What Is DNA and How Does It Store Information?

DNA, short for deoxyribonucleic acid, is the molecule that carries the instructions for building and running living things. It is found in nearly every cell of your body and is passed from parents to offspring, which is why family members often share traits.

Key takeaways

• DNA is a long molecule shaped like a twisted ladder, or double helix.
• Its information is written with four chemical bases: A, T, C and G.
• Genes are sections of DNA that usually contain instructions for making proteins.
• Cells copy their DNA before dividing, with mechanisms that fix most copying errors.
• Genes influence traits, but environment and chance also matter.

Structure of the double helix

DNA is made of two long strands wound around each other. Each strand is a chain of units called nucleotides, and each nucleotide contains a sugar, a phosphate group and one of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The two strands are held together by pairs of bases, with A always pairing with T and C always pairing with G. This pairing rule is key to how DNA is copied. The double-helix structure was described by James Watson and Francis Crick in 1953, building on important X-ray data from Rosalind Franklin and Maurice Wilkins.

DNA as a code

The order of bases along a strand works like letters in a very long text. In humans, the full set of DNA, called the genome, contains roughly three billion base pairs. A gene is a stretch of DNA whose sequence usually specifies how to build a protein, and the human genome contains around twenty thousand protein-coding genes. Much of the rest of the DNA has other roles, such as helping to control when genes are switched on.

From gene to protein

Making a protein happens in two main steps. First, the cell copies a gene into a related molecule called messenger RNA, a process called transcription. Then the cell reads the RNA in groups of three bases, each group specifying one amino acid, and links the amino acids into a chain that folds into a protein. This is called translation. Proteins do most of the work in cells, forming structures, speeding up chemical reactions and carrying signals.

Copying and mistakes

Before a cell divides, it copies all its DNA. The two strands separate, and each acts as a template for a new partner strand. Enzymes proofread the copy and repair most errors. Occasionally a change slips through, called a mutation. Many mutations have no noticeable effect, some are harmful and a few can be beneficial. Mutations in reproductive cells can be inherited, and over long periods they are one source of the variation on which evolution acts.

What DNA can and can't tell us

Because everyone except identical twins has a unique DNA sequence, DNA is used in forensic identification and in tracing family relationships. Consumer DNA tests can estimate ancestry or report certain genetic variants, but their results are statistical and limited. Most traits and health outcomes depend on many genes plus lifestyle and environment, so a test result is rarely a simple yes or no. Anyone with health concerns should talk to a qualified healthcare professional or genetic counsellor.

Chromosomes and genes

Human DNA is packaged into 23 pairs of chromosomes in the nucleus of most cells, one set inherited from each parent. Twenty-two pairs are autosomes, and the last pair are the sex chromosomes, usually XX or XY. If you stretched out the DNA from a single human cell, it would be about two metres long, so cells coil it tightly around proteins called histones. Genes are scattered along the chromosomes, and a gene can be read in different ways to make related proteins.

Beyond the sequence: epigenetics

Genes are not simply switched on forever. Chemical tags attached to DNA or to its packaging proteins can change how active a gene is without altering the underlying sequence. This field is called epigenetics. It helps explain why identical twins, who share almost the same DNA, can become slightly different over time, and why different cell types behave differently. Researchers are studying how diet, stress and environment influence these tags, but many claims in popular media go beyond the evidence.

Variation and ancestry

Humans are genetically very similar, but small differences matter. Most variation comes from single-letter changes called SNPs. Ancestry tests compare your SNPs with reference groups to estimate where your ancestors may have lived. Results are statistical and depend on the company's database, so two companies may give slightly different answers, and the results can change as databases grow. Some tests also trace the maternal line through mitochondrial DNA and the paternal line through the Y chromosome.

Reading the whole genome

The Human Genome Project began in 1990 and published a first draft in 2000, with a nearly complete sequence announced in 2003. In 2022, researchers reported the first truly complete human genome sequence, filling in difficult repetitive regions. The cost of sequencing has fallen dramatically, from billions of dollars to a few hundred, which has made it possible to sequence people's genomes in hospitals to diagnose rare diseases and to guide some cancer treatments.

Gene editing and ethics

Tools such as CRISPR allow scientists to make precise changes to DNA, and approved therapies now treat certain genetic blood disorders. Editing the DNA of embryos or reproductive cells, which would pass on changes to future generations, is highly controversial and restricted or banned in many countries. Genetic data also raises privacy concerns. Laws in some places prohibit certain kinds of genetic discrimination in employment or health insurance, but protections vary, so it is worth understanding how a company will store and share your data before using a testing service.

Frequently asked questions

Do all cells contain the same DNA?

Nearly all body cells carry the same DNA, but different cells switch on different genes, which is why a skin cell and a nerve cell look and behave differently.

Is DNA only in the nucleus?

Most is in the nucleus, but mitochondria also have a small amount of their own DNA.

How much DNA do humans share with each other?

Any two people share the vast majority of their DNA, with differences in a small fraction of positions.

How much DNA is in a single cell?

A typical human cell nucleus contains about three billion base pairs in each set of chromosomes, which would stretch to roughly two metres if uncoiled. Red blood cells have no nucleus and therefore no nuclear DNA.

Can DNA predict my future health?

Only in limited ways. A few conditions are caused by single genes and can be predicted, but most health outcomes depend on many genes, environment and chance. Genetic risk scores can add information, but they are not a diagnosis. A doctor or genetic counsellor can help interpret results.

Do identical twins have exactly the same DNA?

Nearly, but not perfectly. Small differences arise during development and later in life, and their epigenetic patterns can diverge. These differences help explain why identical twins are alike but not identical in health and traits.

Sources and further reading

• DNA (Wikipedia)
• National Human Genome Research Institute