Understanding DNA Structure and Function

Apr 26, 2025

Introduction to DNA

Lecture Contributors

  • Edited by John Reif
  • Natalia Tretyakova, College of Pharmacy, U. of Minnesota
  • Richard Lavery, Institut de Biologie Physico-Chimique, Paris

Overview of DNA Structure

  • DNA is a double helix structure storing genetic code through a sequence of bases.
  • Human genome consists of approximately 3.3 billion base pairs and around 25,000 genes.

DNA Size Scale

  • Chemical bond: 1 Å (10^-10 m)
  • Amino acid: 10 Å (10^-9 m)
  • Globular protein: 100 Å (10^-8 m)
  • Virus: 1000 Å (10^-7 m)
  • Cell nucleus: 1 µm (10^-6 m)
  • Bacterial cell: 5 µm (10^-5 m)
  • Chromosome DNA: 10 cm (10^-1 m)

Nucleotides and DNA Structure

  • Nucleotides form DNA through phosphodiester bonds.
  • DNA strands have directionality (5' to 3').
  • DNA backbone is negatively charged due to phosphate groups.

Base Families

  • Purine: Adenine (A), Guanine (G)
  • Pyrimidine: Thymine (T), Cytosine (C), Uracil (U, in RNA)
  • Purines have a two-ring structure, pyrimidines have a single-ring structure.

Nucleosides and Nucleotides

  • Nucleoside: A base linked to a sugar (ribose or deoxyribose).
  • Nucleotide: Nucleoside with one or more phosphate groups.

DNA and RNA Differences

  • DNA: Contains deoxyribose, bases A, T, G, C
  • RNA: Contains ribose, bases A, U, G, C

DNA Helix Structure

  • B-DNA is the common form, with right-handed helix, antiparallel strands, and base pairs inside the helix.
  • Helix features major and minor grooves, important for protein interactions.
  • Base Pairing: A-T and G-C with hydrogen bonding.
  • DNA stability is maintained by hydrogen bonds, stacking interactions, and electrostatic forces.

Alternative DNA Forms

  • A-DNA: Dehydrated form, right-handed, more compact.
  • Z-DNA: Left-handed, occurs in high salt conditions or specific sequences.

Historical Context of DNA

  • 1865: Genes concept by Gregor Mendel.
  • 1869: "Nuclein" discovery by Johann Friedrich Miescher.
  • 1953: Watson and Crick propose the DNA double helix structure based on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins.

DNA Function and Biological Importance

  • DNA replication, transcription, and translation are central to the central dogma of molecular biology.
  • DNA hybridization involves the formation of double-stranded DNA from complementary strands and is crucial for biological processes and laboratory techniques.

DNA Technology and Applications

  • Peptide Nucleic Acids (PNAs) are synthetic polymers that mimic DNA/RNA but with a peptide-like backbone.
  • Used for high-specificity hybridization and resistant to enzymatic degradation.

Conclusion

  • DNA's structure and functionality are central to understanding biological processes and genetic information storage and transfer.