Organic Chemistry · Biomolecules: Nucleic Acids

Base Pairing in DNA

8 min read

Hydrogen-bond counts per pair are standard textbook facts; the Wallace rule is an empirical estimate for short oligonucleotides, and $T_m$ depends on salt and buffer conditions — verify against current sources before relying on exact values in assessments.

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Base pairing is the hydrogen-bonding pattern that holds the two strands of DNA together and makes the molecule's information redundant: given one strand, the other is fully determined. In the Watson–Crick model (1953), adenine pairs specifically with thymine and guanine pairs specifically with cytosine. The pairs fit the double helix because a purine (two rings) always pairs with a pyrimidine (one ring), keeping the base-pair width constant, and the hydrogen-bond donor/acceptor patterns are complementary: A–T forms two hydrogen bonds, G–C forms three.

Two consequences define the biology of DNA. First, complementarity: the sequence of one strand dictates the sequence of the other, which is exactly what makes replication (Topic 28.3) and transcription possible. Second, : in any double-stranded DNA, the amount of adenine equals thymine and guanine equals cytosine — a direct, measurable consequence of the pairing rules. This topic explains the structural basis of the pairing, why G–C pairs are stronger than A–T pairs, and how pairing controls DNA stability, melting, and mutation.

Why this matters

Replication and heredity: Base pairing is the copying mechanism. Each strand serves as the template for its complement, which is how genetic information is passed to daughter cells with high fidelity.
DNA stability and melting: The $T_m$ (melting temperature) of a DNA duplex depends on its G–C content because G–C pairs have three hydrogen bonds versus two for A–T. PCR primers, probes, and hybridization assays are all designed with this relationship in mind.
Mutations and disease: Rare tautomeric forms of the bases can form nonstandard pairs, and these mispairs, if unrepaired, become mutations. Understanding pairing explains why certain base changes (transitions) are more common than others.
Forensics and diagnostics: DNA fingerprinting, paternity testing, and COVID-style PCR all exploit specific base pairing between a primer/probe and its target.
Exams: Writing complementary strands, applying Chargaff's rules, predicting $T_m$ trends from G–C content, and counting hydrogen bonds between paired strands are classic problems.

The college version

Core Concepts

The hydrogen-bonding rules

In DNA, each base presents a specific pattern of hydrogen-bond donors (N–H) and acceptors (C=O, ring N). The patterns are complementary:

A pairs with T: two hydrogen bonds. Adenine's N-1 and N-6 amino group pair with thymine's O-4 and N-3.
G pairs with C: three hydrogen bonds. Guanine's N-1, N-2 amino group, and O-6 pair with cytosine's N-3, O-2, and N-4 amino group.

The two strands run : one runs 5' → 3' and the other 3' → 5', so the base pairs are read in opposite directions along the two strands. The pairing rules are the same in RNA, except uracil replaces thymine: A pairs with U (two hydrogen bonds), while G pairs with C (three) as in DNA.

Why A–T and G–C and not other combinations

Two geometric facts enforce the pairing rules. First, purine–pyrimidine width: a purine–pyrimidine pair spans the same distance as the other base pairs, keeping the two sugar–phosphate backbones a constant distance apart along the helix. Two purines would bulge the helix; two pyrimidines would pinch it. Second, donor–acceptor complementarity: A–C, G–T, and A–G combinations place donor against donor or acceptor against acceptor, which costs energy and destabilizes the pair. The "correct" pairs are the ones that both fit the geometry and maximize hydrogen bonds.

Chargaff's rules: pairing made visible

Erwin Chargaff's analyses of DNA from many organisms showed that in double-stranded DNA, $[A] = [T]$ and $[G] = [C]$, while the total purine content equals the total pyrimidine content. These rules follow directly from base pairing and are used to find unknown base percentages: if a genome is 30% A, it must be 30% T, leaving 40% for G + C, so 20% each.

Melting temperature and G–C content

Heating DNA breaks the hydrogen bonds and separates the strands — . The temperature at which half the duplex is denatured is the melting temperature, $T_m$. Because G–C pairs contribute three hydrogen bonds and A–T pairs two, a higher G–C fraction raises $T_m$. A common empirical estimate for short duplexes is:

$T m \approx 4(G + C) + 2(A + T) °C$

(the "Wallace rule", for oligonucleotides roughly 14–20 bases long). This relationship is why PCR primers are chosen with balanced G–C content and similar $T_m$ values.

Tautomerism and mispairing

Each base can adopt rare tautomeric forms (e.g., enol instead of keto for G and T). A rare enol of guanine pairs with thymine instead of cytosine — a mispair with the same geometry as a correct pair. If replication fixes the mispair into both daughter strands, a mutation results. This is the chemical origin of spontaneous base-substitution mutations and explains why tautomerization matters beyond organic chemistry class.

