What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Purine Always Pairs with Pyrimidine: Base Pairing Rules

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Base Pairing

The genetic information of all living organisms is stored in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), polymers composed of nucleotide monomers. Each nucleotide consists of three components: a five-carbon sugar (deoxyribose in DNA, ribose in RNA), one to three phosphate groups, and a nitrogenous base. The nitrogenous bases are the information-carrying elements of the molecule, and their specific interactions—known as base pairing—underlie the storage, replication, and transmission of genetic information.

What are Purines and Pyrimidines?

Nitrogenous bases fall into two structural classes based on their ring chemistry. Purines are heterocyclic aromatic compounds with a fused two-ring structure: a six-membered pyrimidine ring fused to a five-membered imidazole ring. The two purine bases found in nucleic acids are adenine (A) and guanine (G) . Adenine is 6-aminopurine, and guanine is 2-amino-6-oxopurine.

Pyrimidines, by contrast, are single six-membered heterocyclic rings containing two nitrogen atoms at positions 1 and 3. The three pyrimidine bases found in nucleic acids are cytosine (C) , thymine (T) , and uracil (U) . Cytosine is 2-oxo-4-aminopyrimidine; thymine is 2,4-dioxo-5-methylpyrimidine; and uracil is 2,4-dioxopyrimidine. Thymine differs from uracil only by the presence of a methyl group at the C5 position. DNA uses thymine, while RNA uses uracil in its place.

The chemical distinction between purines and pyrimidines is fundamental: the two-ring purines are larger molecules than the single-ring pyrimidines. This size difference is the geometric basis for the base pairing rules, as detailed in the next section. For a more detailed structural comparison, see Purine Pyrimidine Bases.

The Rule of Complementary Base Pairing

In double-stranded DNA, the two polynucleotide chains run antiparallel—one in the 5′ to 3′ direction and the other in the 3′ to 5′ direction—and are held together by hydrogen bonds between bases on opposite strands. The pairing is not random. Adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C) . This is the rule of complementary base pairing, also known as Watson-Crick base pairing after James Watson and Francis Crick, who proposed the double-helical model of DNA in 1953.

The rule has two levels of specificity. First, a purine must pair with a pyrimidine—never a purine with a purine, nor a pyrimidine with a pyrimidine. Second, within that constraint, only specific purine-pyrimidine combinations are allowed: A with T (or U in RNA) and G with C. The first constraint is geometric; the second is chemical, determined by the positions of hydrogen bond donors and acceptors on each base.

This complementary pairing is the molecular basis for DNA replication (each strand serves as a template for the synthesis of its complement), transcription (DNA is transcribed into a complementary RNA), and DNA repair (damage on one strand can be corrected using the intact complementary strand as a reference). The rule also explains the phenomenon of Base Pair Substitution, where a single nucleotide change in one strand creates a mismatch that must be resolved by repair machinery.

Structural Basis for Purine-Pyrimidine Pairing

The requirement that a purine always pairs with a pyrimidine is not arbitrary—it is a direct consequence of the geometry of the DNA double helix. The two sugar-phosphate backbones of a DNA duplex are held at a fixed distance from each other by the base pairs that bridge them. For the helix to be regular and stable, this distance must be constant at every position along the molecule.

Molecular Dimensions of Purines and Pyrimidines

A purine base is a fused bicyclic system with a molecular width of approximately 12.0 Å when measured across its long axis. A pyrimidine is a monocyclic system with a width of approximately 9.0 Å. If two purines paired with each other, the combined width would be about 24 Å; if two pyrimidines paired, the combined width would be about 18 Å. Neither of these would fit within the uniform 20 Å diameter of the DNA double helix.

The key insight is that a purine-pyrimidine pair has a combined width of approximately 21 Å (12.0 + 9.0 Å), which matches the measured diameter of B-form DNA—the most common conformation under physiological conditions. This is not a coincidence; it is a structural constraint that has been conserved throughout evolution. The glycosidic bonds (the bonds connecting each base to its sugar) are positioned such that when a purine pairs with a pyrimidine, the two sugar-phosphate backbones are exactly the right distance apart to form a regular helix.

If a purine were to pair with another purine, the backbones would be pushed too far apart, disrupting the helical structure and creating a bulge. If a pyrimidine paired with another pyrimidine, the backbones would be pulled too close together, causing a constriction. Both scenarios would destabilize the duplex and interfere with the packaging of DNA into Chromosome Structure and Chromatin Structure.

