A DNA molecule is shaped like a twisted ladder, a structure scientists call a double helix. Two long strands wind around each other in a spiral, connected by paired chemical “rungs” in the middle. The helix is about 2 nanometers wide, and it completes one full twist every 10 rungs, or base pairs.
The Double Helix Up Close
Picture a ladder, then imagine grabbing both ends and twisting it so the rails spiral around a central axis. That’s the basic geometry of DNA. The two rails of the ladder are the “backbone,” made of alternating sugar and phosphate molecules linked together in a repeating chain. The rungs connecting the two rails are pairs of smaller molecules called bases. DNA uses four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Each rung is always a specific pairing: A pairs with T, and C pairs with G. These pairings are held together by hydrogen bonds, which act like weak but precise clasps. The A-T pair is held by two hydrogen bonds, while the C-G pair is held by three, making C-G connections slightly stronger.
The two backbone strands run in opposite directions, a property called “antiparallel.” Think of it like two lanes of traffic on a highway going opposite ways. The 5′ end of one strand faces the 3′ end of the other (those numbers refer to specific carbon atoms on each sugar molecule). This opposing orientation is essential for how cells copy and read DNA.
Key Measurements
The most common form of DNA in living cells, called B-DNA, has consistent and well-measured proportions. The helix is about 20 angstroms (2 nanometers) in diameter. Each base pair stacks 3.4 angstroms above the last, and one full turn of the helix spans 10 base pairs, covering 34 angstroms of length. These dimensions were first deduced in the early 1950s, partly from an X-ray diffraction image known as Photo 51, taken by Rosalind Franklin. The distinctive X-shaped pattern in that image confirmed the helical shape, and the spacing within the pattern revealed there were 10 base pairs per turn and two strands in the molecule.
Why “Rungs” Only Pair One Way
The reason A always pairs with T, and C always pairs with G, comes down to physical shape and chemistry. A and G are bulkier, double-ring molecules (purines), while T and C are smaller, single-ring molecules (pyrimidines). Every rung of the ladder pairs one large base with one small base, which keeps the helix a uniform width. Beyond size, the atoms on each base are positioned so that hydrogen bonds only form efficiently between A-T and C-G. A mismatched pair simply can’t line up the right atoms to bond properly. This strict pairing is what allows DNA to be copied faithfully: if you know one strand’s sequence, the other strand’s sequence is automatically determined.
How DNA Bends and Packs Into Cells
Despite looking rigid in textbook illustrations, DNA is surprisingly flexible. Its “persistence length,” the scale at which it behaves like a stiff rod, is about 50 nanometers, or roughly 150 base pairs. Anything shorter than that acts relatively rigid, but longer stretches of DNA bend and coil freely. This matters because the DNA in a single human cell stretches about 2 meters in total length, yet it fits inside a nucleus just 6 micrometers across.
To achieve that level of compression, DNA wraps around clusters of proteins called histones. About 147 base pairs of DNA wind 1.7 times around each histone cluster, forming a spool-like unit called a nucleosome. These nucleosomes then stack and fold into increasingly compact fibers. The result is a packaging system that compresses DNA by a factor of roughly 10,000 while still allowing specific genes to be accessed when needed.
Not All DNA Is a Straight Double Helix
B-DNA is the classic form you see in textbooks, but DNA can shift into other shapes depending on conditions. A-DNA is a wider, more compact version that forms when water levels around the molecule drop, or when certain proteins bind to it. Some enzymes temporarily push DNA into an A-form shape during processes like copying the genetic code. Z-DNA is more unusual: its helix spirals to the left instead of the right. It tends to form in stretches where C and G bases alternate repeatedly, and DNA’s natural twisting tension (supercoiling) can drive its formation. An analysis of over one million base pairs of human DNA identified 329 potential Z-DNA-forming sequences, with a strong tendency to cluster near sites where genes get switched on.
Even more exotic shapes exist. G-quadruplexes are four-stranded structures that form in guanine-rich regions of DNA. Four guanine bases can bond together in a flat square, and several of these squares stack on top of each other to create a tower-like structure stabilized by potassium ions. High-throughput sequencing methods have identified roughly 700,000 sequences in the human genome capable of forming these structures. They’re especially concentrated at regulatory regions, including promoters and enhancers, suggesting they help control when genes are turned on or off. The number of active G-quadruplexes varies dramatically between cell types, ranging from about 1,000 to 20,000 depending on the cell line studied.
Circular DNA in Bacteria and Mitochondria
Not all DNA molecules are long, linear strands. Bacterial chromosomes and plasmids (small extra DNA molecules in bacteria) are typically circular, forming closed loops of double-stranded DNA. These circular molecules still have the double-helix structure, but instead of having two free ends, the strand connects back to itself.
Your own cells contain circular DNA too. Mitochondria, the energy-producing structures inside cells, each carry one or more copies of a small circular DNA molecule. Unlike nuclear DNA, mitochondrial DNA isn’t wrapped around histone proteins, leaving it more exposed to damage. It’s inherited exclusively from your mother, and mutations in it can cause a range of metabolic disorders.