What Is a Nucleoside? Structure, Functions, and Drug Uses

A nucleoside is one of biology’s fundamental building blocks: a sugar molecule bonded to a nitrogen-containing base. Attach a phosphate group to that sugar and you get a nucleotide, the unit that strings together to form DNA and RNA. Though the distinction sounds minor, nucleosides occupy a surprisingly central place in physiology, drug design, and modern vaccine technology, often doing things that have nothing to do with storing genetic information.

What Makes a Nucleoside a Nucleoside

Strip a strand of DNA or RNA down to its individual links and you find nucleotides. Remove the phosphate group from a nucleotide and what remains is a nucleoside. It consists of two parts: a five-carbon sugar (ribose in RNA, deoxyribose in DNA) and a nitrogenous base (adenine, guanine, cytosine, uracil, or thymine, depending on the context). The base and sugar are joined by a bond between the base’s nitrogen atom and the sugar’s first carbon. The term “nucleoside” itself dates to the early twentieth century, when Phoebus Levene and Walter Jacobs at the Rockefeller Institute purified these sugar-base units from yeast nucleic acid into crystalline form and proposed the name for the first time.

The identity of the sugar matters enormously. RNA nucleosides carry ribose, which has a hydroxyl group at its 2′ carbon position. DNA nucleosides carry deoxyribose, which lacks that hydroxyl. That single oxygen-hydrogen group makes RNA chemically different from DNA in ways that ripple through molecular biology. Crystal structures of DNA-RNA hybrid duplexes show that the 2′-hydroxyl forms hydrogen bonds with nearby oxygens on the sugar backbone and phosphate groups, locking the molecule into a particular three-dimensional shape known as the A-conformation.

Computational studies on RNA hairpin structures have further shown that these 2′-hydroxyl interactions stabilize RNA folds by a substantial energy margin, making RNA structures both more thermodynamically and kinetically stable than equivalent deoxyribose-containing sequences.

Adenosine and the Body’s Signaling Network

Not every nucleoside sits quietly inside a strand of genetic material. Adenosine, the nucleoside made of adenine bonded to ribose, doubles as a signaling molecule with far-reaching effects across the body. It acts through four membrane receptors scattered across different tissues, and its concentration fluctuates constantly as cells burn energy.

Sleep researchers recognized decades ago that adenosine builds up in the brain during waking hours and drops during sleep. It is now widely accepted as a sleep-regulatory substance that encodes what scientists call “sleep pressure,” the growing urge to sleep the longer you stay awake.

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Because adenosine is produced when cells break down ATP (the molecule that powers almost every cellular process), its extracellular levels directly reflect how hard the brain has been working. The longer you are awake, the more adenosine accumulates, promoting drowsiness by suppressing arousal circuits and activating sleep-promoting pathways.

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Caffeine works precisely by blocking adenosine receptors, preventing the drowsiness signal from landing. And brain cells called astrocytes release their own adenosine, which plays a role in the cognitive and memory deficits that come from sleep deprivation.

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Adenosine’s reach extends well beyond sleep. In the cardiovascular system, it is released during low-oxygen conditions, inflammation, and stress. It slows heart rhythm, produces strong vasodilation (widening of blood vessels) in the coronary, pulmonary, and systemic circulation, and lowers blood pressure.

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Cardiologists routinely exploit this property. Adenosine is used clinically to produce a hyperemic (increased blood flow) response in coronary arteries, particularly during diagnostic stress tests that evaluate microvascular function.

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How Nucleosides Move In and Out of Cells

Because nucleosides are water-soluble molecules that cannot easily cross fatty cell membranes on their own, the body has dedicated transporter proteins for them. In humans, two unrelated protein families handle the job. The concentrative nucleoside transporters (CNTs) use the energy stored in sodium ion gradients to pump nucleosides into cells against their concentration gradient. The equilibrative nucleoside transporters (ENTs) work passively, allowing nucleosides to flow in whichever direction balances the concentration on both sides of the membrane.

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In the gut, these two families divide the labor spatially. Concentrative transporters sit on the inner surface of intestinal cells facing the gut lumen, pulling nucleosides from digested food into the cell. Equilibrative transporters sit on the opposite side, facing the bloodstream, and shuttle nucleosides out into circulation.

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This arrangement matters for drug design, because many nucleoside-based medications rely on the same transporters to enter target cells. A drug that cannot get through the transporter cannot reach its target.