Hydrogen Bonding Patterns

The second level of specificity—why A pairs only with T and G only with C—is determined by the positions of hydrogen bond donors and acceptors on the bases. A hydrogen bond forms when a hydrogen atom covalently bonded to an electronegative atom (donor) is attracted to another electronegative atom (acceptor). In the context of base pairing, the relevant donors are N-H groups and the relevant acceptors are carbonyl oxygens (C=O) and ring nitrogens.

In the Watson-Crick geometry, each base presents a specific pattern of donors and acceptors along its "Watson-Crick edge"—the edge that faces the opposing base in the duplex:

Adenine presents one donor (N6-H) and one acceptor (N1) along its Watson-Crick edge.
Thymine presents one acceptor (O4) and one donor (N3-H) along its edge.
Guanine presents one acceptor (O6), one donor (N1-H), and one acceptor (N3) along its edge.
Cytosine presents one donor (N4-H), one acceptor (N3), and one donor (N2-H) along its edge.

When A pairs with T, the donor-acceptor patterns are complementary: A's N6-H donates to T's O4, and T's N3-H donates to A's N1. When G pairs with C, three complementary interactions form: G's N1-H donates to C's N3, C's N4-H donates to G's O6, and G's N2-H donates to C's O2.

No other combination of bases can satisfy both the geometric constraint (purine-pyrimidine width) and the chemical constraint (complementary donor-acceptor patterns). For example, A paired with C would place two donors opposite each other in one position, and G paired with T would place two acceptors opposite each other—both arrangements are electrostatically unfavorable and cannot form stable hydrogen bonds.

Chargaff's Rules and Experimental Evidence

The base pairing rules were not derived purely from structural reasoning; they were also supported by careful biochemical measurements made before the Watson-Crick model was proposed.

Chargaff's Observations

In the late 1940s, the Austrian-American biochemist Erwin Chargaff and his colleagues at Columbia University analyzed the base composition of DNA from a wide variety of organisms, including bacteria, yeast, and mammals. They used paper chromatography to separate the four bases and spectrophotometry to quantify each one. Their results, published between 1949 and 1953, revealed two consistent patterns:

The amount of adenine equals the amount of thymine (A = T), and the amount of guanine equals the amount of cytosine (G = C) , in every organism examined.
The total amount of purines equals the total amount of pyrimidines (A + G = T + C) , meaning the purine-to-pyrimidine ratio is always 1:1.

These relationships became known as Chargaff's rules. They held regardless of the organism's complexity, from bacteriophages to humans, and regardless of the overall base composition. For example, human DNA is approximately 30% A, 30% T, 20% G, and 20% C, while the DNA of the bacterium Streptomyces griseus is approximately 36% A, 36% T, 14% G, and 14% C. In both cases, A = T and G = C, but the A+T content varies widely between species.

Chargaff's data also showed that the base composition of DNA is species-specific—different organisms have different A+T/G+C ratios—which argued against the earlier tetranucleotide hypothesis (the idea that DNA was a monotonous repeat of all four bases in equal amounts). This species specificity was consistent with DNA being the genetic material, a role that had been demonstrated by Avery, MacLeod, and McCarty in 1944 and by Hershey and Chase in 1952.

Link to Watson-Crick Model

When Watson and Crick were building their model of DNA in early 1953, they were aware of Chargaff's unpublished data showing A = T and G = C. This information was critical: it told them that adenine must pair with thymine and guanine with cytosine, rather than the alternative possibility of A pairing with C and G with T. The complementary donor-acceptor patterns confirmed that A-T and G-C were the only chemically sensible pairs.

The Watson-Crick model, published in Nature in April 1953, incorporated Chargaff's rules as a central feature. The model proposed that the two strands of DNA are held together by specific hydrogen bonds between A and T and between G and C, and that this complementarity provides a mechanism for the accurate replication of genetic information. Chargaff himself was initially skeptical of the model, but subsequent experimental evidence—particularly the X-ray diffraction data of Rosalind Franklin and Maurice Wilkins—confirmed the double-helical structure and the base pairing rules.

Hydrogen Bonding in A-T and G-C Pairs

The stability of the DNA double helix depends on the cumulative effect of many weak interactions, including hydrogen bonds between bases, base stacking interactions (van der Waals forces between the flat aromatic rings of adjacent bases), and electrostatic interactions with the surrounding solvent and ions. The hydrogen bonds between paired bases are individually weak—each contributes only about 1 to 2 kcal/mol of stabilization—but they are numerous and highly specific.

