Chapter 9: Molecular Structure of DNA and RNA

Topics Covered

• Experiments that identified DNA as the genetic material

• Nucleotide structure

• Nucleoside vs. nucleotide and nomenclature

• Chargaff’s rules

• Watson-Crick model of DNA

• DNA vs. RNA

9.1 Identification of DNA As the Genetic Material

• 1869: Friedrich Miescher

  • Extracted nuclei from pus found on hospital bandages.

  • Identified a substance termed “nuclein.”

  • Nuclein was acidic and composed of Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), and Phosphorus (P), which is not found in proteins.

• 1902-1903: Theodor Boveri and Walter Sutton

  • Developed the chromosome theory of inheritance.

  • Observed that chromosomes are passed on from one generation to the next during cell division, indicating they carry genetic material.

  • Recognized that chromosomes contain both DNA and proteins but believed proteins were the carriers of genetic information, as DNA was thought to be too simple (only 4 bases).

• 1928: Frederick Griffith

  • Conducted experiments injecting mice with R (rough) and S (smooth) strains of Streptococcus pneumoniae.

  • Discovered that an unknown substance transformed R strain into S strain, labeled as the “transforming principle.”

• 1944: Oswald Avery, Colin MacLeod, and Maclyn McCarty

  • Identified Griffith’s transforming principle.

  • Found that DNA extracts from the S strain converted the R strain into S, ruling out RNA and proteins as the transforming principle.

• 1952: Alfred Hershey and Martha Chase

  • Used the T2 bacteriophage to determine that DNA, not RNA or proteins, is the genetic material.

  • Conclusion drawn through experiments that tracked labeled DNA and proteins in infected bacteria.

9.3 Nucleotide Structure

• Components of DNA and RNA nucleotides:

  1. Sugars (pentoses):

  • DNA contains deoxyribose.

  • RNA contains ribose.

  1. Bases:

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

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

  1. Phosphate group.

• Nucleoside vs. Nucleotide:

  • A nucleoside consists of a base linked to a sugar.

  • A nucleotide is a nucleoside with one or more phosphate groups attached.

  • The base is attached to the 1′ Carbon (C) of the sugar by an N-glycosidic bond.

  • The phosphate group(s) is always attached to the 5′ Carbon (C).

9.4 Structure of a DNA Strand

• Phosphodiester bonds are responsible for joining nucleotides together.

• Backbone of DNA is formed through these bonds.

• Key components include:

  • Deoxyribose sugar and a phosphate group forming the DNA backbone.

  • Bases (A, T, C, G) extending from the sugar.

9.6 Structure of the DNA Double Helix

• 1953: James Watson and Francis Crick proposed the double helix structure of DNA.

• Key insights for their model derived from:

  1. Base composition studies conducted by Erwin Chargaff, confirming that the amount of purines equals that of pyrimidines, thus # A = # T and # G = # C.

  2. X-ray diffraction data of DNA provided by Rosalind Franklin, indicating DNA is a double helix.

• Chargaff’s Rules:

  • The number of purine bases (A and G) equals the number of pyrimidine bases (C and T/U).

  • Different species exhibit different base compositions.

• Rosalind Franklin's Findings:

  • X-ray crystallography suggested a double helix structure, indicating that:

  • A pair of purines would be too wide.

  • A pair of pyrimidines would be too narrow.

  • A purine paired with a pyrimidine maintained a consistent width compatible with X-ray data.

• Watson-Crick Model Details:

  • One complete turn of the helix spans 3.4 nm.

  • The distance between adjacent bases is 0.34 nm.

  • Complementary base pairing occurs between A-T and G-C through hydrogen bonds.

• Hyperchromatic shifts and melting temperatures (Tm) of DNA show the stability of the DNA structure as influenced by temperature, measured with optical density at 260 nm (OD260).

9.7 RNA Structure Differs from DNA

• Key differences between RNA and DNA include:

  1. Sugar: RNA contains ribose, whereas DNA contains deoxyribose.

  2. Base substitution: RNA uses uracil (U) while DNA uses thymine (T).

  3. Strand structure: RNA is typically single-stranded, while DNA is double-stranded.