Watson-Crick Model of Nucleic Acids

  • Hershey–Chase experiments and Chargaff’s findings
    • Viruses like bacteriophages have a simple structure: a protein coat (capsid) and a nucleic acid core (DNA or RNA).
    • Bacteriophage infection process:
    • Attach to the bacterial surface and inject its nucleic acid into the host cell.
    • Phage DNA uses host machinery to copy itself; host cell lyses, releasing numerous phages.
    • Hershey–Chase experiment setup:
    • Batch 1 labeled phage with radioactive sulfur, 35S^{35}\text{S}, to label the protein coat.
    • Batch 2 labeled phage with radioactive phosphorus, 32P^{32}\text{P}, to label DNA (phosphorus is in DNA but not in protein).
    • After infection, a blender detached phage coats from cells; the mixture was centrifuged.
    • Heavier bacterial cells formed a pellet; lighter phage particles remained in the supernatant.
    • Observations:
    • In the 35S^{35}\text{S} batch, radioactivity was in the supernatant (phage coats) and not in the pellet (bacteria).
    • In the 32P^{32}\text{P} batch, radioactivity was detected in the pellet (bacteria), not in the supernatant.
    • Conclusion:
    • It was the phage DNA that entered the bacterial cells and carried information to produce more phage particles, providing strong evidence that DNA is the genetic material, not proteins. (Figure 14.4)
    • Chargaff’s rules (Erwin Chargaff):
    • The amounts of adenine (A), thymine (T), guanine (G), and cytosine (C) vary by species but not between individuals of the same species.
    • A → T and G → C equivalence: A=T, G=CA = T, \, G = C
    • This finding greatly aided Watson and Crick’s construction of the DNA double-helix model.
  • DNA structure and sequencing (overview of Section 14.2)
    • By the end of the section you should be able to:
    • Describe the structure of DNA.
    • Explain the Sanger method of DNA sequencing.
    • Discuss similarities and differences between eukaryotic and prokaryotic DNA.
  • DNA building blocks: nucleotides
    • Building blocks of DNA: nucleotides composed of three parts:
    • A nitrogenous base (A, T, G, C) (purines: A, GA,\ G; pyrimidines: C, TC,\ T).
    • A five-carbon sugar: deoxyribose in DNA (ribose in RNA).
    • A phosphate group.
    • Nucleotide naming depends on the nitrogenous base.
    • Sugar details:
    • Deoxyribose (DNA) vs ribose (RNA).
    • Bonding and backbone:
    • Nucleotides join via phosphodiester bonds.
    • The phosphate residue attaches to the 5′-OH of one sugar and the 3′-OH of the next sugar, forming a 5′-3′ phosphodiester linkage: 5′-phosphate−OH to 3′-OH of next sugar5'\text{-}\text{phosphate}-\text{OH} \text{ to } 3'\text{-OH} \text{ of next sugar}
    • Purines vs pyrimidines (base structures):
    • Purines: A, G\text{A},\ \text{G} (double-ring)
      • double-ring structure: six-membered ring fused to a five-membered ring
    • Pyrimidines: C,T\text{C},\text{T} (single-ring)
    • Naming by base and sugar:
    • Nucleotide base + deoxyribose + phosphate backbone.
    • Sugar carbon numbering:
    • Carbons on the five-carbon sugar are numbered 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”).
    • Phosphodiester bond specifics:
    • Phosphate group links the 5′-phosphate of one nucleotide to the 3′-OH of the adjacent nucleotide, creating a directional chain with a 5′→3′ polarity.
    • Historical context on DNA structure development (1950s):
    • Francis Crick and James Watson developed DNA structure with input from others:
      • Linus Pauling and Maurice Wilkins were active in the field.
      • Pauling had discovered the secondary structure of proteins via X-ray crystallography.
      • Rosalind Franklin used X-ray diffraction to study DNA; her data contributed to determining the double-helix model.
      • Watson, Crick, and Maurice Wilkins received the Nobel Prize in Medicine in 1962; Franklin had died, and Nobel prizes are not awarded posthumously.
