What Is DNA?

Deoxyribonucleic acid—or DNA— is a molecule that serves as the hereditary material containing biological instructions that make every human and other organism unique. During reproduction, adult organisms pass their DNA and its set of instructions along to their offspring.

DNA molecule illustration

KTSDESIGN/SCIENCE PHOTO LIBRARY

The Structure and Makeup of DNA

DNA is made up of nucleotides, which are essentially chemical building blocks. Nucleotides join together in chains to form a strand of DNA, and contain three parts: a phosphate group, a sugar group, and one of four types of chemical bases:

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)

These chemical bases come together to create the information found in DNA, and stores it in a code, based on their sequence. A human genome—or the full set of instructions from DNA—contains about 3 billion bases and about 20,000 genes on 23 pairs of chromosomes.

Where DNA Is Found

DNA is found in nearly every cell of the human body. It is primarily located in the nucleus (where it is also referred to as "nuclear DNA"), though there is also a small amount in the mitochondria as well. Mitochondria are another part of human cells and are in charge of converting energy from food into a form that can power the cells. Collectively, all the nuclear DNA in an organism is known as its "genome."

How DNA Works

The purpose of DNA is to instruct organisms—including humans—on how to develop, survive, and reproduce. In order for this to happen, DNA sequences—known as "genes"—are converted into proteins, which are complex molecules responsible for carrying out most of the work in human bodies. While genes vary in size—ranging from about 1,000 bases to 1 million bases in humans—they only make up approximately 1% of the DNA sequence. The rest of the DNA sequences regulate when, how, and how much of a protein is made.

It takes two separate steps to make proteins using instructions from DNA. The first is when enzymes read the information delivered in a DNA molecule and then transcribe it to a separate molecule called messenger ribonucleic acid, or mRNA. Once that happens, the information sent by the mRNA molecule is then translated into a language that amino acids—also known as the building blocks of proteins—can understand. The cell applies those instructions in order to link the correct amino acids together to create a specific type of protein. Given that there are 20 types of amino acids that can be put together in many possible orders and combinations, it gives DNA the opportunity to form a wide range of proteins.

The Double Helix

To understand how DNA works, it's important to go back to the four chemical bases mentioned earlier: A, G, C, and T. They each pair up with another base in order to create units called "base pairs." Then, each base also attaches to a sugar molecule and a phosphate molecule, forming a nucleotide. When arranged in two long strands, nucleotides form what looks like a twisted ladder or spiral staircase known as a "double helix." Using the example of a ladder, the base pairs are the rungs, while the sugar and phosphate molecules form the vertical sides of the ladder, holding it all together.

The shape of the double helix is what gives DNA the capability to pass along biological instructions with great accuracy. This is the case because the spiral shape is the reason DNA is able to replicate itself during cell division. When it comes time for a cell to divide, the double helix separates down the middle to become two single strands. From there, the single strands function as templates to form new double helix DNA molecules, which—once the bases are partnered and added to the structure—turns out as a replica of the original DNA molecule.

The History and Discovery of DNA

In 1869, Swiss physician and biochemist Friedrich Miescher discovered a chemical substance in human leucocytes. His research focused on the chemical contents of a cell's nucleus, and in order to get a better look at them, he examined pus on surgical bandages from the local hospital. Pus was known to contain large amounts of leucocytes, so Miescher purified their nuclei to better understand their makeup. In doing so, he was able to isolate a new chemical substance in the nucleus, which he named "nuclein"—but is known today as DNA. While there was a significant amount of research done on nucleic acids during and shortly after Miescher's lifetime, it would take several more decades before scientists understood their significance.

There was a renewed interest in DNA starting in the 1930s, with many major discoveries soon following, including the understanding that DNA was responsible for passing along hereditary characteristics. The structure of DNA was also the subject of research in the 1930s, including that of English physicist and molecular biologist William T. Astbury, who suggested that DNA was a long and helical linear molecule.

The best-known DNA breakthrough came in 1953, when Rosalind Franklin, James Watson, Francis Crick, and Maurice Wilkins conducted research that would result in the discovery of the double helix model of DNA. Using X-ray diffraction patterns and building models, the scientists determined that the double helix structure of DNA enabled it to carry biological information from one generation to the next.

In 1962, Watson, Crick, and Wilkins were awarded the Nobel Prize in medicine for their discovery. Though Franklin would have been eligible to receive the prize, she died in 1958 from ovarian cancer at the age of 37, and the Nobel Prize rules stipulate that the award can't be split among more than three people, or given out after someone has died.

A Word From Verywell

Like many scientists who researched genetics in the field's early days, Watson was known to hold damaging—and scientifically inaccurate—beliefs on race, ethnicity, gender, and sexual identity, among other demographics. While the discoveries he made alongside his colleagues were significant, it's also important to acknowledge aspects of his work that don't hold up today.

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  2. MedlinePlus. What is DNA? Updated September 17, 2020.

  3. Portin P. The birth and development of the DNA theory of inheritance: sixty years since the discovery of the structure of DNAJ Genet. 2014;93(1):293-302. doi:10.1007/s12041-014-0337-4