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ch10 History of DNA

Chapter 10: DNA - The Chemical Nature of the Gene

10.1 Key Characteristics of Genetic Material

  • Complex Information: Genetic material must store complex details necessary for an organism's development and functioning.

  • Faithful Replication: It must replicate accurately to ensure continuity across generations.

  • Phenotype Encoding: It must encode phenotypic features, impacting traits seen in the organism.

  • Capacity to Vary: There should be potential for variation, essential for evolution and adaptation.

10.2 Genetic Information Encoded in DNA/RNA

10.2.1 Early Studies of DNA

  • Miescher: First identified "nuclein" (now known as DNA).

  • Kossel: Discovered that DNA contains four nitrogenous bases.

  • Chargaff's Rules: Identified ratios of bases within DNA.

Timeline of Discoveries

  • 1833: Brown describes the nucleus.

  • 1869: Miescher discovers nuclein in white blood cell nuclei.

  • 1900: Mendel's work is rediscovered; Levene proposes tetranucleotide theory.

  • 1928: Griffith demonstrates the "transforming principle" in experiments.

  • 1952: Hershey and Chase prove DNA is the genetic material in bacteriophage.

  • 1953: Watson and Crick model the structure of DNA.

10.2.2 Chargaff's Rules

  • Adenine (A) = Thymine (T)

  • Guanine (G) = Cytosine (C)

  • Base Composition Table

    • E. coli: A: 26.0%, T: 23.9%, G: 24.9%, C: 25.2%

    • Human: A: 30.3%, T: 30.3%, G: 19.5%, C: 19.9%

10.2.3 Transformation Principle

  • Griffith's Experiment: Identified that a substance from heat-killed virulent bacteria could genetically transform non-virulent bacteria.

    • Conclusion: The transforming substance was DNA.

10.2.4 The Hershey-Chase Experiment

  • Investigated whether DNA or protein is the genetic material in bacteriophages.

  • Used radioactive isotopes to differentiate between DNA and protein.

  • Conclusion: DNA is the genetic material, as only it was found in progeny phages.

10.2.5 RNA as Genetic Material

  • RNA can serve as genetic material in some viruses, e.g., Tobacco Mosaic Virus (TMV).

  • Experiment: Hybrid TMV (mixing RNA and proteins of different types) confirmed RNA determines progeny characteristics.

10.3 DNA Structure: Double Helix

Primary Structure

  • Deoxyribonucleotides: Consist of a sugar, phosphate, and a nitrogenous base.

    • Base Types: Purines (A, G) and pyrimidines (C, T).

Secondary Structure

  • Double Helix: Formed by complementary and antiparallel strands via phosphodiester and hydrogen bonds.

  • Key Features: Antiparallel strands crucial for hydrogen bonding and base pairing, leading to the double-helix structure.

10.4 Special Structures in DNA & RNA

Hairpin Structures

  • Formed in single strands where nucleotide sequences are inverted complements; common in RNA folding.

H-DNA Formation

  • Three-stranded DNA structures when one strand pairs with double-stranded DNA within the same molecule, frequently seen in long sequences of purines or pyrimidines.

DNA Methylation

  • Addition of methyl groups to nucleotide bases can influence gene expression and DNA structure, particularly in eukaryotes.