CELS191 Lecture 10: DNA Structure

Lecture Overview

• Course: CELS191 - Molecular Biology & Genetics
• Instructor: Dr Annika Bokor, Department of Biochemistry
• Contact: annika.bokor@otago.ac.nz
• Focus: Change in understanding of DNA structure and function.

Learning Objectives

• Describe the components of DNA and the Watson-Crick model's main features.
  • Orientation of monomers
  • Geometry of the molecule
  • Base pairing specifics
• Understand semiconservative DNA replication and its significance in genetic inheritance.

DNA Overview

• Function of DNA: Acts as the genetic material, carrying hereditary information.

Historical Background

• Key Developments in DNA Research:
  • Identification of nucleic acids: two types - DNA and RNA.
  • Chromosomes recognized as carriers of genetic information.
  • Composition of chromosomes: Proteins and deoxyribonucleic acid (DNA).

Phoebus Levene's Contributions

• Identified the basic structure of nucleic acids.
• Proposed first tetranucleotide hypothesis (incorrect): nucleotides are present in equal ratios (A, T, C, G).
• Conclusion: Speculated proteins were the genetic material.

Chargaff's Discoveries

• Found variability in DNA composition among species, supporting the theory that DNA is indeed the genetic material.
• Chargaff's Rules:
  • First Rule: [A] = [T] and [G] = [C]
  • Second Rule: Composition of DNA varies between species.

DNA Structure

• Key Contributors: Watson, Crick, Wilkins, and Franklin.
  • Rosalind Franklin's X-ray diffraction provided critical insight into DNA's helical structure (Photo 51).
• Characteristics of DNA:
  • Double-stranded helix
  • Antiparallel strands: One strand runs 5' o 3' while the other runs 3' o 5'.
  • Nucleotide units joined by phosphodiester bonds.
  • Bases are stabilized by hydrogen bonds
  • A pairs with T (two hydrogen bonds)
  • G pairs with C (three hydrogen bonds)

DNA Synthesis

• Direction of Synthesis: Always in the 5' o 3' direction.
• Importance of helical structure:
  • Provides stability and protection for the genetic code contained within the bases.

Semiconservative Replication

• Each original strand of DNA serves as a template to construct new strands.
• Significance: Ensures precise copying of genetic information for cell division.
• Occurs during cell division (mitosis), necessary for growth and repair.

Summary of Key Features

• DNA molecule characteristics:
  • Double-stranded helical structure with sugar-phosphate backbone on the outside and bases on the inside.
  • Stabilized by hydrogen bonding between complementary bases.
  • Allows for genetic information inheritance through semiconservative DNA replication.

Revision Questions

1. How are the polynucleotide chains organized in a DNA molecule?
2. In which direction does a DNA chain grow during synthesis?
3. How is the DNA helix stabilized?
4. Who are the key components located inside and outside of the DNA helix?
5. Explain how semiconservative DNA replication allows genetic information to be passed to new cells and the next generation.