A-T Pair: Two Hydrogen Bonds

The adenine-thymine pair is held together by two hydrogen bonds. The specific interactions are:

Adenine N6-H (donor) to Thymine O4 (acceptor)
Thymine N3-H (donor) to Adenine N1 (acceptor)

The geometry of these bonds is nearly linear, with the hydrogen atom positioned approximately 1.8 to 2.0 Å from the acceptor atom. The N-H···O and N-H···N distances are consistent with standard hydrogen bond lengths observed in small-molecule crystal structures.

Because the A-T pair has only two hydrogen bonds, it is less stable than the G-C pair. The melting temperature (Tₘ) of a DNA duplex—the temperature at which half of the double-stranded molecules separate into single strands—increases with G-C content. In a typical buffer containing 10 mM sodium phosphate (pH 7.0) and 1 mM EDTA, a 20-base-pair duplex with 50% G-C content will have a Tₘ of approximately 55 to 60°C, while a duplex with 70% G-C content will melt at approximately 65 to 70°C. This difference is exploited in experimental design: PCR primers are typically designed with a G-C content of 40 to 60% to ensure that the melting temperatures of the forward and reverse primers are within 1 to 2°C of each other.

G-C Pair: Three Hydrogen Bonds

The guanine-cytosine pair is held together by three hydrogen bonds:

Guanine N1-H (donor) to Cytosine N3 (acceptor)
Cytosine N4-H (donor) to Guanine O6 (acceptor)
Guanine N2-H (donor) to Cytosine O2 (acceptor)

The third hydrogen bond, involving guanine's N2 amino group and cytosine's O2 carbonyl oxygen, is the key difference between the G-C and A-T pairs. This additional bond makes the G-C pair approximately 1.5 to 2.0 kcal/mol more stable than the A-T pair under standard conditions.

The higher stability of G-C pairs has several biological consequences. Regions of the genome that are G-C-rich are more resistant to denaturation and are often found in regulatory regions, where they may help maintain the double-stranded state. Conversely, A-T-rich regions are more easily melted and are commonly found at origins of replication, where strand separation is required to initiate DNA synthesis. The relationship between base composition and helix stability is also relevant to DNA Supercoiling, as the torsional stress required to unwind a duplex depends on its base composition.

Exceptions and Variations in RNA

The base pairing rules as described above apply to double-stranded DNA in its canonical B-form. However, RNA and certain structural contexts introduce variations that are important to understand.

RNA Base Pairing

In RNA, thymine is replaced by uracil. Uracil is identical to thymine except that it lacks the methyl group at the C5 position. This substitution does not change the hydrogen bonding pattern: adenine pairs with uracil (A-U) in RNA, forming two hydrogen bonds exactly analogous to the A-T pair in DNA. The N3-H of uracil donates to N1 of adenine, and the N6-H of adenine donates to O4 of uracil.

Guanine-cytosine pairing in RNA is identical to that in DNA, with three hydrogen bonds. This means that the relative stability of base pairs in RNA duplexes follows the same order as in DNA: G-C pairs are more stable than A-U pairs.

RNA molecules are typically single-stranded, but they fold into complex secondary structures through intramolecular base pairing. Common motifs include the stem-loop (a double-stranded stem formed by complementary sequences within the same molecule, capped by a single-stranded loop) and the pseudoknot (a structure in which the loop of one stem-loop pairs with a complementary sequence elsewhere in the molecule). These structures are essential for the function of transfer RNA (tRNA), ribosomal RNA (rRNA), and many regulatory RNAs.

Non-Standard Pairing in tRNA

While Watson-Crick base pairs dominate in RNA secondary structure, non-standard base pairs also occur, particularly in tRNA. Transfer RNA molecules are approximately 76 nucleotides long and fold into a cloverleaf secondary structure with three stem-loops and a central loop. The three-dimensional L-shaped structure is stabilized by a variety of non-Watson-Crick interactions.

The most common non-standard pair in tRNA is the G-U wobble pair. In this pair, guanine and uracil form two hydrogen bonds: G's N1-H donates to U's O2, and U's N3-H donates to G's O6. The geometry is slightly different from a standard Watson-Crick pair—the bases are shifted relative to each other—but the pair is stable and fits within the double helix with minimal distortion.

