Molecular Structure of DNA and RNA Notes

Molecular Structure of DNA and RNA

Identification of DNA as the Genetic Material

  • Criteria for Genetic Material:

  • Information: Contains necessary information for making an entire organism.

  • Transmission: Passed from parent to offspring.

  • Replication: Must be copied for transmission.

  • Variation: Capable of changes to explain phenotypic variation.

  • Historical Source of Evidence:

  • Data from many geneticists, including Mendel, supported these properties.

  • Chemical nature of genetic material identified via various experimental approaches.

Griffith’s Experiments on Genetic Transformation

  • Organism Studied:

  • Streptococcus pneumoniae (pneumococci) with two strains:

    • Type S: Smooth, secretes a polysaccharide capsule, protects from immune system.

    • Type R: Rough, no capsule secretion.

  • Key Experiment Steps:

  1. Injection of Live Type S: Mouse dies; Type S recovered.

  2. Injection of Live Type R: Mouse survives; no bacteria recovered.

  3. Injection of Heat-Killed Type S: Mouse survives; no bacteria recovered.

  4. Injection of Live Type R + Heat-Killed Type S: Mouse dies; Type S recovered.

  • Conclusion: A transforming principle from dead Type S turned Type R into Type S.

Avery, MacLeod, and McCarty Experiments

  • Objective: Identify the transforming substance in Griffith's experiments.

  • Key Findings:

  • Prepared cell extracts from Type S and tested macromolecules.

  • Only the DNA extract could transform Type R to Type S.

  • Treatment with DNase eliminated transformation, whereas RNase or protease did not.

Evidence from Hershey and Chase

  • Experiment Focus: Investigated whether DNA or proteins were genetic material of T2 bacteriophage.

  • Methodology:

  • Labeled DNA with 32P and proteins with 35S.

  • After infection of E. coli, more 32P found in bacterial cells, indicating DNA is the genetic material.

Overview of DNA and RNA Structure

  • Discovery:

  • DNA (Deoxyribonucleic Acid) discovered by Friedrich Miescher in 1869; termed "nuclein."

  • Characteristics:

  • Nucleotides are building blocks of nucleic acids (DNA and RNA).

  • DNA forms a double helix with two strands; RNA usually single-stranded but can form short double strands.

Nucleotide Structure

  • Components:

  • Phosphate group

  • Pentose sugar:

    • Ribose in RNA.

    • Deoxyribose in DNA.

  • Nitrogenous bases:

    • Purines: Adenine (A), Guanine (G).

    • Pyrimidines: Cytosine (C), Thymine (T) in DNA, Uracil (U) in RNA.

Structure of DNA Strand

  • Linkage:

  • Nucleotides linked by phosphodiester bonds (linking 5' phosphate to 3' hydroxyl).

  • The backbone consists of phosphate and sugar, with bases projecting outwards.

Discovery of the Double Helix

  • Key Contributors:

  • Watson and Crick (1953), using data from Rosalind Franklin's X-ray diffraction.

  • Chargaff's rule: %A = %T and %C = %G, was crucial in elucidating DNA's structure.

  • Notably, Franklin's contributions to X-ray techniques enabled visualization of helical DNA structure.

Structure of the DNA Double Helix

  • Key Features:

  • Right-handed helix, antiparallel strands (one 5' to 3', the other 3' to 5').

  • Hydrogen bonds between complementary base pairs (A–T and C–G).

  • Base stacking stabilizes the double helix structure.

Types of DNA Double Helices

  • B DNA:

  • Predominant form in living cells, bases perpendicular to helical axis, 10 base pairs per turn.

  • Z DNA:

  • Left-handed helix, forms under specific sequences and conditions; may play roles in regulation and chromatin structure.

RNA Structure

  • Differences from DNA:

  • Uses uracil instead of thymine.

  • Contains ribose sugar with a hydroxyl group at the 2' position.

  • Can form complex secondary structures due to base-pairing (A–U, C–G).

  • Types of RNA:

  • Various RNA molecules can have loops and stems; example structures include bulge loops and internal loops.

This outline provides a comprehensive overview of DNA and RNA structure, their discovery, and key experiments that led to the identification of DNA as genetic material, tailored for in-depth understanding.