DNA Structure and Function Lecture Notes

Introduction to DNA

• DNA stands for Deoxyribonucleic Acid.
• It is a crucial biological molecule responsible for storing genetic information within cells.
• Both eukaryotic and prokaryotic cells use DNA to synthesize proteins and enzymes necessary for survival.
• During reproduction, cells replicate DNA to pass it to offspring.

Composition of DNA

• DNA is a polymer made up of individual subunits called nucleotides.
• Each nucleotide consists of:
  1. A deoxyribose sugar (five-membered sugar)
  2. A nitrogenous base
  3. A phosphate group

Nitrogenous Bases

• Four types of nitrogenous bases:
  • Adenine (A) and Guanine (G) - Purines (two-ring structures)
  • Cytosine (C) and Thymine (T) - Pyrimidines (single-ring structures)

Nucleotide Bonding

• Nucleotides bond via phosphodiester bonds.
• Phosphodiester bond occurs between:
  • The third carbon of one sugar
  • The fifth carbon of another sugar (3' to 5' bond).
• Nucleotides are often named by their nitrogenous base (e.g., guanine nucleotide).

Double-Stranded DNA

• DNA typically forms a double-stranded structure known as the double helix.
• Two complementary DNA strands bond via hydrogen bonds between nitrogenous bases.

Complementary Base Pairing

• Guanine (G) pairs with Cytosine (C) - forms three hydrogen bonds.
• Adenine (A) pairs with Thymine (T) - forms two hydrogen bonds.
• More G-C pairs imply stronger DNA bonding.

Directionality and Anti-Parallel Structure

• DNA strands are anti-parallel:
  • One strand runs 5' to 3'
  • The complementary strand runs 3' to 5'
• This arrangement facilitates optimal hydrogen bonding.

Structural Arrangement

• Outer structure: Sugar and phosphate groups are on the outside.
• Inner structure: Nitrogenous bases are protected inside.
• Anti-parallel structure ensures stability and prevents spontaneous unzipping.

Temperature and DNA Stability

• High temperatures can break hydrogen bonds despite the protective structure.

Summary

• DNA as a polymer consists of nucleotides with a sugar, nitrogenous base, and phosphate group.
• Purines include adenine and guanine; pyrimidines include thymine and cytosine.
• DNA strands bond via hydrogen bonds between nitrogenous bases and phosphodiester bonds between nucleotides.