Deoxyribonucleic acid, or DNA, is a long molecule shaped like a twisted ladder. Two strands wind around each other in a structure called a double helix, held together by precise chemical pairing between the “rungs.” This architecture allows DNA to store the genetic instructions for building and maintaining every cell in your body. Understanding its structure starts with the smallest repeating unit: the nucleotide.
The Nucleotide: DNA’s Building Block
Every strand of DNA is a chain of smaller units called nucleotides. Each nucleotide has three parts: a sugar, a phosphate group, and a nitrogen-containing base. The sugar in DNA is deoxyribose, a five-carbon ring that gives the molecule its full name. A phosphate group attaches to one side of the sugar, and a base attaches to the other.
There are four bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These fall into two categories based on their size. Adenine and guanine are purines, built from a fused double ring of carbon and nitrogen atoms. Cytosine and thymine are pyrimidines, smaller molecules with just a single six-sided ring. This size difference matters because a purine always pairs with a pyrimidine, keeping the width of the double helix consistent from one rung to the next.
How Nucleotides Link Into a Strand
Nucleotides connect end to end through what’s called a phosphodiester bond. The phosphate group on one nucleotide attaches to the sugar of the next, creating a backbone that alternates between sugar and phosphate like links in a chain. This bonding gives each strand a built-in direction: one end has a free phosphate (called the 5′ end), and the other has a free sugar hydroxyl group (the 3′ end). Cells read and copy DNA in a specific direction along this backbone, so the polarity is essential for every process that uses genetic information.
Base Pairing and Hydrogen Bonds
The two strands of DNA are not identical. They are complementary, meaning the sequence of bases on one strand dictates the sequence on the other. Adenine always pairs with thymine, and cytosine always pairs with guanine. James Watson and Francis Crick first described this pairing in 1953, and it explained an observation the biochemist Erwin Chargaff had made years earlier: in any sample of DNA, the amount of adenine equals the amount of thymine, and the amount of cytosine equals the amount of guanine.
These base pairs are held together by hydrogen bonds, a type of weak attraction between atoms. An A-T pair is held by two hydrogen bonds, while a C-G pair is held by three. That extra bond makes C-G pairs slightly stronger. Across an entire genome with billions of base pairs, this difference influences how easily certain stretches of DNA separate during copying or gene activation.
The Double Helix Shape
The two connected strands twist around a shared central axis, forming the famous double helix. They run in opposite directions, one oriented 5′ to 3′ and the other 3′ to 5′. This antiparallel arrangement is critical for DNA replication and for the enzymes that read genetic information. If the strands ran in the same direction, the molecular machinery that copies DNA would not function.
In its most common form (called B-DNA), the helix is right-handed, meaning it spirals clockwise when viewed from above. The helix is about 2 nanometers wide. Each base pair is stacked roughly 0.34 nanometers above the last, and one full turn of the helix spans about 10 base pairs, or 3.4 nanometers. These dimensions are remarkably consistent, which is why a purine always pairs with a pyrimidine: two purines would bulge the helix, and two pyrimidines would leave a gap.
Major and Minor Grooves
Because the two sugar-phosphate backbones don’t sit directly opposite each other on the helix, the twisting creates two channels that spiral along the outside of the molecule. The wider one is the major groove, roughly 11 to 12 angstroms across. The narrower one is the minor groove, ranging from about 4.6 to 7.5 angstroms wide depending on the local base sequence.
These grooves are where proteins interact with DNA. Transcription factors, the proteins that switch genes on or off, typically read the chemical pattern of bases exposed in the major groove, where there’s enough variation between base pairs to allow specific recognition. The minor groove, by contrast, reveals less about which bases are present, so proteins that bind there tend to be less sequence-specific. Architectural proteins that bend or reshape DNA, for instance, often work through the minor groove.
Alternative Forms of the Helix
B-DNA is the standard form found under normal cellular conditions, but DNA can adopt other shapes. A-DNA is a shorter, wider right-handed helix that forms when DNA is dehydrated or in certain protein complexes. Its base pairs tilt relative to the helix axis, giving it a more compact look.
Z-DNA is the most visually distinct variant. It twists in the opposite direction, forming a left-handed helix with a zig-zag pattern in its backbone. The bases alternate between two different orientations along the strand, unlike B-DNA where they all face the same way. Z-DNA is also more elongated, with a greater distance between stacked base pairs, and it lacks a major groove entirely. Z-DNA tends to form in stretches where cytosine and guanine alternate, and cells appear to use it in regulating gene activity and immune signaling.
How DNA Packs Into Chromosomes
The double helix by itself is extraordinarily long. The DNA in a single human cell, stretched end to end, would reach about two meters. To fit inside a cell nucleus just a few millionths of a meter across, the helix wraps around protein spools called histones. A segment of 145 to 147 base pairs coils about 1.7 times around a cluster of eight histone proteins, forming a structure called a nucleosome. This is the first level of compaction.
Nucleosomes then stack and fold into progressively denser fibers, eventually producing the tightly packed chromosomes visible under a microscope during cell division. The way DNA wraps around histones also controls which genes are accessible. Genes buried deep in tightly packed nucleosomes are effectively silenced, while those in loosely packed regions can be read and turned into proteins. So the structure of DNA matters not just at the molecular level but at every scale of organization, from individual base pairs to entire chromosomes.
How the Structure Was Discovered
Watson and Crick published their model of the double helix in 1953, but the discovery depended on experimental work by others. Rosalind Franklin, a crystallographer at King’s College London, produced X-ray diffraction images of DNA that revealed key structural details. Her most famous image, Photograph 51, showed the characteristic X-shaped pattern of a helix and provided measurements of the molecule’s dimensions and symmetry. Franklin determined that the DNA unit cell had a specific type of symmetry (called C2), which later served as powerful confirmation that the Watson-Crick model was correct. Watson was shown this image without Franklin’s knowledge, a fact that has shaped discussions about scientific credit ever since. Franklin died in 1958 and was not included in the Nobel Prize awarded to Watson, Crick, and Maurice Wilkins in 1962.