Common Confusions

Do Not Confuse	With	Difference
A–T vs A–U	Same pairing in DNA and RNA	DNA uses T; RNA uses U; both pair with A via two hydrogen bonds
Hydrogen bonds (between strands)	Phosphodiester bonds (within a strand)	H-bonds are weak, noncovalent, and hold strands together; phosphodiester bonds are covalent and build the backbone
Complementary strand	Identical strand	The complement has swapped bases and reversed direction; it is not the same sequence
G–C content and $T_m$	G–C content and helix width	G–C content changes melting temperature, not helix geometry; width is fixed by purine–pyrimidine pairing
Chargaff's rules	Any single-stranded DNA	Chargaff's equalities hold only for double-stranded DNA; single-stranded DNA can have any composition
Tautomer	Resonance form	Tautomers differ in atom positions (proton moves, bond shifts); resonance forms differ only in electron placement
Mutation from mispairing	Mutation from chemical damage	Mispairing arises from tautomers or replication errors; damage (UV, oxidation) chemically alters bases — different mechanisms, same outcome

Eli explains

The same idea, in plain words

Explain it like I’m 10

DNA is like a zipper with two rows of teeth that always match: a big tooth (A) only clicks with its special small tooth (T), and another big tooth (G) only clicks with its small tooth (C). The two rows face opposite directions, like two people shaking hands. If you know one row of teeth, you can always predict the other row — that's how a cell copies its DNA to make a new cell.

Worked example

Example 1: Applying Chargaff's rules

A double-stranded DNA sample is 28% adenine. What are the percentages of T, G, and C?

Write the rules first: $[A] = [T]$, $[G] = [C]$, and $[A] + [T] + [G] + [C] = 100\%$.

Substitute $[A] = 28\%$:

$[T] = 28 %, [G] + [C] = 100 % - 56 % = 44 %$

Since $[G] = [C]$:

$[G] = [C] = 44 % 2 = 22 %$

Answer: T = 28%, G = 22%, C = 22%. Notice that only A's percentage is needed — pairing rules determine everything else.

Example 2: Hydrogen bonds in a genome segment

A double-stranded DNA region is 1000 base pairs long with 40% G–C pairs. Calculate the total number of hydrogen bonds holding the two strands together.

Write the relationship first:

$H = 3N GC + 2N AT$

where $N{GC}\ = 0.40 \times 1000 = 400$ and $N{AT} = 1000 - 400 = 600$.

Substitute:

$H = 3(400) + 2(600) = 1200 + 1200 = 2400$

Answer: 2400 hydrogen bonds. The same reasoning explains why G–C-rich regions are harder to denature and melt at higher temperature.

Example 3: Writing the complementary strand

Write the complementary strand for 5'-A T G C C G T A-3', labeling its direction.

Apply pairing rules base by base, then reverse direction because the complement runs antiparallel:

$5' -A T G C C G T A-3' \to 3' -T A C G G C A T-5'$

Answer: The complement is 3'-TACGGCAT-5', conventionally rewritten as 5'-TACGGCA T-3' when read in the standard direction. The classic exam trap is forgetting to reverse the direction when writing the answer 5' → 3'.

Key takeaways

Watson–Crick pairs: A–T (2 H-bonds), G–C (3 H-bonds); in RNA, A–U (2 H-bonds).
Purine always pairs with pyrimidine — constant helix width.
Strands are antiparallel (5' ↔ 3' and 3' ↔ 5'); sequences are written 5' → 3' by convention.
Chargaff: $[A] = [T]$, $[G] = [C]$; total purines = total pyrimidines.
Higher G–C content → higher $T_m$ (more hydrogen bonds); Wallace rule: $T_m \approx 4(G{+}C) + 2(A{+}T)\ \text{°C}$ for short oligos.
Keto tautomers dominate; rare enol/imine tautomers cause mispairing and mutations.
Complementarity makes replication and transcription possible: each strand is a template for the other.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

Which base pairs form two hydrogen bonds, and which form three?
Show answer
A–T (and A–U in RNA) form two hydrogen bonds; G–C forms three.
A DNA duplex is 35% cytosine. What are the percentages of G, A, and T?
Show answer
G = 35% (pairs with C); remaining 30% is A + T, so A = 15% and T = 15%.
Write the (with direction) of 5'-G G A T C C-3'.
Show answer
3'-C C T A G G-5' (equivalently 5'-G G A T C C-3' read antiparallel; written 5' → 3' the complement is 5'-G G A T C C-3' reversed: 5'-C C T A G G-3'). The key: A pairs with T, G pairs with C, and the direction reverses.
Why must a purine pair with a pyrimidine?
Show answer
Purine–pyrimidine pairs keep the sugar–phosphate backbones a constant distance apart; two purines would bulge the helix and two pyrimidines would pinch it.
Two 20-base DNA duplexes have the same length but different sequences. One is 70% G–C, the other 30%. Which has the higher $T_m$, and why?
Show answer
The 70% G–C duplex: G–C pairs contribute three hydrogen bonds each versus two for A–T, so more hydrogen bonds must be broken and the duplex melts at a higher temperature.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Base pair: Two bases held together by hydrogen bonds (A–T, G–C)
Complementary strand: The strand whose sequence is dictated by base pairing
Antiparallel: Two strands running in opposite 5' → 3' directions
Hydrogen bond: Weak electrostatic interaction between N–H and N/O acceptors
Chargaff's rules: $[A] = [T]$ and $[G] = [C]$ in duplex DNA
Melting temperature (Tm): Temperature at which half the duplex denatures
Tautomer: Isomer differing by proton position (keto vs enol)
Denaturation: Separation of the two strands by heat or pH

Sources & references

openstax.org — Organic Chemistry

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