G-U wobble pairs are functionally important in the wobble hypothesis, proposed by Francis Crick in 1966 to explain the degeneracy of the genetic code. The third position of a codon (the "wobble position") can sometimes pair with the first position of the anticodon using non-standard pairing. For example, the tRNA that carries the amino acid alanine has the anticodon 3′-CGA-5′, which pairs with the codon 5′-GCU-3′. The G at the first position of the anticodon can pair with either C or U at the third position of the codon, allowing one tRNA to recognize two codons.

Other non-standard pairs found in tRNA include G-A, A-C, and U-U pairs, which occur in specific structural contexts and are stabilized by the surrounding three-dimensional environment. These exceptions do not violate the fundamental rule that purines pair with pyrimidines in standard Watson-Crick geometry; they represent additional, structurally distinct modes of interaction.

Methods Used to Study Base Pairing

The base pairing rules have been confirmed and refined using a variety of experimental techniques, each providing complementary information about the structure and dynamics of nucleic acids.

X-Ray Crystallography

X-ray crystallography has been the primary method for determining the three-dimensional structure of DNA and RNA at atomic resolution. In this technique, a purified nucleic acid sample is crystallized, and the crystal is exposed to a beam of X-rays. The diffraction pattern produced by the crystal is recorded on a detector, and the electron density map is reconstructed using Fourier transform mathematics.

The first X-ray diffraction patterns of DNA were obtained by Rosalind Franklin and Raymond Gosling at King's College London in the early 1950s. Franklin's "Photo 51," an X-ray diffraction image of B-form DNA, showed a clear cross-shaped pattern characteristic of a helical molecule with a 3.4 Å repeat (the distance between adjacent base pairs) and a 34 Å pitch (the length of one complete turn of the helix). These data, along with Chargaff's rules, were instrumental in the Watson-Crick model.

Modern crystallographic studies have provided high-resolution structures of DNA duplexes with specific sequences, revealing the precise geometry of A-T and G-C pairs. For example, the structure of a B-DNA dodecamer determined by Richard Dickerson and Horace Drew in 1981 showed that the average rise per base pair is 3.4 Å, the average twist is 36° (10 base pairs per turn), and the base pairs are nearly planar, with a slight propeller twist of about 15°.

Nuclear Magnetic Resonance (NMR)

Nuclear magnetic resonance spectroscopy is used to study nucleic acid structure in solution, complementing crystallographic studies of solid samples. NMR exploits the magnetic properties of certain atomic nuclei—particularly ¹H, ¹³C, ¹⁵N, and ³¹P—to determine the distances between atoms and the angles between bonds.

In a typical NMR experiment, a DNA or RNA sample at a concentration of 0.5 to 2.0 mM is placed in a strong magnetic field (typically 500 to 900 MHz for modern instruments). Radiofrequency pulses excite the nuclei, and the resulting signals are detected and analyzed. The nuclear Overhauser effect (NOE) provides information about through-space distances between protons: a strong NOE is observed for protons within 3 to 4 Å of each other, which is exactly the distance range relevant for base pairing.

NMR studies have confirmed the hydrogen bonding patterns in A-T and G-C pairs and have been particularly valuable for studying the dynamics of base pairing. For example, imino proton exchange experiments have shown that individual base pairs open transiently on a millisecond timescale, even in stable duplexes, and that the rate of opening depends on the local sequence context.

UV Spectroscopy

Ultraviolet absorption spectroscopy is a simple and widely used method for studying base pairing and helix stability. Nucleic acids absorb UV light maximally at 260 nm, and the absorbance of a DNA solution decreases by 20 to 40% when the DNA transitions from single-stranded to double-stranded form. This phenomenon, known as the hyperchromic effect, occurs because base stacking in the duplex reduces the absorbance of the bases.

A UV melting curve is obtained by measuring the absorbance at 260 nm as the temperature is increased at a constant rate (typically 0.5 to 1.0°C per minute). The temperature at which the absorbance increase is halfway between the fully double-stranded and fully single-stranded values is the melting temperature (Tₘ). Melting curves provide a direct measure of duplex stability and can be used to:

Determine the Tₘ of a specific duplex, which depends on its length, sequence, and salt concentration.
Compare the stability of A-T and G-C pairs by measuring Tₘ values for duplexes with different base compositions.
Monitor the binding of ligands (such as drugs or proteins) to DNA, which typically increases Tₘ.

