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Nucleotide Structure
and Base Pairing

Explore the molecular building blocks of DNA and RNA, and discover how complementary base pairing enables the storage and transfer of genetic information.

Biochemistry

What Is a Nucleotide?

A nucleotide is the fundamental monomer of nucleic acids (DNA and RNA). Every nucleotide contains exactly three components joined by covalent bonds:

Phosphate Group
A phosphorus atom bonded to four oxygens (\( \text{PO}_4^{3-} \)). Links nucleotides together via phosphodiester bonds.
Pentose Sugar
A five-carbon sugar: deoxyribose in DNA or ribose in RNA. The sugar forms the backbone.
Nitrogenous Base
A nitrogen-containing ring structure that carries the genetic code. There are five types in total.

Anatomy of a Nucleotide

Phosphate
\( \text{PO}_4^{3-} \)
 5' bond Pentose Sugar 5'C 1'C 3'C 1' bond Nitrogenous Base A, T, G, C (DNA) A, U, G, C (RNA) 3' OH Single Nucleotide 5' → 3' direction

The Five Nitrogenous Bases

Bases fall into two structural categories based on their ring structure. This distinction is critical for understanding base pairing.

Purines (Double Ring)
Adenine (A) and Guanine (G): two fused rings (a six-membered and a five-membered ring). Larger molecules.
Pyrimidines (Single Ring)
Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA: one six-membered ring. Smaller molecules.

Purine vs Pyrimidine Rings

Purine 6-ring 5-ring N N N N Adenine (A) Guanine (G) Pyrimidine 6-ring N N Cytosine (C) Thymine (T) / Uracil (U) vs Larger (2 rings) Smaller (1 ring) Pairing: always 1 purine + 1 pyrimidine

Deoxyribose vs Ribose

Deoxyribose (DNA) O 3' 2' 1' 4' O H OH No OH here! Ribose (RNA) O 3' 2' 1' 4' O OH OH OH here! Key: 2' carbon hydroxyl group

Chargaff's Rules of Base Pairing

In 1950, Erwin Chargaff discovered that in any DNA molecule, the amount of each base follows precise ratios. These rules are the foundation of complementary base pairing:

1
Adenine pairs with Thymine (A = T). They are connected by 2 hydrogen bonds.
2
Guanine pairs with Cytosine (G = C). They are connected by 3 hydrogen bonds.
3
Therefore: \( [A] = [T] \) and \( [G] = [C] \). The ratio \( \frac{[A] + [G]}{[T] + [C]} = 1 \) always holds.

Complementary Base Pairing

DNA Base Pairs A Adenine Purine T Thymine Pyrimidine 2 H-bonds G Guanine Purine C Cytosine Pyrimidine 3 H-bonds G-C bonds are stronger than A-T bonds

DNA vs RNA: Key Differences

Feature DNA RNA
Sugar Deoxyribose Ribose
Bases A, T, G, C A, U, G, C
Strands Double-stranded Single-stranded
Stability More stable Less stable
Location Nucleus Nucleus + Cytoplasm
Function Long-term storage Protein synthesis

The Phosphodiester Backbone

P Sugar A 5' end 3'→5' P Sugar T P Sugar G 3' end Backbone Phosphodiester bond: 3' OH + 5' phosphate Releases water (condensation reaction)

Why Base Pairing Matters

1
DNA Replication: Each strand serves as a template. Knowing one strand's sequence immediately tells you the complementary strand.
2
Transcription: RNA polymerase uses base pairing to create an mRNA copy of a DNA gene (A→U, T→A, G→C, C→G).
3
Error Detection: Mismatched bases distort the helix shape, allowing repair enzymes to find and fix mutations.
4
Constant Helix Width: Purine + pyrimidine pairing keeps the double helix a uniform width of about 2 nm.

How to Find the Complementary Strand

1
Identify each base in the given strand, reading from 5' to 3'.
2
Apply pairing rules: A↔T and G↔C (for DNA). A↔U and G↔C (for RNA).
3
Write the complement in antiparallel direction: the new strand runs 3' to 5' when the template runs 5' to 3'.
4
Verify with Chargaff's Rule: Count each base. \( [A] = [T] \) and \( [G] = [C] \) should hold.

Example 1: DNA Complementary Strand

Problem: Given the DNA template strand 5'-ATCGGA-3', write the complementary strand.

5' A T C G G A 3' Template 3' T A G C C T 5' Complement Answer: 3'-TAGCCT-5' (or written conventionally: 5'-TCCGAT-3')

Example 2: Chargaff's Rule Calculation

Problem: A DNA sample contains 22% Adenine. What are the percentages of the other three bases?

1
By Chargaff's rule: \( [A] = [T] \), so Thymine = 22%.
2
Total A + T = \( 22\% + 22\% = 44\% \).
3
Remaining: \( 100\% - 44\% = 56\% \) for G + C.
4
By Chargaff's rule: \( [G] = [C] \), so each = \( \frac{56\%}{2} = 28\% \).

Example 3: mRNA Transcription

Problem: A DNA template strand reads 3'-TACGGA-5'. What is the mRNA sequence?

3' T A C G G A 5' DNA Template 5' A U G C C U 3' mRNA Transcription Rules: T → A, A → U (not T!), C → G, G → C Answer: 5'-AUGCCU-3'

Common Mistakes to Avoid

✕ Pairing A with C or G with T
A always pairs with T (DNA) or U (RNA). G always pairs with C. No exceptions in standard Watson-Crick pairing.
✕ Using T in RNA
RNA uses Uracil (U), never Thymine. When transcribing, DNA's A becomes U in RNA, not T.
✕ Forgetting Antiparallel
DNA strands run in opposite directions (5'→3' and 3'→5'). Writing both strands the same direction is incorrect.
✕ Mixing Up Sugar Types
DNA has deoxyribose (no 2' OH), RNA has ribose (with 2' OH). Confusing them changes the molecule's identity.

Summary of Key Concepts

Concept Detail Key Fact
Nucleotide Parts Phosphate + Sugar + Base 3 components
Purines A and G Double ring
Pyrimidines C, T (DNA), U (RNA) Single ring
A-T / A-U bonds 2 hydrogen bonds Weaker pair
G-C bonds 3 hydrogen bonds Stronger pair
Backbone bond Phosphodiester 3' to 5' linkage
Strand direction Antiparallel 5'→3' and 3'→5'

Question 1 of 5

Which three components make up a single nucleotide?

Question 2 of 5

A DNA sample contains 30% cytosine. What percentage of the sample is adenine?

Question 3 of 5

How many hydrogen bonds form between guanine and cytosine?

Question 4 of 5

If a DNA template strand reads 3'-GCATAG-5', what is the corresponding mRNA sequence?

Question 5 of 5

What is the key structural difference between deoxyribose and ribose?