DNA is a long, twisted molecule shaped like a spiraling ladder. Its structure has two key parts: a sturdy outer framework made of sugars and phosphates, and inner “rungs” made of paired chemical bases that carry genetic information. This design, called the double helix, allows DNA to store the instructions for building and running every cell in your body while also being simple enough to copy itself reliably each time a cell divides.
The Building Blocks: Nucleotides
DNA is built from repeating units called nucleotides. Each nucleotide has three components: a sugar molecule called deoxyribose, a phosphate group, and one of four nitrogen-containing bases. Those four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Think of nucleotides like beads on a string, where each bead carries one of four possible letters. The specific order of those letters along the strand is what encodes your genes.
The Sugar-Phosphate Backbone
The outer rails of the DNA “ladder” are chains of alternating sugar and phosphate molecules. Each phosphate group links to two sugars, connecting the third carbon on one sugar to the fifth carbon on the next. This bond gives each strand a built-in direction, like a one-way street. Scientists label the two ends of a strand as the 5′ (five-prime) end and the 3′ (three-prime) end based on which carbon atom is exposed.
This backbone is what gives DNA its structural stability. The phosphate linkages carry a negative electrical charge, which helps the molecule interact with surrounding water and stay chemically stable in the watery environment inside cells. The bases, meanwhile, point inward from the backbone like the teeth of a zipper, ready to pair up with bases on the opposite strand.
How the Two Strands Pair Together
The most elegant part of DNA’s structure is its base pairing. The bases on one strand connect to the bases on the opposite strand through hydrogen bonds, but only in specific combinations: A always pairs with T, and C always pairs with G. This means if you know the sequence of one strand, you automatically know the sequence of the other. A strand reading ATCG will pair with a strand reading TAGC.
This complementary pairing also explains something biochemists noticed long before the structure was solved. 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 ratios, known as Chargaff’s rules, were a critical clue that helped James Watson and Francis Crick work out the double helix model in 1953, building on X-ray images produced by Rosalind Franklin that confirmed the two backbones sat on the outside of the molecule.
The Double Helix Shape
The two strands don’t simply sit side by side. They wind around each other in a spiral, creating the famous double helix. The diameter of this helix is uniform at 2 nanometers (about 50,000 times narrower than a human hair). Each base pair is separated from the next by 0.34 nanometers, and the helix completes one full twist every 3.4 nanometers, which works out to 10 base pairs per turn.
The two strands run in opposite directions. One strand goes from 5′ to 3′ while the other goes from 3′ to 5′. This antiparallel arrangement is essential for the base pairs to fit neatly between the two backbones and for the enzymes that copy DNA to do their work correctly. Picture two spiral staircases intertwined, one running up and the other running down.
Major and Minor Grooves
Because the two backbone strands don’t sit directly across from each other on the helix, the twisting creates two channels that spiral along the surface. One is wider (the major groove) and one is narrower (the minor groove). These aren’t just decorative features. They’re where proteins physically grip the DNA molecule.
Proteins that need to read specific genetic sequences typically dock into the major groove, where the arrangement of atoms on the base pairs is more distinctive and easier to identify. Transcription factors, for instance, use the major groove to locate particular gene sequences and switch them on or off. Proteins that bind DNA more generally, without caring much about the sequence, tend to interact with the minor groove instead. The width of these grooves can vary depending on the local base sequence. Stretches rich in A and T bases tend to produce especially narrow minor grooves with a stronger negative charge, which attracts certain positively charged parts of proteins.
Three Forms of the Helix
The classic DNA shape most people picture is called B-DNA, and it’s the form DNA takes under normal conditions inside cells. But DNA can adopt other configurations depending on its environment and sequence.
- B-DNA is right-handed (the helix spirals clockwise when viewed from above), 2.0 nanometers wide, with a rise of 0.34 nanometers per base pair. This is the standard form.
- A-DNA is also right-handed but slightly wider at 2.3 nanometers, with a more compact rise of 0.26 nanometers per base pair. DNA can shift into this form in dehydrated conditions.
- Z-DNA is the outlier: a left-handed helix that’s narrower (1.8 nanometers) and more stretched out, with 0.45 nanometers between base pairs. Short stretches of Z-DNA appear in cells and may play a role in regulating gene activity.
How DNA Packs Into Chromosomes
A single DNA molecule in a human cell can be several centimeters long if stretched out, yet it needs to fit inside a cell nucleus just a few thousandths of a millimeter across. The solution is an elaborate packaging system.
The first level of compaction involves wrapping the DNA around clusters of proteins called histones. Eight histone proteins form a spool, and about 146 to 147 base pairs of DNA wind around each spool. This unit, called a nucleosome, looks like a bead on a string when viewed under an electron microscope. Millions of nucleosomes coil the DNA into a fiber about 30 nanometers wide, called chromatin. From there, the chromatin folds and loops into progressively tighter arrangements until it reaches the dense, compact form we recognize as a chromosome during cell division.
This packaging isn’t just about saving space. How tightly a region of DNA is wound around its histones affects whether genes in that region can be read by the cell. Loosely packed chromatin tends to contain active genes, while tightly packed chromatin silences them. The structure of DNA, from its atomic-level base pairs all the way up to its chromosome-level folding, is inseparable from how it functions.