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Nucleic Acid Structure
and Base Pairing

Master the molecular architecture of DNA and RNA, understand how complementary base pairing drives replication, transcription, and translation.

MCAT Biochemistry Content Category 1B

What Are Nucleic Acids?

Nucleic acids are biological macromolecules that store, transmit, and express genetic information. The two major types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

DNA
Double-stranded helix storing the complete genetic blueprint of an organism
RNA
Single-stranded molecule that carries out instructions encoded in DNA

The Nucleotide Monomer

Every nucleotide is composed of three components. Understanding these building blocks is the key to understanding all nucleic acid structure.

Phosphate Group Pentose Sugar Nitrogenous Base
\( \text{PO}_4^{3-} \)
Ribose / Deoxyribose A, G, C, T (or U) 5' bond N-glycosidic bond

The Five Nitrogenous Bases

Bases fall into two structural categories based on ring number. This distinction matters because base pairing always joins a purine with a pyrimidine.

Purines (2 rings)
Adenine (A) and Guanine (G): double-ring structures with a fused imidazole and pyrimidine ring
Pyrimidines (1 ring)
Cytosine (C), Thymine (T), and Uracil (U): single six-membered ring structures

The DNA Double Helix

A T G C A G C T A C G T C G 2 H-bonds 3 H-bonds
\(5'\)
\(3'\)
\(3'\)
\(5'\)
 Sugar- Phosphate Backbone Sugar- Phosphate Backbone

Chargaff's Rules of Base Pairing

In any double-stranded DNA molecule, the amount of adenine equals thymine, and the amount of guanine equals cytosine. This is the quantitative basis of Watson-Crick base pairing.

\(\text{A} = \text{T}\)
Connected by 2 hydrogen bonds. The percentage of A always equals the percentage of T.
\(\text{G} \equiv \text{C}\)
Connected by 3 hydrogen bonds. The percentage of G always equals the percentage of C.

Hydrogen Bonds in Base Pairing

A Purine T Pyrimidine H H 2 H-bonds G Purine C Pyrimidine H H H 3 H-bonds Purine always pairs with Pyrimidine
This keeps the helix diameter constant at \(\sim 2 \text{ nm}\)

Key Distinction: DNA vs RNA

Feature DNA RNA
Sugar Deoxyribose Ribose
Strands Double-stranded Single-stranded
Bases \(\text{A, T, G, C}\) \(\text{A, }\)\(\text{U}\)\(\text{, G, C}\)
Base Pairing \(\text{A-T, G-C}\) \(\text{A-}\)\(\text{U}\)\(\text{, G-C}\)
Stability Very stable (dsDNA) Less stable, more reactive
\(2'\)-OH Absent (H only) Present
Function Long-term storage Diverse: mRNA, tRNA, rRNA

The Phosphodiester Bond

Sugar 1
\(5'\)
 P Base
\(3'\)
 P Phosphodiester Bond
\(5'\)
 Sugar 2 Base
\(3'\) end

Types of RNA

mRNA
Carries genetic code from DNA to ribosomes. Codons are read in the \(5' \to 3'\) direction.
tRNA
Adaptor molecule with an anticodon loop and amino acid attachment at the \(3'\) CCA end.
rRNA
Structural and catalytic component of ribosomes. Catalyzes peptide bond formation (ribozyme).

Denaturation and Reannealing

When dsDNA is heated or exposed to extreme pH, the hydrogen bonds between bases break and the strands separate. This is denaturation (melting).

Native dsDNA Heat Denatured Cool Reannealed

How to Find a Complementary Sequence

1
Identify the template strand and note its \(5' \to 3'\) direction. Always read the given strand's polarity first.
2
Apply base pairing rules: \(\text{A}\) pairs with \(\text{T}\) (DNA) or \(\text{U}\) (RNA); \(\text{G}\) pairs with \(\text{C}\). Replace each base with its complement.
3
Reverse the polarity: The complementary strand runs antiparallel. If the template is \(5' \to 3'\), write the complement as \(3' \to 5'\).
4
Convert if needed: For mRNA, replace \(\text{T}\) with \(\text{U}\). The mRNA sequence matches the coding strand (with \(\text{U}\) instead of \(\text{T}\)).

