2.1 Nucleotide Structure

The Building Blocks of DNA

Nucleotides are the monomers that make up nucleic acids. Each nucleotide consists of three components: a nitrogenous base, a five-carbon sugar, and one or more phosphate groups.

Nucleotide Components

Nitrogenous Bases

Purines (2 rings)

  • Adenine (A): Pairs with T (or U in RNA)
  • Guanine (G): Pairs with C

Pyrimidines (1 ring)

  • Cytosine (C): Pairs with G
  • Thymine (T): DNA only, pairs with A
  • Uracil (U): RNA only, pairs with A

Pentose Sugar

Deoxyribose (DNA)

H at 2' position. More stable.

Ribose (RNA)

OH at 2' position. More reactive.

Phosphate Group

  • • Attached to 5' carbon of sugar
  • • Negative charge at physiological pH
  • • Forms phosphodiester bonds between nucleotides
  • • High-energy bonds in NTPs (ATP, GTP)

Chargaff's Rules

Erwin Chargaff (1950) discovered key regularities in DNA composition:

[A] = [T]

Adenine equals Thymine

[G] = [C]

Guanine equals Cytosine

This provided key evidence for complementary base pairing in the double helix.

← Part 2 OverviewThe Double Helix →