how genetic information is encoded, and how it is passed along from one generation to the next.