Once inside, cells use nucleosides as raw material. Cells maintain their pools of purine nucleotides (the building blocks containing adenine or guanine) through two routes: building them from scratch or recycling free bases and nucleosides scavenged from the environment. Recent work tracing these pathways across tissues and tumors found that adenine and inosine are the most effective circulating precursors for supplying purine nucleotides, while hypoxanthine, long assumed to be a major salvage fuel, is rapidly broken down and poorly recycled in living organisms.

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Nucleoside Analogs as Antiviral Drugs

One of the most powerful applications of nucleoside chemistry is in drug design. Because viruses must copy their own genetic material to reproduce, and because that copying depends on nucleotides, a molecule that looks just enough like a natural nucleoside to fool the virus’s machinery but then gums up the works can be a potent antiviral. These are nucleoside analogs, and they have been a mainstay of antiviral therapy for decades.

HIV treatment, for example, relies heavily on nucleoside analog reverse transcriptase inhibitors, or NRTIs. These drugs mimic natural nucleosides closely enough that the virus’s reverse transcriptase enzyme incorporates them into the growing DNA chain.

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Once incorporated, they block further extension of the chain in two ways: by sitting in the DNA and preventing the next nucleotide from being added, and by directly inhibiting the reverse transcriptase enzyme itself.

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Remdesivir, developed to treat RNA viruses, uses a related but distinct strategy. Its active form, remdesivir monophosphate, gets incorporated into the viral RNA strand by the virus’s own RNA-dependent RNA polymerase. But the drug carries a cyano group, a small chemical appendage, that physically collides with part of the polymerase after several more nucleotides have been added. The collision prevents the enzyme from sliding forward along the RNA template, stalling the copying machinery in place.

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Fighting Cancer With Modified Nucleosides

The same logic that makes nucleoside analogs effective against viruses works against cancer, since cancer cells also depend on rapid DNA copying. Cytarabine and gemcitabine are two anticancer drugs built on the nucleoside scaffold, both carrying modifications at the 2′ position of the sugar ring. Despite having all the chemical groups needed for DNA synthesis, they jam the machinery once they get woven into a growing DNA strand. Crystal structures show that cytarabine’s hydroxyl group locks the sugar into an unfavorable shape, while gemcitabine’s fluorine atoms disrupt the normal shape-shifting the sugar needs to undergo during replication.

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Gemcitabine has an additional trick. The cell’s DNA copying machinery includes a proofreading function that normally detects and removes incorrectly placed nucleotides. Gemcitabine’s phosphorylated forms actively suppress this proofreading ability, preventing the cell from removing the drug after it has been incorporated. The more phosphate groups attached, the stronger this suppression. The result is that the faulty nucleoside stays embedded in the DNA, permanently blocking further synthesis.

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Drug resistance is a persistent challenge with nucleoside-based chemotherapy. For years, researchers assumed resistance arose mainly from defects in the transporter proteins that carry nucleosides into cells or from reduced activity of the kinase enzymes that activate them. More recently, a family of membrane pumps called ABC transporters has been recognized as another culprit. These pumps can actively expel the phosphorylated forms of nucleoside drugs from inside the cell, reducing the drug concentration before it can do its job.

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Modified Nucleosides and mRNA Vaccines

The COVID-19 pandemic brought nucleoside chemistry to global attention through mRNA vaccines, even if most people did not realize it at the time. The central problem with injecting synthetic mRNA into a person is that the immune system treats unmodified RNA as a danger signal, triggering inflammatory responses that destroy the molecule before it can do its job. The breakthrough was replacing one of the four standard RNA nucleosides with a modified version.

Pioneering work showed that swapping uridine for pseudouridine in synthetic mRNA dramatically reduced the immune system’s inflammatory response. In animal experiments, only unmodified mRNA triggered high levels of interferon-alpha, a marker of innate immune activation; pseudouridine-containing mRNA did not, even at higher doses. The modification also increased the mRNA’s stability and the amount of protein it produced.

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A further refinement used N1-methylpseudouridine instead, which outperformed plain pseudouridine. In cell lines and mice, mRNA containing this modification produced even more protein and triggered even less innate immune activation, partly by evading detection by a sensor called Toll-like receptor 3.

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N1-methylpseudouridine is the modification used in the Pfizer-BioNTech and Moderna COVID-19 vaccines, making it arguably the most widely administered nucleoside modification in human history.

Nucleoside Modifications in Antisense Therapies

Beyond vaccines, modified nucleosides underpin another class of medicines: antisense oligonucleotides, short synthetic stretches of nucleic acid designed to bind specific mRNA molecules and silence the genes they encode. Unmodified oligonucleotides break down rapidly in the body, digested by enzymes called nucleases. Over the past two decades, chemists have found that modifications at the 2′ position of the sugar dramatically improve resistance to enzymatic degradation, strengthen binding to the target mRNA, and improve how the drug moves through the body.

