Nucleotides are the small molecules that make up DNA and RNA, the two major nucleic acids found in living cells. DNA stores hereditary information, while RNA helps use that information to make proteins and carry out other cellular functions. The sequence of nucleotides in these molecules provides cells with a chemical system for storing, copying, and using biological information.
A nucleotide has three basic parts: a nitrogenous base, a five-carbon sugar, and one or more phosphate groups. The particular base and sugar determine the nucleotide’s identity and help determine how it behaves in DNA or RNA.
Understanding nucleotides makes the structure and function of DNA and RNA much easier to understand.
What is a nucleotide?
A nucleotide is an organic molecule composed of three components:
- A nitrogenous base
- A five-carbon sugar, also called a pentose
- At least one phosphate group
Nucleotides can exist individually inside cells or be linked together into long chains. When many nucleotides are joined by chemical bonds, they form a polynucleotide, such as a strand of DNA or RNA.
The three components serve different purposes. The nitrogenous base carries much of the sequence information. The sugar forms part of the structural framework of the nucleic acid strand. Phosphate groups connect neighboring sugars and contribute to the overall chemical properties of the molecule.
A nucleotide without a phosphate group is called a nucleoside. In other words, a nucleoside consists of a sugar plus a nitrogenous base, while adding one or more phosphate groups produces a nucleotide.
The three parts of a nucleotide
Nitrogenous bases
The nitrogenous bases found in DNA and RNA belong to two chemical families: purines and pyrimidines.
Purines have a two-ring structure. The two purines used in nucleic acids are:
- Adenine (A)
- Guanine (G)
Pyrimidines have a single-ring structure. DNA contains:
- Cytosine (C)
- Thymine (T)
RNA contains:
- Cytosine (C)
- Uracil (U)
Thus, DNA uses four principal bases—adenine, guanine, cytosine, and thymine—while RNA uses adenine, guanine, cytosine, and uracil.
The base is especially important because its sequence along a nucleic acid strand encodes biological information. In DNA, for example, a particular stretch of bases can contain the information needed to produce a functional RNA molecule and, in many cases, ultimately a protein.
Five-carbon sugars
The sugar differs between DNA and RNA.
DNA contains deoxyribose, while RNA contains ribose. The names are closely related because deoxyribose differs from ribose by having one less oxygen atom at a particular carbon position.
This seemingly small chemical difference has important consequences. DNA’s deoxyribose-containing structure contributes to its greater chemical stability, which is useful for a molecule whose primary role is long-term information storage. RNA’s ribose makes RNA chemically more reactive and contributes to the wide range of structures and functions that RNA can have.
A nucleotide’s base is attached to its sugar, forming a nucleoside. Phosphate groups can then be attached to the sugar to produce a nucleotide.
Phosphate groups
Phosphate groups give nucleotides an important structural role in nucleic acids.
When nucleotides are joined together, a phosphate group links the sugar of one nucleotide to the sugar of the next. These connections form the sugar-phosphate backbone of DNA and RNA.
The bases project from this backbone. Their sequence varies along the strand, whereas the repeating sugar-phosphate framework provides structural continuity.
The phosphate groups also contribute negative electrical charge to nucleic acids. This is one reason DNA and RNA interact with positively charged molecules and ions in cells.
How nucleotides form DNA and RNA
DNA and RNA are polymers, meaning they are long molecules assembled from repeating smaller units. Their repeating units are nucleotides.
During nucleic acid synthesis, the phosphate group of one nucleotide becomes connected to the sugar of another through a phosphodiester bond. Repeated formation of these bonds produces the sugar-phosphate backbone.
The resulting strand has a direction, conventionally described from its 5′ end to its 3′ end. These labels refer to particular carbon positions in the sugar. The direction matters because enzymes that synthesize nucleic acids generally add new nucleotides to the 3′ end of a growing strand.
This directional organization is fundamental to DNA replication and RNA synthesis.
DNA and RNA use different nucleotides
The basic nucleotide design is shared by DNA and RNA, but the two molecules differ in their sugars and one of their bases.
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Purines | Adenine, guanine | Adenine, guanine |
| Pyrimidines | Cytosine, thymine | Cytosine, uracil |
| Typical structure | Usually double-stranded | Usually single-stranded |
| Major biological role | Long-term genetic information storage | Information transfer, protein production, regulation, catalysis, and other functions |
DNA’s two strands are held together in part by interactions between complementary bases. Adenine pairs with thymine, while guanine pairs with cytosine. In RNA, uracil takes the place of thymine, so adenine pairs with uracil when complementary RNA strands interact.
The pairing rules allow one strand of DNA to serve as a template for producing another complementary strand during replication. They also allow DNA information to be copied into RNA during transcription.