Example 1: Complementary DNA Strand

Given the DNA template strand: \(3'\)-ATGCCGTA-\(5'\), find the complementary strand.

1
Apply pairing: \(\text{A} \to \text{T}, \text{T} \to \text{A}, \text{G} \to \text{C}, \text{C} \to \text{G}, \text{C} \to \text{G}, \text{G} \to \text{C}, \text{T} \to \text{A}, \text{A} \to \text{T}\)
2
Write antiparallel: Complement reads \(5'\)-TACGGCAT-\(3'\)

Example 2: DNA to mRNA

Given the DNA template strand: \(3'\)-TACGCAATT-\(5'\), what is the mRNA sequence?

1
Complement with RNA rules: \(\text{T} \to \text{A}, \text{A} \to \text{U}, \text{C} \to \text{G}, \text{G} \to \text{C}, \text{C} \to \text{G}, \text{A} \to \text{U}, \text{A} \to \text{U}, \text{T} \to \text{A}, \text{T} \to \text{A}\)
2
Write antiparallel (\(5' \to 3'\)): \(5'\)-AUGCGUUAA-\(3'\)
3
Note: AUG is the start codon (Met) and UAA is a stop codon. This is a complete open reading frame.

Example 3: Chargaff's Rule Calculation

A dsDNA sample contains 22% adenine. What are the percentages of the other three bases?

1
\(\%\text{A} = \%\text{T} = 22\%\) (Chargaff's rule: adenine always equals thymine)
2
\(\%\text{A} + \%\text{T} = 44\%\), so \(\%\text{G} + \%\text{C} = 100\% - 44\% = 56\%\)
3
\(\%\text{G} = \%\text{C} = 28\%\) (guanine equals cytosine)

Common Mistakes

✕ Applying Chargaff to ssRNA
Chargaff's rules (\(\text{A}=\text{T}, \text{G}=\text{C}\)) apply only to double-stranded DNA. In single-stranded nucleic acids, there is no obligate base pairing.
✕ Wrong Polarity Direction
Forgetting that strands are antiparallel: writing the complement in the same direction as the template instead of reversing it.
✕ Using T in RNA
Writing thymine (T) in an RNA sequence instead of uracil (\(\text{U}\)). RNA uses \(\text{U}\) wherever DNA uses \(\text{T}\).

Summary Table

Concept Key Fact MCAT Relevance
Nucleotide Base + Sugar + Phosphate Monomer identification
Purines A, G (2 rings) Purine/pyrimidine size matching
Pyrimidines C, T, U (1 ring) \(\text{T}\) in DNA, \(\text{U}\) in RNA
H-bonds \(\text{A-T}: 2, \text{G-C}: 3\) \(T_m\) and stability questions
Chargaff's \(\%\text{A} = \%\text{T}\), \(\%\text{G} = \%\text{C}\) Calculation problems
Antiparallel \(5' \to 3'\) vs \(3' \to 5'\) Replication and transcription
\(2'\)-OH Present in RNA only Stability and hydrolysis

Question 1 of 5

A double-stranded DNA molecule is found to contain 30% cytosine. What percentage of the molecule is adenine?

Question 2 of 5

Which structural feature makes RNA more susceptible to hydrolysis than DNA?

Question 3 of 5

Given the DNA template strand \(3'\)-GCATTAGC-\(5'\), what is the corresponding mRNA sequence?

Question 4 of 5

Two DNA samples have the same length but different melting temperatures. Sample A has a \(T_m\) of \(85^\circ\text{C}\) and Sample B has a \(T_m\) of \(72^\circ\text{C}\). Which statement best explains this difference?

Question 5 of 5

Adenine and guanine are classified as purines because they: