Molecular Structure of DNA and RNA

Molecular Structure of DNA and RNA

Introduction to Molecular Genetics

  • Molecular genetics focuses on DNA structure and function at the molecular level.
  • Advances in techniques have enhanced understanding of:
    • Molecular genetics: Study of the molecular basis of genetic inheritance.
    • Transmission genetics: Study of how traits are inherited.
    • Population genetics: Study of genetic variation within populations.
  • Knowledge of genetics heavily depends on understanding DNA and RNA structure.

9.1 Identification of DNA as Genetic Material

Essential Criteria for Genetic Material:
  • Information: Must contain necessary information for organism development.
  • Transmission: Must be capable of being passed from parent to offspring.
  • Replication: Must be able to replicate to ensure continuity in inheritance.
  • Variation: Must allow changes to account for species variations.
  • Early data (e.g., Mendel's experiments) aligned with these properties, but identification of DNA included experimental approaches beyond genetic crosses.

Griffith's Experiments

  • Studied Streptococcus pneumoniae, identifying two strains:
    • Type S (Smooth): Produces capsule, protects from immune response.
    • Type R (Rough): No capsule, does not evade immune system.
Griffith's Conclusion:
  • Transforming Principle: Dead Type S bacteria transformed Type R to Type S.
  • Transformation was caused by a substance (transforming principle) from dead S bacteria.

The Transforming Principle

  • Fulfilled the genetic material criteria:
    • Acquired information to produce capsules.
    • Variation in capsule formation.
    • Replication capabilities.

Avery, MacLeod and McCarty Experiments

  • Aimed to identify the genetic material responsible for transformation.
  • They purified macromolecules from Type S cells.
  • Only DNA extract could transform Type R bacteria to Type S; RNase and protease treatment had no effect, but DNase destroyed transformation ability.

Hershey and Chase Experiment

  • Used T2 phage to identify DNA as genetic material.
  • Employed radioisotopes to label DNA and proteins separately.
  • Infection of bacterial cells showed DNA, not protein, entered the cells, confirming DNA as the genetic material.

9.2 Overview of DNA and RNA Structure

  • Nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
  • First identified by Friedrich Miescher (1869) as “nuclein.”
  • Nucleic acids release H+ in water, hence their name.
Nucleotide Structure
  • Components of a nucleotide:
    • Phosphate group
    • Pentose sugar (Ribose in RNA; Deoxyribose in DNA)
    • Nitrogenous base
    • Purines: Adenine (A), Guanine (G)
    • Pyrimidines: Cytosine (C), Thymine (T) in DNA; Uracil (U) in RNA.
Levels of Nucleic Acid Structure
  • Nucleotides -> Single strand -> Double helix -> Three-dimensional structure.

9.4 Structure of a DNA Strand

  • Nucleotides linked by phosphodiester linkages (between 5′ and 3′ carbons).
  • Directionality: 5′ to 3′.
  • Backbone formed by alternating sugar and phosphate groups with bases projecting outward.

Discovery of the Double Helix

  • Watson and Crick (1953) credited for elucidating DNA structure; inspired by earlier studies.
  • Contributions from:
    • Linus Pauling: Identified protein secondary structures.
    • Rosalind Franklin: Conducted X-ray diffraction studies, revealing DNA's helical structure.
    • Erwin Chargaff: Discovered Chargaff's rules (A=T, C=G) essential for double helix formation.

9.6 Structure of the DNA Double Helix

  • Two complementary strands twisted into a right-handed double helix.
  • Antiparallel orientation: one strand 5′ to 3′ and another 3′ to 5′.
  • Structure stabilized by:
    • Hydrogen bonds between bases (A to T has 2; C to G has 3).
    • Base stacking interactions enhance stability.
DNA Grooves
  • Major and minor grooves allow protein interactions with specific sequences.

9.7 RNA Structure

  • RNA primary structure resembles DNA but includes Uracil instead of Thymine and possesses ribose sugar with a 2′ OH.
  • RNA typically single-stranded but may form short double-stranded regions through complementary base-pairing (A-U, C-G).
Types of RNA Structures
  • Secondary structures in RNA include loops and stems that arise from base-pairing.
  • Tertiary structure influenced by interactions with various molecules and proteins.