What Are the Monomers for Nucleic Acids?

Nucleotides are the monomers of nucleic acids. Every strand of DNA and RNA is built from nucleotide subunits linked end to end, much like beads on a string. Each nucleotide is itself a small molecule made of three parts: a sugar, a phosphate group, and a nitrogen-containing base. The specifics of those three components differ between DNA and RNA, and the differences matter more than you might expect.

What a Nucleotide Actually Looks Like

A nucleotide has three pieces snapped together. The central piece is a five-carbon sugar. Hanging off one side of the sugar is a phosphate group, which carries a negative charge and gives nucleic acids their acidic character. Attached to another position on the sugar is a nitrogenous base, a flat, ring-shaped molecule that varies from one nucleotide to the next. The base is the part that encodes information; the sugar and phosphate form the structural backbone once nucleotides are chained together.

When nucleotides polymerize into a nucleic acid strand, the phosphate of one nucleotide bonds to the sugar of the next. This creates a repeating sugar-phosphate-sugar-phosphate backbone with bases projecting off to the side. The backbone is chemically uniform along the entire strand; all the variation, and therefore all the genetic information, comes from the sequence of bases.

DNA Nucleotides Versus RNA Nucleotides

The sugar in a DNA nucleotide is deoxyribose. The sugar in an RNA nucleotide is ribose. The only structural difference between the two sugars is a single oxygen atom: ribose has a hydroxyl group (an oxygen bonded to a hydrogen) at the 2′ position of the ring, while deoxyribose has just a hydrogen there. The prefix “deoxy” literally means “lacking an oxygen.”¹

That missing oxygen has real consequences. The extra hydroxyl on ribose makes RNA more chemically reactive, which is one reason RNA molecules tend to be shorter-lived than DNA. It also affects the three-dimensional shapes RNA can fold into. Molecular simulations have shown that 2′-hydroxyl groups help stabilize specific RNA folding structures, like hairpin loops, that would not hold together as well in an equivalent DNA sequence.¹ DNA’s chemical stability, by contrast, makes it better suited for long-term information storage, which is presumably why evolution settled on deoxyribose for the genome.

The Five Standard Bases

Both DNA and RNA use four bases each, and three of those are shared. The two categories of bases are purines (larger, two-ring structures) and pyrimidines (smaller, single-ring structures). DNA uses the purines adenine (A) and guanine (G) along with the pyrimidines cytosine (C) and thymine (T). RNA swaps out thymine for a closely related pyrimidine called uracil (U). So in total, five different bases appear across the two types of nucleic acid: A, G, C, T, and U.

The bases pair up in a specific way that makes the double-helix structure of DNA possible. Adenine pairs with thymine (or uracil, in RNA), and guanine pairs with cytosine. These pairings are driven by hydrogen bonds between the bases. Guanine and cytosine form three hydrogen bonds, making their pairing stronger than the two-bond pairing between adenine and thymine.² This is why DNA regions rich in G-C pairs are harder to pull apart (they require more heat to “melt”) than regions rich in A-T pairs. It also means the base composition of a genome has practical implications for laboratory work involving DNA.

Each base has a characteristic pattern of ultraviolet light absorption, peaking around 260 nanometers. This property is so reliable that researchers routinely measure the concentration and purity of nucleic acid samples by shining UV light through them. The specific absorption profile of each base depends on which structural form the base takes, and even a single base like guanine can adopt subtly different shapes depending on its chemical environment.³

How Nucleotides Are Linked Into Chains

Inside cells, nucleic acids are assembled by enzymes called polymerases. The raw material for this assembly is not a plain nucleotide but a nucleoside triphosphate, a nucleotide carrying three phosphate groups instead of one.⁴ The extra phosphates store energy, and when a polymerase adds a nucleotide to a growing chain, it snaps off two of those phosphates. The energy released drives the bond formation.

DNA polymerases, the enzymes that copy DNA, work by reading one strand and building a complementary strand alongside it. They require metal ions, typically magnesium, to catalyze the reaction. For decades, the standard model held that two magnesium ions were needed at the active site. More recent crystallographic work has identified a third magnesium ion that appears during the reaction, though its exact role is still being worked out.⁵ The bond that forms between nucleotides is called a phosphodiester bond, and it is this bond, repeated thousands or billions of times, that holds an entire chromosome together.

