What is DNA?
DNA, the hereditary material in humans and almost all other organisms, is a double‑helix of nucleotides that stores genetic information in a sequence of four bases.
Location of DNA
Nearly every cell in a person’s body contains the same DNA. Most of it resides in the cell nucleus (called nuclear DNA), while a small amount is found in the mitochondria (called mitochondrial DNA or mtDNA). Mitochondria are structures that convert the energy from food into a form that cells can use.
Chemical Composition
DNA is encoded by a sequence of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA contains about 3 billion bases, and more than 99 percent of those bases are identical in all people. The order of the bases functions like letters in a language, encoding information for building and maintaining an organism.
Structure of the Double Helix
Base pairs are formed by complementary bases: A pairs with T and C pairs with G. Each base is attached to a sugar and a phosphate molecule, together forming a nucleotide. Two strands of nucleotides run in opposite directions and wind around each other to form a double helix. The base pairs form the rungs of a ladder, while the sugar‑phosphate backbones form the sides.
Replication
DNA can replicate, making copies of itself. Each strand of the double helix can serve as a template for duplicating the sequence of bases, which is essential when cells divide so that each new cell receives an exact copy of the DNA.
Key Facts
- DNA is the hereditary material in humans and almost all other organisms.
- Most DNA is located in the nucleus; a small amount is in mitochondria.
- Human DNA has ~3 billion bases; >99 % are identical among people.
- Base pairs: A‑T, C‑G.
- Structure: double helix, sugar‑phosphate backbone.
- DNA replicates during cell division.
Further Resources
For more information about DNA:
- National Human Genome Research Institute (NHGRI) fact sheet on Deoxyribonucleic Acid (DNA).
- NHGRI Talking Glossary of Genomic and Genetic Terms – information on DNA and mitochondrial DNA.
- StatedClearly video: “What is DNA and How Does it Work?”
- Arizona State University “Ask a Biologist” website – basic explanation and illustration of DNA.
Related Topics in the Cells and DNA Chapter
- What is a cell?
- What is DNA?
- What is a gene?
- What is a chromosome?
- How many chromosomes do people have?
- What is noncoding DNA?
Other chapters in Help Me Understand Genetics are also available.
Credit: U.S. National Library of Medicine.