    • DNA’s double-helix organization: two strands twisted into a right-handed helix.
    • Base pairing rules and anti-parallel strands:
    • Purine–pyrimidine pairing ensures uniform diameter: A!:!T, G!:!CA!:!T,\ G!:!C
  • Base pairing, structure, and dimensions of DNA
    • Complementary base pairing:
    • Adenine pairs with thymine (A–T) and cytosine pairs with guanine (C–G).
    • Hydrogen bonding stabilizes base pairs: A–T:2 H-bonds; C–G:3 H-bonds.\text{A} \text{–} \text{T}: 2\ \text{H-bonds};\ \text{C} \text{–} \text{G}: 3\ \text{H-bonds}.
    • Strands are anti-parallel: the 3′ end of one strand runs opposite the 5′ end of the other strand.
    • Backbone vs bases:
    • The sugar–phosphate backbone forms the outside of the molecule; bases stack inside the helix.
    • Geometric parameters:
    • Distance between base pairs: 0.34 nm0.34\ \text{nm} per base pair.
    • Rise per helical turn: 3.4 nm3.4\ \text{nm}; with that, there are 10 bp per turn.10\ \text{bp per turn}.
    • Diameter of the double helix: 2 nm.2\ \text{nm}.
    • Major and minor grooves:
    • Twisting of the two strands creates major and minor grooves which serve as binding sites for DNA-binding proteins during transcription and replication.
  • DNA sequencing technologies and Sanger method
    • Historical context: sequencing was expensive and slow until automated methods emerged in the 1990s.
    • Fred Sanger developed the chain-termination sequencing technique (Sanger sequencing) used for the Human Genome Project.
    • Core idea: use chain terminators (dideoxynucleotides, ddNTPs) that lack a 3′-OH group, preventing further elongation when incorporated.
    • ddNTP principle:
    • ddNTPs differ from normal deoxynucleotides by lacking the 3′-OH group, so once a ddNTP is incorporated, no further nucleotides can be added.
    • Reaction setup:
    • DNA is denatured to separate strands.
    • Four separate tubes contain primer, DNA polymerase, and the four normal nucleotides (A, T, G, C).
    • Each tube also contains a limited amount of one of the ddNTPs (labeled A, T, G, or C) to terminate synthesis at varying lengths.
    • Fluorescently labeled ddNTPs allow detection of terminated fragments in a single reaction.
    • Readout and automation:
    • Fragments are separated by capillary electrophoresis based on size.
    • A laser scanner detects the fluorescent labels to read the sequence (electropherogram).
    • Significance:
    • Sanger sequencing earned a Nobel Prize in Chemistry in 1980 for this work.
    • The approach accelerated genome sequencing, leading to the idea of a ~10001000 genome in a short time (the “$1000 in one day$” slogan).
  • DNA sequencing readout and gel electrophoresis fundamentals
    • Capillary electrophoresis workflow (as in Figure 14.8):
    • Denature DNA; divide into four tubes; add primer, polymerase, four normal nucleotides; add a ddNTP to each tube with a distinct fluorescent label.
    • Chain elongation proceeds until a ddNTP is incorporated, terminating the strand.
    • Fragments are separated by size via capillary electrophoresis.
    • A laser scanner reads the fluorescence to determine the sequence.
    • Gel electrophoresis basics (Figure 14.9):
    • Agarose gel acts as a molecular sieve to separate DNA fragments by size.
    • DNA is negatively charged and migrates toward the positive electrode.
    • Smaller fragments migrate farther from the loading well than larger fragments.
    • Gels are stained with a DNA-specific dye for visualization.
  • DNA packaging in cells: differences between prokaryotes and eukaryotes
    • Prokaryotic genome organization:
    • Typically a single, circular chromosome located in the nucleoid (a region in the cytoplasm).
    • DNA and RNA synthesis occur simultaneously in the cytoplasm (no nucleus).
    • Example genome size: about 4.6×1064.6\times 10^{6} base pairs (~1.1 mm if stretched out).
