1.5.2 Nucleotide Structure & the Phosphodiester Bond

Both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers of nucleotides. Each nucleotide is composed of three main components:

1. A pentose sugar (5-carbon sugar),
2. A phosphate group, and
3. A nitrogen-containing organic base.

Components of DNA and RNA Nucleotides:

• DNA Nucleotide:
  • Contains a deoxyribose sugar.
  • Has a phosphate group.
  • Includes one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or thymine (T).
• RNA Nucleotide:
  • Contains a ribose sugar.
  • Has a phosphate group.
  • Includes one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or uracil (U) (in place of thymine).

Phosphodiester Bond and Formation of Polynucleotides:

• Nucleotides join together through phosphodiester bonds in a condensation reaction, releasing water.
• The bond forms between the phosphate group of one nucleotide and the sugar of another.
• This linkage creates a sugar-phosphate backbone, resulting in a chain of nucleotides known as a polynucleotide.

Structure of DNA and RNA:

• DNA:
  • DNA consists of two polynucleotide strands that coil to form a double helix.
  • The two strands are held together by hydrogen bonds between complementary bases:
  • Adenine pairs with Thymine (A-T) through two hydrogen bonds.
  • Guanine pairs with Cytosine (G-C) through three hydrogen bonds, which contribute to DNA stability.
  • DNA is a highly stable molecule because:
  • The phosphodiester backbone protects the nitrogenous bases within the helix.
  • A high proportion of G-C pairs (with three hydrogen bonds) increases stability.
• RNA:
  • RNA is typically a single polynucleotide chain, making it shorter and less stable than DNA.

Base Pairing Rules:

• Adenine (A) ALWAYS pairs with Thymine (T) in DNA.
• Guanine (G) ALWAYS pairs with Cytosine (C).

This structure allows DNA to serve as a stable genetic material, while RNA acts as a messenger, transferring information from DNA for protein synthesis.