DNA facts for kids

For a non-technical introduction to the topic, see Introduction to genetics. For other uses, see DNA (disambiguation).

The structure of the DNA double helix. Chemical atoms make up the shapes of the two base pairs.

Simplified diagram

Deoxyribonucleic acid, usually called DNA, is the instruction manual for living things. It is a long molecule called a polymer. It has two chains called polynucleotide strands. These chains wrap around each other to make a twisted ladder shape called a double helix. DNA carries all the genetic directions that living things need. These directions guide how every organism grows, lives, and survives. Even many kinds of virus particles carry DNA instructions. Along with RNA, DNA is a special substance known as a nucleic acid. Living things also depend on other large macromolecule structures. These important building materials include proteins, lipids, and complex sugars called polysaccharides. Together, these molecules form the basic foundations needed for all known forms of life.

The two strands of DNA are built from smaller parts called nucleotide units. Every single nucleotide has three parts. First, it contains a sugar called deoxyribose. Second, it has a chemical piece called a phosphate group. Third, it carries a special part called a nucleobase. Scientists group these nucleobases into four kinds: adenine (A), cytosine (C), guanine (G), and thymine (T). Strong chemical links called covalent bonds join the nucleotides together. Scientists call this connection a phosphodiester linkage. This connection creates a sturdy side rail called a sugar-phosphate backbone. The bases on one strand connect to bases on the partner strand. They join using hydrogen bonds according to strict rules called base pairing. Adenine always connects with thymine, and cytosine always connects with guanine. These paired bases hold the entire double helix together like steps on a spiral staircase.

Structure and Physical Properties of DNA

Chemical structure of DNA; hydrogen bonds show as dotted lines. Each strand has a 5' end and a 3' end.

DNA is a very long molecule made from linked repeating pieces. Each building block is a monomer called a nucleotide. When a nucleobase links only to a sugar, scientists call it a nucleoside. When you add a phosphate group, it becomes a complete nucleotide. A long chain made of many nucleotides joined together is called a biopolymer. DNA does not normally float around as a single strand. Instead, two matching strands wrap together into a famous twisted shape called a double helix. This long molecular spiral is flexible. It can twist into tight loops, coils, and other shapes inside living cells.

The two outer sides of the DNA ladder form the molecule's structural support. Each side is made of alternating sugar and phosphate groups. The sugar used in DNA is 2-deoxyribose. It is a five-carbon ring called a pentose sugar. Phosphate groups link these sugar rings together between their third and fifth carbon atoms. Scientists refer to these positions as the 3′-end and the 5′-end. This setup gives every strand of DNA an orderly direction. Scientists call this property directionality. The two strands run side by side but point in opposite directions. Because they run in reverse ways, scientists describe them as antiparallel. In contrast, RNA uses a slightly different sugar called ribose.

The Four Basic Building Blocks

A section of DNA showing bases between the two spiraling strands.

The steps of the DNA ladder are made from four main types of bases. These bases are adenine (A), cytosine (C), guanine (G), and thymine (T). Chemists divide these four bases into two different chemical families. Adenine and guanine belong to a family of double-ring molecules called purines. Cytosine and thymine belong to a family of single-ring molecules called pyrimidines. In RNA, a base called uracil (U) takes the place of thymine. Uracil lacks a tiny chemical tag called a methyl group. Scientists can also build artificial nucleic acid analogues in modern laboratories. These human-made parts help researchers study how natural nucleic acids behave.

Chemical Grooves in the Double Helix

DNA major and minor grooves provide a binding site for cellular proteins.

As the two DNA strands twist around each other, gaps appear between the backbones. Scientists call these empty spaces grooves. These spaces act as a binding site where other cell parts can touch the bases. Because the strands do not sit directly opposite each other, the spaces have different sizes. The larger opening is called the major groove. The smaller opening is called the minor groove. In the major groove, the edges of the bases are easier to reach. Special proteins called transcription factors usually touch the bases inside this wide major groove. This allows helper proteins to read the DNA sequence without unwinding the double helix.

Rules of Base Pairing

A base pair showing hydrogen bond lines between matching chemicals.

The bases inside the DNA ladder stick together in very specific ways. A purine base on one strand always binds to a pyrimidine base opposite it. This matching system is called complementary base pairing. Adenine pairs only with thymine, forming two hydrogen bonds. Cytosine pairs only with guanine, forming three hydrogen bonds. This classic shape is called a Watson-Crick base pair. Another rarer style is called a Hoogsteen base pair. DNA strands with a high GC-content are held together very tightly. Hydrogen bonds are weaker than a strong covalent bond. Because of this, heat or cell enzymes can unzip the two strands like a jacket.

DNA Supercoiling and Twisting

DNA can twist tightly upon itself like a wound-up rubber band. Scientists call this tight winding DNA supercoiling. When DNA sits in its relaxed state, the strand turns once every ten base pairs. When twisted in the direction of the spiral, it forms positive supercoiling. In this state, the bases lock tightly together. If twisted in the opposite direction, it forms negative supercoiling. Negative supercoiling makes it easier for the cell to pull the strands apart. Special cellular helpers called topoisomerase enzymes help control this twisting. These enzyme helpers keep DNA from tangling during normal daily life.