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More extreme modifications lock the sugar ring into a fixed shape by chemically bridging the 2′ and 4′ positions, creating so-called bridged nucleic acids. These show extremely tight binding to target RNA and strong nuclease resistance.

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A newer approach explores modifications on the nucleobase itself rather than the sugar. Selenium-modified thymidine nucleosides, for example, sit in a position on the nucleic acid duplex that does not distort its shape, yet they are exceptionally resistant to nuclease digestion while still allowing the target-destroying enzyme RNase H to do its work, a combination that sugar modifications sometimes compromise.

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How Modified Nucleosides Shape Translation Accuracy

Living cells add chemical modifications to their own nucleosides as well, and the most densely decorated molecules are transfer RNAs (tRNAs), the adaptors that read the genetic code and deliver amino acids during protein synthesis. Modifications at a specific position in the tRNA’s anticodon loop, position 34 (the “wobble” position), can change which genetic code words the tRNA recognizes. In both human and bacterial tRNAs, certain sulfur and methyl modifications at this position shift the chemical equilibrium of the nucleoside’s base from one form to another, allowing the tRNA to pair with codons it would not normally match while maintaining the geometric shape the ribosome requires for accurate decoding.

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Removing these modifications does not always produce the same result. Research in yeast found that when modifications were stripped from some tRNAs, translation errors increased, meaning the modification normally helped the ribosome pick the right amino acid. But removing the same type of modification from other tRNAs actually decreased errors, because the unmodified tRNA became so weak a reader that it could no longer compete against the correct tRNA for a given codon.

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The upshot is that nucleoside modifications do not simply “improve” or “worsen” translation. They tune it in context-dependent ways, and the effect depends on the strength of the underlying codon-anticodon pairing.

Nucleosides as Cancer Biomarkers

When cells turn over RNA, modified nucleosides are released into the bloodstream and eventually excreted in urine. Because cancer cells generally have higher rates of RNA metabolism, researchers have investigated whether elevated levels of specific modified nucleosides in urine could serve as cancer biomarkers. A growing body of work has established links between certain modified nucleoside levels in blood and urine and cancer onset or progression.

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In bladder cancer specifically, urinary modified nucleosides have shown promise for both diagnosis and follow-up monitoring, with two particular modifications, 1-methyladenosine and 1-methylinosine, identified as the most clinically promising candidates for further study.

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Detecting and quantifying these modifications requires sensitive analytical techniques. Mass spectrometry coupled with various chromatographic separation methods can now identify dozens of modified nucleosides simultaneously from tiny RNA samples. Researchers have profiled both canonical and modified nucleosides from digested RNA using supercritical fluid chromatography alongside more traditional liquid chromatography, finding that combining both techniques gives the broadest view of the modification landscape.

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Newer micro-flow methods can work with as little as 25 nanograms of tRNA, opening the door to monitoring dynamic nucleoside modification changes in very limited biological samples.

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Epigenetic Marks on DNA Nucleosides

Cells also modify nucleosides that are already embedded in their DNA, and these modifications carry regulatory information that does not change the genetic sequence itself. The best-known example is 5-methylcytosine, a modified form of the cytosine nucleoside in which a methyl group is attached to the base. This mark typically silences nearby genes. A family of enzymes called TET proteins can then oxidize 5-methylcytosine through a series of further modified forms, producing 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine, which are intermediates on the path toward removing the methyl mark entirely and reactivating the gene.

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Each of these intermediate forms may also carry its own biological signals, though exactly how cells read and respond to them is still being worked out.

Nucleosides and the Origin of Life

The question of how nucleosides first formed on a lifeless Earth remains one of the great puzzles in chemistry. Getting a sugar and a base to join under plausible prebiotic conditions is not straightforward, and researchers have spent decades proposing and testing possible routes. The challenge has driven creative chemistry, but no single pathway has been accepted as the definitive answer.

An intriguing line of evidence comes from meteorites. Analyses of carbonaceous meteorites have identified not only amino acids but also sugar-like molecules, activated phosphates, and nucleobases as indigenous components, meaning they formed in space rather than from Earth-based contamination.

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Since meteorites have been raining down on Earth since the planet’s formation, it is plausible that some of life’s molecular precursors, including components needed to assemble nucleosides, were delivered from space. Whether this delivery was a minor supplement to local chemistry or a critical ingredient remains an open question, but it adds an extraterrestrial dimension to a story most people assume is purely about Earth.