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What is DNA?

Updated: Jan 17, 2022

DNA stands for deoxyribonucleic acid, which is a molecule that contains the instructions an organism needs to develop, live and reproduce. These instructions are found inside every cell and are passed down from parents to their offspring.

What is DNA made of?

DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C).

Nucleotides are attached together to form two long strands that spiral to create a structure called a double helix. If you think of the double-helix structure as a ladder, the phosphate and sugar molecules would be the sides, while the base pairs would be the rungs. The bases on one strand pair with the bases on another strand: Adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C).

Human DNA is made up of around 3 billion base pairs, and more than 99% of those bases are the same in all people, according to the U.S. National Library of Medicine (NLM).

Similar to the way the order of letters in the alphabet can be used to form words, the order of nitrogen bases in a DNA sequence forms genes, which, in the language of the cell, tell cells how to make proteins. The shorthand for this process is that genes "encode" proteins. But DNA is not the direct template for protein production. To make a protein, the cell makes a copy of the gene, using not DNA but ribonucleic acid, or RNA. This RNA copy, called messenger RNA, tells the cell's protein-making machinery which amino acids to string together into a protein, according to "Biochemistry" (W. H. Freeman and Company, 2002).

DNA molecules are long — so long, in fact, that they can't fit into cells without the right packaging. To fit inside cells, DNA is coiled tightly to form structures called chromosomes. Each chromosome contains a single DNA molecule. Humans have 23 pairs of chromosomes, which are found inside each cell's nucleus.

DNA was first observed by Swiss biochemist Friedrich Miescher in 1869, according to a paper published in 2005 in the journal Developmental Biology. Miescher used biochemical methods to isolate DNA — which he called nuclein — from white blood cells and sperm, and determined that it was very different from protein. (The term "nucleic acid" derives from "nuclein.") But for many years, researchers did not realize the importance of this molecule.

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In 1952, chemist Rosalind Franklin, who was working in the lab of biophysicist Maurice Wilkins, used X-ray diffraction — a way of determining the structure of a molecule by the way X-rays bounce off it — to learn that DNA had a helical structure. Franklin documented this structure in what became known as Photo 51.

In 1953, Wilkins showed the photo to biologists James Watson and Francis Crick— without Franklin's knowledge. Armed with the information that DNA was a double helix and previous reports that the bases adenine and thymine occurred in equal amounts within DNA, as did guanine and cytosine, Watson and Crick published a landmark 1953 paper in the journal Nature. In that paper, they proposed a model of DNA as we now know it: a double helical ladder with sugar-phosphate sides and rungs made up of A-T and G-C base pairs. They also suggested that, based on their proposed structure, DNA could be copied — and, therefore, passed on.

Watson, Crick and Wilkins were awarded the Nobel Prize in medicine in 1962 "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material." Franklin was not included in the award, even though her work was integral to the research.

How does DNA function?

Genes encode proteins that perform all sorts of functions for humans (and other living beings). The human gene HBA1, for example, contains instructions for building the protein alpha globin, which is a component of hemoglobin, the oxygen-carrying protein in red blood cells, according to the NLM. To take another example, the gene OR6A2 encodes an olfactory receptor, a protein that detects odors in the nose, scientists reported in 2021 in the journal Gene. Depending on which version of OR6A2 you have, you may love cilantro or think it tastes like soap, according to a study published in 2012 in the journal Flavour.

DNA testing

A person's DNA contains information about their heritage, and it can sometimes reveal whether they are at an elevated risk for certain diseases. DNA tests, or genetic tests, are used for a variety of reasons, including to diagnose genetic disorders, to determine whether a person is a carrier of a genetic mutation that they could pass on to their children and to examine whether a person is at risk for a genetic disease. For instance, certain mutations in the BRCA1 and BRCA2 genes are known to increase the risk of breast and ovarian cancers, and analysis of these genes in a genetic test can reveal whether a person has these mutations.

Genetic test results can have implications for a person's health, and the tests are often provided along with genetic counseling to help individuals understand the results and consequences

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