Different Shapes of the Double Helix

From left to right, the structures of A, B and Z-DNA.

DNA can change its overall shape depending on its surroundings. Scientists call these different physical arrangements conformations. The most common shape found inside living cells is called B-DNA. In B-DNA, the double helix twists to the right like a normal wood screw. Another form, called A-DNA, is wider and appears when there is less water around. A third type, called Z-DNA, twists to the left instead of the right. The cell uses left-handed Z-DNA in special places to help manage certain active genes.

Telomeres and Quadruplex Structures

DNA quadruplex formed by telomere repeats with protective folded shapes.

The ends of long linear chromosomes have special protective caps called telomeres. These caps stop the cell from mistaking the ends of DNA for broken pieces. Normal copying proteins cannot reach the very tip of a linear chromosome strand. A special enzyme called telomerase helps maintain these tips so genes do not vanish. In human cells, telomeres have repeating sequences of bases rich in guanine. Four guanine bases can join together into a flat square called a guanine tetrad. These flat units stack up to form a sturdy shape called a G-quadruplex. Telomeres can also fold into a giant closed ring called a D-loop.

How Cells Organize and Protect Genetic Information

A human karyotype showing matched homologous chromosome pairs.

Every cell in an organism needs a complete set of genetic instructions. Scientists call this entire collection of genetic material a genome. In a human cell, the main genome contains billions of base pairs of DNA. If you stretched out all the DNA from one human cell, it would measure about two meters. The largest single human package is chromosome 1, which holds hundreds of millions of base pairs. Cells must pack this long thread into microscopic spaces through DNA condensation. In eukaryotic organisms, most DNA sits safely inside a central room called the cell nucleus.

Chromosomes and Chromatin Packaging

DNA wrapping around histone proteins to make chromatin.

To keep long strands from tangling, eukaryotic cells wind their DNA around spool-like proteins. These tiny packing proteins are called histones. Two turns of DNA wrap around a cluster of histones to make a nucleosome. Together, the DNA strands and their spooling proteins form a material called chromatin. Tight packing keeps resting DNA safe from harm and takes up very little space. Adding tiny chemical marks can change how tightly the histones hug the DNA. For example, processes like acetylation, phosphorylation, and methylation loosen or tighten the spooling. When histones loosen, reading proteins can reach the genes much more easily.

DNA in Different Kinds of Cells

Living things store their genetic material in different compartments. Eukaryotes include all animals, plants, fungi, and tiny single-celled protists. Most of their DNA is packed into the nucleus as nuclear DNA. However, tiny powerhouses called mitochondria hold their own mitochondrial DNA. Green plant cells also carry chloroplast DNA inside their light-harvesting chloroplasts. In contrast, simple prokaryotes like bacteria and archaea do not have a nucleus. Instead, their DNA floats in the main cell fluid within a zone called the nucleoid. Many bacteria also carry small extra rings of DNA called plasmids.

Protecting DNA from Damage

DNA molecules face chemical attacks and damage every single day. A substance that alters the genetic code is called a mutagen. High-energy rays such as ultraviolet light and medical X-rays can break the chemical backbone. For instance, strong sunlight can cause neighboring bases to stick together as thymine dimers. Damaging oxygen molecules called free radicals and chemicals like hydrogen peroxide also harm bases. Some flat chemicals slide between base steps in an action called intercalation. A damaged base that changes unexpectedly is called a mutation. Cells use active DNA repair systems to find and fix damaged spots quickly.

How DNA Works in Living Organisms

How DNA fits inside chromosomes in a eukaryotic cell.

DNA acts like a library of recipes that tell the cell how to build things. A single recipe that holds instructions for a trait is called a gene. The physical appearance or behavior produced by these genes is called a phenotype. The transfer of instructions from parent to child is called genetic transmission. To use these instructions, the cell reads the order of bases along the strand. This order of letters is known as the nucleic acid sequence. Helper molecules make working copies of these genes to build cellular tools.

Transcription: Making RNA Messages

When a cell needs a specific protein, it makes an RNA copy of that gene. This copying process is called transcription. An enzyme called RNA polymerase binds to a starting signal on DNA called a promoter. The enzyme unzips the two strands and reads the template sequence. It strings together matching RNA nucleotides to form a strand of messenger RNA. During this step, the enzyme puts uracil in places where thymine would normally go. The polymerase continues reading until it reaches a stop sign called a terminator. Once finished, the new messenger RNA floats away to be read by protein-making factories.

Translation and the Genetic Code

The cell uses the messenger RNA instructions to build working proteins. This manufacturing step is called translation. A cell factory called a ribosome attaches to the messenger RNA strand. The ribosome reads the bases in three-letter groups called codons. This universal language system is known as the genetic code. Each codon stands for one specific building block called an amino acid. Another helper molecule called transfer RNA brings the matching amino acid to the factory. The ribosome links these amino acids together to make a protein sequence. The finished protein folds up and begins its job in the cell.

DNA Replication: Making Exact Copies

DNA replication unzips the double helix to build two new strands.

