DNA Structure, Chargaff’s Rules, & The Franklin–Watson–Crick Story

DNA vs. RNA: Core Differences

• Both are nucleic acids built from nucleotide sub-units, yet differ in sugar, bases, and functional roles.
  • DNA = Deoxyribonucleic Acid
  • Sugar: deoxyribose (pentose; 5-carbon).
  • Bases: adenine (A), thymine (T), guanine (G), cytosine (C).
  • Typically double-stranded; long-term genetic storage.
  • RNA = Ribonucleic Acid
  • Sugar: ribose (pentose; 5-carbon).
  • Bases: adenine (A), uracil (U), guanine (G), cytosine (C).
    • No thymine in RNA.
  • Usually single-stranded; versatile roles: messenger (mRNA), ribosomal (rRNA), transfer (tRNA), catalytic (ribozymes), etc.
• Functional summary
  • DNA: blueprint that must be replicated accurately and transmitted generationally, yet flexible enough to mutate.
  • RNA: can be transcribed from DNA, translated into protein, or (in some viruses) even serve directly as genetic material capable of self-replication.

What Makes a Molecule “Genetic Material”?

• Must:
  • Replicate accurately during every cell division.
  • Be heritable—pass faithfully from generation to generation.
  • Allow mutations that introduce variability for evolution/adaptation.

Nucleotide Chemistry & Classification

• A nucleotide = pentose sugar + phosphate group + nitrogenous base.
• Bases fall into two structural classes:
  • Purines (double-ring; “short name, BIG structure”): Adenine (A), Guanine (G).
  • Pyrimidines (single-ring; “long name, small structure”): Cytosine (C), Thymine (T), Uracil (U).

Chargaff’s Rules (Erwin Chargaff)

• Experimental comparison of many species’ DNA revealed:
  • $(\%A) \approx (\%T)$
  • $(\%G) \approx (\%C)$
• Base ratios are consistent within a species but vary between species → molecular basis of species diversity.
• Practical implication: knowing one base percentage lets you deduce its complement’s percentage.
• Example exercise promised in lecture: If $A = 30\%$, then $T = 30\%$, and $G + C = 40\%.\;$ Therefore, $G = C = 20\%.\;$ (Detailed problems to follow in class.)

Scale of Genetic Information

• Average eukaryotic genome size cited: $1.4 \times 10^8$ base pairs.
• Total possible sequences of that length: $4^{140000000}$ – astronomically large, underscoring DNA’s capacity to encode vast diversity.

Historical Race to Uncover DNA Structure (1940s-1950s)

• Post-WWII scientific focus shifted from the Manhattan Project to the “molecule of inheritance” question—protein vs. nucleic acid.
• Key contributors & timeline highlights:
  • Erwin Chargaff – quantitative base studies (late 1940s).
  • Rosalind Franklin – expert in the new field of X-ray crystallography; worked at King’s College London.
  • Produced famous Photo 51 (high-resolution X-ray diffraction image of DNA).
  • Her data implied a helical structure but did not yet resolve whether bases faced inward/outward or helix number.
  • Maurice Wilkins – colleague at King’s; conflict over lab hierarchy and Franklin’s independence as a female scientist in the 1950s.
  • Without Franklin’s consent, shared Photo 51 with James Watson & Francis Crick at Cambridge.
  • Watson & Crick – built physical wire-and-ball models, integrating:
  • Chargaff’s base-ratio constraints.
  • Franklin’s X-ray dimensions (helical repeat, diameter, spacing).
  • Chemical intuition about hydrogen bonding and steric fit.
  • Deduced a double helix with antiparallel sugar-phosphate backbones and base pairs internal.
  • 1953: Published landmark Nature paper describing the model.
  • 1962: Nobel Prize in Physiology or Medicine awarded to Watson, Crick, and Wilkins.
    • Franklin had died in 1958 (ovarian cancer, likely radiation-linked).
    • Nobel rules forbid posthumous awards → ethical controversy.
    • One laureate reportedly contemplated returning the medal amid public criticism.

Ethical, Social, and Gender Dimensions

• Franklin’s exclusion illustrates systemic gender bias in mid-20th-century science:
  • Women lacked formal recognition, voting rights in various contexts, and often lab leadership.
• Issues raised:
  • Data ownership & scientific credit.
  • Radiation safety (parallels to Marie Curie; early X-ray work).
  • Ongoing discussions of equity and recognition in STEM.

Features of the Watson–Crick Double Helix

• Visualized as a twisted ladder:
  • Sides (rails): alternating sugar–phosphate backbone.
  • Rungs: complementary base pairs hydrogen-bonded inside.
• Base Pairing Specificity (enforces uniform helix width):
  • Purine (large) pairs with pyrimidine (small):
  • A ↔ T (2 H-bonds).
  • G ↔ C (3 H-bonds).
  • Purine–purine or pyrimidine–pyrimidine pairing would distort width; Franklin’s image confirmed constant diameter.
• Antiparallel orientation:
  • Strands run in opposite chemical directions.
  • One strand: 5′ → 3′.
  • Complement: 3′ → 5′.
• Replication directionality:
  • DNA polymerases add nucleotides only to a free 3′-OH → synthesis always proceeds 5′ → 3′ on the new strand.
  • Template is “read” 3′ → 5′.
• Hydrogen bonding + base complementarity satisfy replication fidelity demands set out earlier.

Looking Ahead in Course

• Next lecture: “deep dive” into mechanisms of DNA replication:
  • Origin sites, replication forks, leading vs. lagging strands, enzymes (helicase, primase, DNA pol, ligase).
  • Connection back to antiparallel constraints.
• Upcoming problem sets:
  • Chargaff percentage puzzles.
  • Predicting effects of point mutations on H-bonding and helix stability.
• Pending administrative note: Instructor will check whether recent test grades have been posted.

Quick Memory Aids & Examples Mentioned

• Purines: “PURe As Gold” = Purine, Adenine, Guanine.
• Purine has shorter word, larger two-ring; Pyrimidine has longer word, smaller single-ring.
• Double helix analogy: twisted ladder with rails (backbone) & rungs (base pairs).

Real-World & Philosophical Significance

• Understanding DNA structure underpins modern genetics, biotechnology, forensic science, and medicine (e.g., CRISPR editing).
• Ethical lessons from Franklin’s story echo in contemporary debates on:
  • Data sharing & collaboration norms.
  • Recognition of marginalized groups in scientific discovery.
  • Safe lab practices around radiation and other hazards.