In a typical experiment, a 15-base-pair duplex in 10 mM sodium phosphate buffer (pH 7.0) containing 100 mM NaCl will have a Tₘ of approximately 45 to 55°C, depending on its G-C content. Increasing the NaCl concentration to 1 M raises the Tₘ by approximately 15 to 20°C, because the sodium ions screen the electrostatic repulsion between the negatively charged phosphate groups on the two strands.

Common Misconceptions and Pitfalls

Students frequently encounter specific misunderstandings when learning about base pairing. Addressing these directly will help you avoid common exam errors.

Misconception: Any Purine with Any Pyrimidine

The statement "purine always pairs with pyrimidine" is correct but incomplete. It does not mean that any purine can pair with any pyrimidine. The pairing is specific: adenine pairs only with thymine (or uracil), and guanine pairs only with cytosine. The specificity is determined by the complementary hydrogen bonding patterns, not merely by the size constraint.

A common exam question asks students to identify the correct base pair in a given sequence. For example, if one strand has the sequence 5′-AGCT-3′, the complementary strand is 3′-TCGA-5′. A student who mistakenly pairs A with C and G with T would write 3′-TCGA-5′ incorrectly as 3′-CAGT-5′. The correct answer requires applying both the purine-pyrimidine rule and the specific A-T and G-C pairing rules.

Misconception: Hydrogen Bond Count

Students sometimes confuse the number of hydrogen bonds in A-T and G-C pairs. The correct values are:

Base Pair	Number of Hydrogen Bonds	Donor-Acceptor Interactions
A-T (or A-U)	2	A N6-H → T O4; T N3-H → A N1
G-C	3	G N1-H → C N3; C N4-H → G O6; G N2-H → C O2

A useful mnemonic: G-C has three bonds, and "G" and "C" are the only bases with three letters in their names that are also the only pair with three hydrogen bonds. Alternatively, remember that guanine and cytosine are the "stronger" pair, and the word "strong" has three syllables if you stretch it—whatever works for you.

Misconception: Hydrogen Bonds Are the Only Stabilizing Force

While hydrogen bonds are essential for base pair specificity, they are not the primary source of double-helix stability. The major stabilizing force in DNA is base stacking—the van der Waals interactions and hydrophobic effects that arise when the flat aromatic rings of adjacent bases stack on top of each other in the helix interior. Base stacking contributes approximately 5 to 10 kcal/mol per base pair, while hydrogen bonding contributes only 1 to 2 kcal/mol per pair.

This distinction matters for understanding why DNA is stable: if hydrogen bonds were the only stabilizing force, the double helix would dissociate at physiological temperatures. The hydrophobic interior of the helix, created by base stacking, excludes water and provides the thermodynamic driving force for duplex formation.

Misconception: The Rule Applies Only to DNA

The purine-pyrimidine pairing rule applies to RNA as well, with the substitution of uracil for thymine. In RNA, adenine pairs with uracil, and guanine pairs with cytosine. The rule also applies to synthetic nucleic acid analogs, such as peptide nucleic acids (PNAs) and locked nucleic acids (LNAs), which form duplexes with complementary DNA or RNA strands using the same base pairing rules.

Practical Summary and Exam Tips

Key Points to Remember

Purines are two-ring bases (adenine, guanine); pyrimidines are one-ring bases (cytosine, thymine, uracil).
A purine always pairs with a pyrimidine to maintain the uniform 20 Å diameter of the DNA double helix.
Adenine pairs with thymine (or uracil in RNA) via two hydrogen bonds.
Guanine pairs with cytosine via three hydrogen bonds.
Chargaff's rules state that A = T and G = C in double-stranded DNA, reflecting the complementary pairing.
The G-C pair is more stable than the A-T pair due to the additional hydrogen bond.
RNA uses uracil instead of thymine, and A-U pairs form two hydrogen bonds.

How to Approach Exam Questions

When solving base pairing problems, follow these steps:

Identify the direction of the strand. DNA strands are antiparallel, so the complementary strand runs in the opposite direction. If the template strand is written 5′ to 3′, the complement must be written 3′ to 5′.
Apply the specific pairing rules. Replace each A with T, each T with A, each G with C, and each C with G. For RNA, replace T with U.
Check the purine-pyrimidine balance. Verify that each purine in one strand is paired with a pyrimidine in the other, and vice versa.
Calculate G-C content if asked. The percentage of G-C pairs in a duplex is the same on both strands, since G always pairs with C.
Relate G-C content to melting temperature. Higher G-C content means higher Tₘ, because G-C pairs have three hydrogen bonds.