Nucleotides That Are Not Part of DNA or RNA

It would be easy to think of nucleotides as purely genetic building blocks, but cells use them for much more. The most familiar example is ATP, adenosine triphosphate, which is the primary energy currency of nearly all living cells. ATP is simply an adenine nucleotide with three phosphate groups. When a cell needs energy, it breaks off one or two of those phosphates, releasing the stored energy and leaving behind ADP or AMP. The purine salvage pathway then recycles those breakdown products back into ATP, a process that is especially critical in energy-hungry organs like the brain and heart.⁶

Nucleotides also serve as signaling molecules. Cyclic AMP, made from ATP by stripping away two phosphates and forming an internal ring, is one of the most important “second messengers” in cell communication. More recently, researchers discovered that mammalian cells produce cyclic GMP-AMP (cGAMP) from ATP and GTP when they detect foreign DNA floating in the cell’s interior. cGAMP triggers an immune signaling cascade that leads to the production of interferon, a key antiviral defense molecule.⁷ So nucleotides do double and triple duty: they store genetic information, power cellular reactions, and relay signals between molecular pathways.

Modified Nucleotides in RNA

The five standard bases are not the end of the story. RNA, in particular, carries a surprising number of chemical modifications. More than 100 different types of chemical modifications to RNA have been documented.⁸ These are not mutations or errors. They are deliberate chemical tweaks, added by specialized enzymes after the RNA strand has been synthesized.

One of the most abundant of these modifications is pseudouridine, sometimes called the “fifth nucleotide” of RNA. Pseudouridine is chemically identical to uridine except for a change in how the base is attached to the sugar: the bond rotates from a nitrogen-carbon link to a carbon-carbon link. This small rearrangement subtly changes the behavior of the nucleotide, affecting how the RNA folds and how stable it is. Pseudouridine was long thought to appear only in structural RNAs like ribosomal RNA and transfer RNA, but newer sequencing methods have found it in messenger RNA as well, opening up a field sometimes called “RNA epigenetics.”⁸ If you followed the development of mRNA vaccines, you may have encountered pseudouridine in that context: the synthetic mRNA in some COVID-19 vaccines uses modified uridine to help the molecule evade the immune system and last longer inside cells.

Nucleotide Analogs as Drugs

Because nucleotides are essential for copying genetic material, they make an attractive target for antiviral and anticancer drugs. The basic strategy is to feed cells (or viruses) a slightly altered nucleotide that looks real enough to be incorporated into a growing DNA or RNA strand but then gums up the machinery. These altered versions are called nucleoside or nucleotide analogs.

Antiviral nucleoside analogs have been used effectively against HIV, hepatitis C, herpesviruses, Ebola, and SARS-CoV-2.⁹ The drug remdesivir, used early in the COVID-19 pandemic, works by mimicking adenosine and being incorporated into the viral RNA strand by the virus’s own copying enzyme. Once there, it disrupts further chain extension. Similarly, acyclovir (used against herpes) and zidovudine (AZT, used against HIV) are chain terminators: once the polymerase adds them to the growing strand, the chain cannot continue because these analogs lack the chemical group needed for the next bond.

One challenge with these drugs is collateral damage. Human cells also use nucleotides for their own DNA replication, and some analogs get mistakenly incorporated into host DNA, particularly mitochondrial DNA. Cells rely on repair enzymes like TDP1 to remove these misincorporated analogs and fix the strand. When TDP1 is absent or impaired, the damage accumulates, which can lead to mitochondrial dysfunction, an important consideration in the long-term side effects of some antiviral treatments.¹⁰

How Cells Keep Their Nucleotide Supply Clean

Even ordinary metabolism produces reactive oxygen species that can damage free-floating nucleotides before they ever make it into DNA. One of the most common forms of oxidative damage converts dGTP (the guanine nucleotide used in DNA synthesis) into 8-oxo-dGTP, a corrupted form that can mispair with adenine instead of cytosine. If 8-oxo-dGTP gets incorporated into DNA, it causes mutations.