Nucleotides do more than build nucleic acids
Nucleotides are not simply construction materials for DNA and RNA. Individual nucleotides and closely related molecules also perform important jobs throughout the cell.
One of the best-known examples is ATP, or adenosine triphosphate. ATP is a nucleotide derivative containing adenine, ribose, and three phosphate groups. Cells use ATP extensively to transfer chemical energy and drive processes that require energy.
Other nucleotide-containing molecules participate in cellular signaling and metabolism. For example, certain nucleotide derivatives act as molecular switches or as parts of larger molecules involved in transferring chemical groups and electrons.
This broader role explains why nucleotides are central to cell biology even when they are not being incorporated into DNA or RNA.
Nucleosides, nucleotides, and nucleotide triphosphates
These terms are closely related but should not be treated as interchangeable.
A nucleoside contains a nitrogenous base and a sugar.
A nucleotide is a nucleoside with one or more phosphate groups attached.
Nucleotides may contain one, two, or three phosphate groups and are commonly described as monophosphates, diphosphates, and triphosphates.
For example, adenosine is a nucleoside consisting of adenine and ribose. Adding one phosphate produces AMP, two produces ADP, and three produces ATP.
The triphosphate forms of nucleotides are particularly important during nucleic acid synthesis. DNA and RNA polymerases use nucleotide triphosphates as substrates, incorporating one nucleotide into the growing strand while releasing part of the incoming molecule’s phosphate groups.
How nucleotide sequence stores biological information
A single nucleotide contains limited information: its identity is determined by its base, sugar, and phosphate arrangement. The extraordinary information capacity of DNA comes from the sequence of many nucleotides.
Consider a DNA strand containing a long sequence of A, T, G, and C. The order of those bases can distinguish one genetic sequence from another. Cells read these sequences through molecular processes that copy or interpret the information.
Genes are stretches of DNA whose information can be used to produce functional biological products, including proteins and functional RNA molecules. During gene expression, the nucleotide sequence of DNA can be transcribed into an RNA sequence. For protein-coding genes, the RNA sequence is then used to determine the order of amino acids in a protein.
The information is therefore not stored in the phosphate groups or sugars themselves. Their repeating arrangement provides the molecular framework, while the changing sequence of bases carries the sequence information.
Why complementary base pairing matters
Base pairing gives nucleic acids a powerful way to copy and organize information.
The shapes and chemical properties of the bases favor particular pairings. In standard DNA, adenine pairs with thymine and guanine pairs with cytosine. Because each base has a defined partner, knowing the sequence of one DNA strand allows the complementary sequence to be determined.
This principle is essential to DNA replication. When the two strands of DNA separate, each can serve as a template for building a new complementary strand. The result is two DNA molecules that, under normal replication, preserve the original sequence information.
Complementary pairing also plays important roles in transcription and in the folding and function of RNA molecules.
DNA nucleotides and RNA nucleotides in cells
Cells maintain pools of nucleotide-related molecules that can be used for different purposes. DNA synthesis requires deoxyribonucleotide triphosphates, commonly abbreviated dNTPs, including dATP, dGTP, dCTP, and dTTP. RNA synthesis uses ribonucleotide triphosphates, including ATP, GTP, CTP, and UTP.
The distinction between ribonucleotides and deoxyribonucleotides is important because DNA and RNA polymerases must use the appropriate substrates when constructing each type of nucleic acid.
Cells also continually make, break down, and recycle nucleotides. Nucleotide metabolism connects nucleic acid production with broader cellular metabolism, allowing cells to adjust nucleotide availability to their needs.
Why nucleotide chemistry matters to genetics
Changes in nucleotide sequence are the basis of genetic variation. A mutation is a change in the DNA sequence. Depending on where the change occurs and how it affects gene function, a mutation may have little effect, alter a biological trait, impair cellular function, or contribute to disease.
DNA sequence can change through processes such as copying errors, chemical damage, and environmental influences. Cells have multiple systems for detecting and repairing DNA damage, helping preserve genetic information.
Nucleotide chemistry also underlies modern genetic technologies. Methods for copying, sequencing, and manipulating DNA all depend on the ability to distinguish, join, or otherwise work with nucleotides and their sequences.
The essential idea
Nucleotides are the fundamental molecular units from which DNA and RNA are built. Each consists of a nitrogenous base, a five-carbon sugar, and one or more phosphate groups. DNA uses deoxyribose and thymine; RNA uses ribose and uracil.
When nucleotides join through phosphodiester bonds, they create the sugar-phosphate backbone of a nucleic acid strand. The sequence of their nitrogenous bases provides the information that cells copy, read, and regulate. At the same time, nucleotide molecules such as ATP have important functions outside the DNA and RNA chains themselves.
That combination of structure, sequence, and chemical versatility makes nucleotides fundamental not only to heredity, but to the chemistry and operation of living cells.
