The molecule that stores instructions
DNA, short for deoxyribonucleic acid, is a molecule found in nearly every cell of living things. It stores the instructions that cells use to build and run an organism. Passed on in reproduction, it is also the physical basis of inheritance, as described in the article on inherited traits.
In humans and other animals, most DNA sits in the cell’s nucleus, packed into chromosomes. Smaller amounts are in the mitochondria, the structures that supply cells with energy. Bacteria, which have no nucleus, carry their DNA in the main body of the cell.
The double helix
DNA is a long, thin molecule made of two strands that wind around each other in a spiral, a shape called the double helix. Each strand is a chain of smaller units called nucleotides. A nucleotide has three parts: a sugar, a phosphate group, and one of four chemical bases. The sugar and phosphate parts form the backbone of each strand, and the bases point inward.
The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). They pair up across the two strands in a fixed pattern: A always pairs with T, and C always pairs with G. Weak chemical attractions called hydrogen bonds hold each pair together. Because of this rule, each strand carries the information needed to rebuild the other.
The double helix structure was proposed in 1953 by James Watson and Francis Crick, who built on X-ray images of DNA, including important work by Rosalind Franklin and Maurice Wilkins. How much credit each person deserves has been debated by historians since.
DNA as a code
The order of the bases along a strand is the information. It is a bit like writing with a four-letter alphabet. This is a useful contrast with binary code, which uses only two symbols; DNA’s four-symbol code can also encode vast amounts of information in a very small space.
The human genome, the full set of DNA in a cell, is made up of roughly three billion base pairs. Stretched out end to end, the DNA in a single cell would be a couple of meters long, and it fits inside a nucleus only because it is wound tightly around proteins and coiled into chromosomes.
What genes are
A gene is a stretch of DNA that carries the instructions for making a particular product, usually a protein. Humans have roughly 20,000 protein-coding genes, though exact counts depend on how a gene is defined and they are still refined by researchers. Genes make up only a small part of the genome. Other regions help control when and where genes are switched on, and the role of some DNA is still being investigated.
Proteins do much of the work in the body. They build structures, carry molecules, act as enzymes that speed up chemical reactions, and send signals between cells. Because the proteins a cell makes shape what that cell does, the difference between, say, a skin cell and a nerve cell comes largely from which genes are active in each, not from different DNA.
From gene to protein
The cell reads a gene in two main steps.
- Transcription. Inside the nucleus, an enzyme copies the gene’s sequence into a related molecule, RNA, which is a single-stranded messenger.
- Translation. The messenger RNA moves to a ribosome, a molecular machine that reads it three bases at a time. Each three-base group, called a codon, specifies a particular amino acid, or signals where to stop. The ribosome links amino acids together in order to build a protein chain, which then folds into a specific three-dimensional shape.
The same genetic code is used, with few exceptions, by almost all living things, which is one of the strongest pieces of evidence for their shared ancestry.
Copying DNA
Before a cell divides, it must duplicate its DNA. Enzymes unwind the helix, and each original strand serves as a template for building a new partner strand, using the base-pairing rule. The result is two identical double helices, one for each new cell. The process includes proofreading mechanisms that fix most copying mistakes, though not all.
Mutations
A change in the DNA sequence is called a mutation. Mutations can come from copying errors or from damage caused by things such as ultraviolet light or certain chemicals. Many have no noticeable effect, some are harmful, and a few can be beneficial in a given environment. Mutations that occur in eggs or sperm can be passed on to children. Over many generations, such variation is the raw material for natural selection, central to the work of Charles Darwin, though he knew nothing of DNA itself.
Two unrelated people share very nearly all of their DNA. The small fraction that differs accounts for much of the variation between individuals, along with environmental influences.
Why it matters
Knowledge of DNA underlies many advances in medical science, including genetic testing, understanding inherited disease, and some modern medicine such as treatments tailored to particular genetic changes. It is also used in forensics and in tracing family and population history. These uses raise questions about privacy and ethics that societies are still working out.
This article is general educational information, not medical or genetic advice. People with questions about their own health or family history can speak with a qualified health professional or genetic counselor.