Cells have a dedicated cleanup system for this. An enzyme called MTH1 (MutT homolog 1 in humans) scans the nucleotide pool and breaks down 8-oxo-dGTP into 8-oxo-dGMP, a form that can no longer be used for DNA synthesis.¹¹ This process is sometimes called “nucleotide pool sanitization,” and it is remarkably conserved across species: bacteria have a version of it (MutT), and humans have their own equivalent. If MTH1 fails, backup systems like the OGG1 glycosylase can catch the damaged base after it has been incorporated, but the first line of defense is always to destroy the bad nucleotide before it gets anywhere near the DNA strand.¹²

What Happens to Nucleotides You Eat

When you eat foods containing nucleic acids, like meat, fish, legumes, and mushrooms, digestive enzymes break the nucleic acids down into individual nucleosides and free bases. These breakdown products are absorbed in the intestine by specialized transporter proteins. One set of transporters (concentrative nucleoside transporters) sits on the side of intestinal cells facing the gut and actively pulls nucleosides in. A different set (equilibrative nucleoside transporters) sits on the opposite side and lets nucleosides pass into the bloodstream.¹³

Your body does not generally rely on dietary nucleotides for DNA and RNA synthesis, though. Most cells can build nucleotides from scratch using simpler molecules like amino acids, carbon dioxide, and sugar phosphates. The dietary nucleotides you absorb are mostly recycled through salvage pathways or broken down further. There are some situations where dietary nucleotides may matter more, such as in rapidly dividing cells of the gut lining or in the immune system during periods of high demand, but for most healthy adults, nucleotide intake from food is not a nutritional concern.

Expanding the Genetic Alphabet

For billions of years, all life on Earth has used the same four-base system: A, T (or U), G, and C. Synthetic biologists have asked whether that alphabet could be expanded. In 2014, researchers created a living bacterium whose DNA contained not just the natural base pairs but also an unnatural base pair, formed between two synthetic molecules called d5SICS and dNaM.¹⁴ Unlike natural base pairs, which pair through hydrogen bonds, these synthetic bases stick together through hydrophobic interactions, essentially avoiding water rather than actively bonding to each other.

Lab tests showed that DNA containing the d5SICS-dNaM pair could be replicated by standard PCR (the workhorse technique for copying DNA) with greater than 99.9% accuracy across a wide variety of surrounding sequence contexts.¹⁵ Subsequent work identified an even better-performing pair called dTPT3-dNaM, which is replicated with higher fidelity still.¹⁶ A six-letter genetic alphabet dramatically increases the number of possible codons (from 64 to 216), which could in principle allow cells to produce proteins containing unnatural amino acids. The practical applications are still mostly in the research phase, but the fact that an expanded monomer set works at all inside a living cell is one of the more striking developments in recent molecular biology.

Where Nucleotides Came From in the First Place

One of the harder puzzles in origin-of-life research is how nucleotides first appeared on a planet with no biology to make them. Modern cells synthesize nucleotides through elaborate enzyme-driven pathways. Before life existed, those enzymes were not available, so nucleotides had to arise through simpler chemistry. Experimental studies have shown that plausible precursors to nucleotides can form under conditions meant to simulate early Earth, but the full pathway from simple chemicals to a functional, self-replicating nucleic acid remains a continuing challenge.¹⁷

The difficulty is that nucleotides are themselves fairly complex molecules. Getting a sugar, a base, and a phosphate group to form and then link up correctly without an enzyme to guide the process is not trivial. Some researchers have explored pathways in which the sugar and base are built up together, part by part, rather than assembled from pre-formed components. Others have investigated whether simpler, non-standard nucleotides might have come first, with the modern set evolving later. The question of how nucleotides form under abiotic conditions, perhaps 3.5 to 4 billion years ago, remains one of the central open problems in prebiotic chemistry.¹⁸ The fact that nucleoside triphosphates were apparently needed for polymerization even in prebiotic scenarios adds another layer to the puzzle, since triphosphates are themselves energy-rich and not easily formed without catalysis.⁴