Inside almost every cell of your body is a molecule about 6 feet (2 meters) long, folded into a space far smaller than the head of a pin. It contains the instructions for building and running a human being: how your cells grow, what color your eyes are, how your body fights infection and much more. This molecule is DNA, and the instructions it carries are organized into genes. Understanding DNA has transformed biology and medicine, from tracing human ancestry to diagnosing diseases and developing new treatments. This explainer describes what DNA is, how it works and how traits are passed from parents to children.
What is DNA?
DNA stands for deoxyribonucleic acid. It is a long molecule made of two strands that twist around each other to form a shape called a double helix, like a twisted ladder.
Each strand is a chain of building blocks called nucleotides. Each nucleotide contains:
- A sugar called deoxyribose.
- A phosphate group.
- One of four chemical bases: adenine (A), thymine (T), guanine (G) or cytosine (C).
The sugars and phosphates form the sides of the ladder, and the bases form the rungs. The two strands are held together by pairs of bases, which always pair in the same way:
- A pairs with T.
- G pairs with C.
This complementary base pairing is the key to how DNA is copied and how it stores information.
Discovering the double helix
Scientists knew by the 1940s that DNA carried genetic information, but its structure was unknown. In 1953, James Watson and Francis Crick proposed the double helix model, building on X-ray images of DNA produced by Rosalind Franklin and Maurice Wilkins. Franklin's famous "Photo 51" provided crucial evidence of the helical structure. Watson, Crick and Wilkins received the Nobel Prize in 1962; Franklin had died in 1958, and Nobel Prizes are not awarded posthumously. Her contribution is now widely recognized.
The genetic code
The sequence of bases along a DNA strand is like a long message written in an alphabet of four letters: A, T, G and C. The human genome, the complete set of DNA in a human cell, contains about 3 billion base pairs.
Information in DNA is read in groups of three bases, called codons. Each codon corresponds to a particular amino acid, the building blocks of proteins, or to a signal to start or stop. With four bases, there are 64 possible codons, coding for 20 amino acids plus stop signals. This code is nearly universal across all living things, from bacteria to humans, which is strong evidence that all life shares a common origin.
What is a gene?
A gene is a segment of DNA that contains the instructions for making a specific product, usually a protein. Humans have about 20,000 protein-coding genes. Surprisingly, these genes make up only about 1 to 2 percent of the genome. The rest includes regions that regulate when and where genes are switched on, sequences that produce functional RNA molecules, structural regions and sequences whose functions are still being studied.
Chromosomes
DNA in human cells is packaged into structures called chromosomes. The DNA is wrapped around proteins called histones, coiling and folding into compact shapes.
Humans have 46 chromosomes in most cells, arranged in 23 pairs. One chromosome of each pair comes from the mother and one from the father. Twenty-two pairs are called autosomes. The 23rd pair are the sex chromosomes: typically XX in females and XY in males.
From DNA to protein
Proteins do most of the work in cells. They build structures, speed up chemical reactions as enzymes, carry signals and fight infections. Making a protein from a gene involves two main steps, summarized in what is called the central dogma of molecular biology: DNA → RNA → protein.
Transcription
In the cell nucleus, an enzyme copies the sequence of a gene from DNA into a molecule called messenger RNA (mRNA). RNA is similar to DNA but single-stranded and uses the base uracil (U) instead of thymine.
Translation
The mRNA travels out of the nucleus to structures called ribosomes. There, the ribosome reads the mRNA codon by codon. Molecules called transfer RNA (tRNA) bring the matching amino acids, which are joined together in a chain. The chain folds into a specific three-dimensional shape, becoming a functional protein.
Different cells in your body contain the same DNA, but they switch different genes on or off. That is why a skin cell, a muscle cell and a nerve cell look and act so differently. The study of how gene activity is regulated without changing the DNA sequence, through chemical tags on DNA and histones, is called epigenetics.
How DNA is copied
Every time a cell divides, it must copy its entire DNA so that each new cell receives a complete set. This process is called DNA replication.
- The double helix unwinds, and the two strands separate.
- Each strand serves as a template.
- Enzymes called DNA polymerases add new nucleotides following the base-pairing rules: A with T and G with C.
- The result is two identical DNA molecules, each with one original strand and one new strand.
Replication is remarkably accurate. DNA polymerases also proofread their work, and repair systems fix most errors. Still, occasional mistakes remain.
Inheritance: how traits are passed on
Children inherit half of their DNA from each parent. Reproductive cells, eggs and sperm, are formed through a special type of cell division called meiosis, which halves the number of chromosomes and shuffles genetic material, so each egg or sperm carries a unique combination of genes. When an egg and sperm combine at fertilization, the resulting cell has a full set of 46 chromosomes.
Mendel's discoveries
The basic rules of inheritance were discovered in the 1860s by Gregor Mendel, a monk who experimented with pea plants. He found that traits are inherited as discrete units, now called genes, and that each individual has two copies of each gene, one from each parent.
Different versions of a gene are called alleles. Some alleles are dominant, meaning one copy is enough to produce a trait, while others are recessive, meaning two copies are needed. For example, a recessive allele may cause a genetic condition only when a person inherits it from both parents.
Most human traits, such as height, skin color and many disease risks, are influenced by many genes as well as the environment. These are called polygenic traits. Even eye color, often presented as a simple example, is influenced by several genes.
Mutations
A mutation is a change in the DNA sequence. Mutations can happen during DNA replication or be caused by factors such as ultraviolet light, certain chemicals or radiation.
- Many mutations have no effect.
- Some are harmful and can cause genetic diseases or contribute to cancer.
- Some are beneficial, providing the variation on which natural selection acts.
Mutations in reproductive cells can be passed on to children; mutations in other body cells affect only the individual.
Genetic technologies
Understanding DNA has led to powerful technologies:
- DNA sequencing reads the order of bases. The Human Genome Project, completed in 2003, sequenced most of the human genome. Today, a person's genome can be sequenced in a day or two for a fraction of the original cost.
- Genetic testing can identify inherited conditions, guide medical treatment and reveal ancestry.
- DNA fingerprinting identifies individuals in forensic investigations and paternity tests.
- CRISPR-Cas9, a gene-editing tool developed in 2012 by Jennifer Doudna and Emmanuelle Charpentier, who received the 2020 Nobel Prize in Chemistry, allows scientists to cut and modify DNA precisely. It is used in research and has led to approved treatments for some genetic blood disorders.
- mRNA vaccines use genetic instructions to teach the body's cells to make a harmless protein that trains the immune system.
These technologies raise important ethical questions about privacy, consent and how far genetic modification should go, which societies continue to debate.
Questions readers ask
What is the difference between DNA and genes? DNA is the molecule that carries genetic information. Genes are specific segments of DNA that contain instructions for making particular products, usually proteins.
How much DNA do humans share with each other? Any two people share about 99.9 percent of their DNA sequence. The small differences contribute to individual variation.
How many chromosomes do humans have? Usually 46, arranged in 23 pairs.
What are mutations? Changes in the DNA sequence. They can be harmless, harmful or occasionally beneficial.
What is CRISPR? A gene-editing technology that allows scientists to precisely cut and change DNA sequences.
In short
DNA is a double-helix molecule made of four bases, A, T, G and C, that store genetic instructions in their sequence. Genes are segments of DNA that are transcribed into RNA and translated into proteins, which do most of the work in cells. DNA is copied before cells divide and passed to children through eggs and sperm, half from each parent. Mutations create variation, sometimes causing disease and sometimes fueling evolution. From Mendel's peas to CRISPR, understanding DNA has revealed the code of life and opened new possibilities for medicine and science.