What is DNA?
Quick answer
DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for the growth, function and reproduction of almost all living things. It’s shaped like a twisted ladder, a double helix, and its code is written with four chemical bases: A, T, C and G. Sections of DNA called genes tell cells how to make proteins.
The double helix
| Part | What it is |
|---|---|
| Sides of the ladder | Alternating sugar and phosphate |
| Rungs | Pairs of bases |
| Base pairs | A pairs with T, C pairs with G |
Because each base has a fixed partner, one strand can serve as a template to copy the other. This is how DNA is copied when cells divide; see why cells divide.
DNA by the numbers
| Fact | Figure |
|---|---|
| Base pairs in the human genome | About 3 billion |
| Protein-coding genes | About 20,000 |
| DNA in one human cell, stretched out | About 2 m |
| DNA shared between any two people | About 99.9% |
Who discovered it?
| Year | Discovery |
|---|---|
| 1869 | Friedrich Miescher isolates “nuclein” from white blood cells |
| 1944 | Experiments show DNA, not protein, carries genetic information |
| 1952 | Rosalind Franklin’s X-ray image, “Photo 51,” reveals a helix |
| 1953 | James Watson and Francis Crick publish the double-helix model |
| 2003 | The Human Genome Project releases a near-complete human genome |
Watson, Crick and Maurice Wilkins won the 1962 Nobel Prize. Franklin had died in 1958, and her key role was long underrecognized.
From DNA to protein
- A gene’s DNA code is copied into a messenger molecule, RNA.
- RNA travels to the cell’s protein factories, ribosomes.
- Every three bases code for one amino acid; see what amino acids are.
- The amino acids are linked into a protein.
Where DNA is found
In plants and animals, most DNA sits in the cell’s nucleus, wound into chromosomes; humans have 46. A little is also in the mitochondria. Bacteria keep their DNA loose in the cell; see which organisms lack a nucleus.
Uses of DNA today
- Forensics: identifying people from tiny samples
- Ancestry tests
- Medicine: diagnosing genetic conditions and designing treatments
- Ancient DNA: studying extinct species and early humans
Inheritance was first worked out before anyone knew about DNA; see Gregor Mendel’s pea plants.
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