For example, consider the DNA sequence 5′-ATGCGTAA-3′. The complementary strand is 3′-TACGCATT-5′. The G-C content is 3 out of 8 base pairs, or 37.5%. If you are asked to predict which of two duplexes has a higher melting temperature, choose the one with more G-C pairs.

Frequently Asked Questions

Does a purine always pair with a pyrimidine?

Yes, in standard Watson-Crick base pairing, a purine always pairs with a pyrimidine. This is a geometric requirement: the combined width of a purine-pyrimidine pair matches the 20 Å diameter of the DNA double helix. A purine-purine pair would be too wide, and a pyrimidine-pyrimidine pair would be too narrow, both of which would disrupt the regular helical structure.

Why does a purine always pair with a pyrimidine?

The reason is structural. The DNA double helix has a uniform diameter because the two sugar-phosphate backbones are held at a constant distance by the base pairs. A purine (two-ring structure, approximately 12.0 Å wide) paired with a pyrimidine (one-ring structure, approximately 9.0 Å wide) gives a combined width of approximately 21 Å, which matches the measured helix diameter. This arrangement also positions the glycosidic bonds (connecting bases to sugars) at the correct distance for the backbones to form a regular helix.

What happens if two purines pair together?

If two purines paired together, the combined width would be approximately 24 Å, which is too wide for the DNA double helix. The sugar-phosphate backbones would be pushed apart, creating a local distortion or bulge in the helix. Such mispairs are recognized by DNA repair machinery and are typically corrected. If not repaired, they can lead to mutations, which may be associated with disease. The study of such mutations falls under Base Pair Substitution.

Are there exceptions to the purine-pyrimidine rule?

In standard Watson-Crick base pairing, the rule is absolute. However, non-standard base pairs do occur in specific contexts. The most common is the G-U wobble pair in RNA, which is important for the wobble hypothesis of codon-anticodon recognition. Other non-Watson-Crick pairs (such as G-A, A-C, and U-U) occur in tRNA and other structured RNAs, where they are stabilized by the surrounding three-dimensional environment. These are exceptions to the standard rule but do not apply to the canonical double helix.

How many hydrogen bonds are in A-T and G-C pairs?

The A-T pair (and the A-U pair in RNA) has two hydrogen bonds. The G-C pair has three hydrogen bonds. This difference in hydrogen bond number explains why G-C-rich DNA has a higher melting temperature than A-T-rich DNA of the same length.

What did Chargaff's rules state about base pairing?

Chargaff's rules, based on experimental measurements of DNA base composition, state that in double-stranded DNA, the amount of adenine equals the amount of thymine (A = T), and the amount of guanine equals the amount of cytosine (G = C). Consequently, the total purine content equals the total pyrimidine content (A + G = T + C). These observations were consistent with—and helped inspire—the Watson-Crick base pairing rules.

Does the rule apply to RNA?

Yes, with one modification: RNA uses uracil instead of thymine. In RNA, adenine pairs with uracil (A-U) via two hydrogen bonds, and guanine pairs with cytosine (G-C) via three hydrogen bonds. The same geometric constraint applies: a purine must pair with a pyrimidine to maintain the proper width of the duplex, which in RNA is typically in the A-form geometry.

Key Takeaways

Purines (adenine, guanine) are two-ring bases; pyrimidines (cytosine, thymine, uracil) are one-ring bases.
In Watson-Crick base pairing, a purine always pairs with a pyrimidine to maintain the uniform 20 Å diameter of the DNA double helix.
The specific pairs are A-T (two hydrogen bonds) and G-C (three hydrogen bonds) in DNA, and A-U (two hydrogen bonds) and G-C (three hydrogen bonds) in RNA.
The specificity of pairing is determined by complementary hydrogen bond donor-acceptor patterns on each base.
Chargaff's rules (A = T, G = C) provided experimental evidence for complementary base pairing before the Watson-Crick model was proposed.
G-C pairs are more stable than A-T pairs due to the additional hydrogen bond, which affects DNA melting temperature and is exploited in experimental design.
Non-Watson-Crick pairs, such as G-U wobble pairs in tRNA, occur in specific RNA contexts but do not apply to the canonical double helix.