Before a cell can divide into two new cells, it must copy its entire genome. This duplication process is called DNA replication. First, an enzyme called helicase unwinds the double helix like a zipper. This separation creates an open area known as a replication fork. Next, an enzyme called DNA polymerase moves along each exposed strand. The polymerase reads each base and attaches the matching complementary nucleotide. Because DNA strands run in opposite directions, the two sides copy differently. One side forms smoothly as the leading strand. The other side forms in short pieces called Okazaki fragments. A helper enzyme called DNA ligase joins these short pieces together into one continuous thread. Many polymerases also have a proofreading ability to fix any mistaken bases immediately.

Crossing Over and Genetic Variety

A Holliday junction formed during DNA recombination.

Chromosomes usually stay in their own separate zones called chromosome territories. However, during sexual reproduction, matching chromosomes pair up to trade pieces. This mixing process is called chromosomal crossover or genetic recombination. When matching chromosomes trade equal parts, scientists call it homologous recombination. Special enzyme tools called recombinases cut and paste the strands together. During this trade, the four strands cross over in an X-shaped structure called a Holliday junction. This shuffling creates fresh combinations of genes in children. These new combinations help drive natural selection and support the long-term survival of species.

Scientific Discoveries and the History of DNA

Maclyn McCarty with Francis Crick and James Watson.

Scientists spent many decades discovering what DNA is and how it functions. In 1869, a Swiss doctor named Friedrich Miescher first found DNA in cells. He called this mysterious substance "nuclein" because it came from the nucleus. In 1878, Albrecht Kossel separated the substance from proteins and identified its pure nucleobases. Later, chemist Phoebus Levene discovered how sugars, phosphates, and bases connect together into nucleotides. In 1928, Frederick Griffith showed that harmless bacteria could absorb a mystery factor from heat-killed bacteria. This famous test was known as Griffith's experiment.

In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty proved that Griffith's factor was DNA. Their classic study was called the Avery–MacLeod–McCarty experiment. In 1950, chemist Erwin Chargaff made another major breakthrough known as Chargaff's rules. He discovered that in any DNA sample, adenine amounts always match thymine amounts. Similarly, guanine amounts always match cytosine amounts. In 1952, researchers Alfred Hershey and Martha Chase confirmed that DNA carries hereditary traits. Their famous test was called the Hershey–Chase experiment.

$Photo 51 x-ray diffraction image$

Photo 51, an X-ray diffraction image made by Rosalind Franklin and Raymond Gosling.

During the early 1950s, scientists raced to discover the three-dimensional shape of DNA. In London, Rosalind Franklin and student Raymond Gosling used X-ray diffraction to photograph DNA crystals. Their clearest image was labeled Photo 51. Franklin recognized that the sugar-phosphate rails had to sit on the outside of the molecule. Meanwhile, at Cambridge, James Watson and Francis Crick built scale physical models out of metal pieces. In 1953, using Franklin's data, Watson and Crick solved the puzzle of the double helix. In 1962, Watson, Crick, and colleague Maurice Wilkins shared the Nobel Prize in Physiology or Medicine.

Modern Technology and Uses of DNA

Nanoscale shapes built through DNA nanotechnology.

Today, scientists can read, copy, and change DNA in laboratories around the world. Researchers use a method called phenol-chloroform extraction to pull pure DNA out of cells. They can snip DNA at exact sequences using molecular scissors called restriction enzymes. Then they use DNA ligase to paste fragments together into new arrangements. A human-made sequence built this way is called Recombinant DNA. Scientists can place these new instructions into harmless host bacteria using a viral vector. This process allows researchers to produce life-saving medicines and improve crops grown in agriculture.

Identifying Organisms and Solving Mysteries

Every person has a unique DNA pattern, except for identical twins. Scientists can identify people using a technique called DNA profiling, or DNA fingerprinting. In 1984, British scientist Sir Alec Jeffreys developed this method to help solve difficult identity questions. Technicians look at repeated sections of DNA known as short tandem repeats. Because these sections vary in length, they create a unique pattern for each individual. Today, doctors use this matching system in DNA paternity testing to establish family relationships. Biologists also collect shed cells from soil and water, which they call environmental DNA. This helps them track rare wild animals without disturbing them.

DNA Nanotechnology and Computing

Engineers can also use DNA as a tiny building material instead of an instruction book. This high-tech field is called DNA nanotechnology. Because matching bases attract each other reliably, scientists can program DNA strands to self-assemble. Scientists fold long strands into custom microscopic shapes using a method called DNA origami. Researchers have built tiny gears, boxes, and nanomechanical devices out of DNA. DNA can also be used as a dense form of digital data storage. A tiny drop of DNA can store vast libraries of digital text and images for centuries.

Studying the Past

Because genetic instructions change very slowly over time, DNA acts like a history book. By comparing the genomes of different species, scientists study evolutionary relationships called phylogeny. Scientists can also extract Ancient DNA from preserved bones and fossils. For example, researchers have sequenced complete genomes from ancient frozen remains of the woolly mammoth. These ancient genetic codes show us how extinct animals lived and how modern animals evolved over millions of years.