    • Supercoiling theory: DNA is twisted to fit inside the compact cell.
    • Proteins involved in supercoiling include DNA gyrase which maintains supercoiling state (under-wound or over-wound relative to relaxed form).
    • Eukaryotic genome organization:
    • Chromosomes are linear DNA molecules packaged with histone proteins to form chromatin.
    • The basic unit of packaging is the nucleosome: DNA wrapped around an octamer of histone proteins; linker DNA connects nucleosomes, creating a “beads on a string” structure.
    • Nucleosome packaging compacts DNA into a 30 nm fiber for higher-order organization.
    • During metaphase, chromosomes are highly condensed (approximately 700 nm in width) and associated with scaffold proteins.
    • Chromatin states:
      • Heterochromatin: densely packed, gene-poor or gene-silent regions, often located near centromeres and telomeres.
      • Euchromatin: less densely packed, gene-rich, transcriptionally active regions.
  • Neanderthal genome project and implications
    • The Neanderthal genome draft was published in 2010 by Richard E. Green et al.
    • Neanderthals: close ancestors of modern humans, lived in Europe and Western Asia; disappeared ~3×1043\times 10^{4} years ago.
    • Key findings from the Neanderthal genome study:
    • Sequenced roughly four billion base pairs from almost 40,000-year-old fossils using advanced ancient DNA techniques.
    • Neanderthal DNA shows 2–3% greater similarity to non-African modern humans than to Africans, suggesting admixture between Neanderthals and modern humans outside Africa.
    • Some DNA segments in Europeans and Asians are more similar to Neanderthal sequences than to other modern human sequences, indicating introgression.
    • Neanderthals are surprisingly closely related to Papuans as to Chinese or French populations, implying widespread gene flow before the divergence of these groups.
    • Genes in modern humans bearing Neanderthal-derived variants are linked to cranial structure, metabolism, skin morphology, and cognitive development.
    • RUNX2 gene shows differences between modern humans and Neanderthals; RUNX2 is associated with frontal bone development, rib cage shape, and dental features, suggesting a possible role in the evolution of the modern human cranium and upper body.
    • Public engagement:
    • Svante Pääbo’s TED talk discusses Neanderthal genome research (linked in the text).
  • DNA packaging and organization: key questions and concepts
    • Comparison of prokaryotic and eukaryotic DNA packaging highlights:
    • Prokaryotes: simple, compact genome organization without a nucleus; transcription and translation can be coupled.
    • Eukaryotes: more elaborate packaging with nucleosomes and higher-order chromatin structures; separate compartments for DNA/RNA synthesis and protein synthesis.
    • Why separation might be advantageous or disadvantageous: regulatory complexity vs. efficiency of gene expression.
  • Basics of DNA replication and models (introduction in Section 14.3)
    • The double-helix structure suggests a replication mechanism where two strands separate and each serves as a template for a new complementary strand.
    • Historical debate about replication models included three major hypotheses:
    • Conservative model: two old strands remain intact as a separate duplex, and two new strands form a new duplex.
    • Semi-conservative model: each daughter DNA molecule consists of one old strand and one new strand.
    • Dispersive model: daughter DNA molecules contain interspersed segments of old and new DNA.
    • Meselson–Stahl experiments (broad context referenced): designed to distinguish among these models, ultimately supporting the semi-conservative mechanism (details would follow in the next sections).
  • Miscellaneous notes and context
    • The text references multiple figures (e.g., Figures 14.4, 14.6, 14.7, 14.8, 14.9, 14.10, 14.11) that illustrate the concepts described above.
    • Terminology to remember:
    • Nucleotide components: base (A, T, G, C), deoxyribose, phosphate.
    • 5′-3′ phosphodiester bond connects nucleotides.
    • Antiparallel strands, major/minor grooves, and the uniform DNA diameter via proper base pairing.
    • Sanger sequencing vs. modern high-throughput methods: Sanger remains a foundational concept for understanding DNA sequencing.
  • Connections to broader themes
    • Foundational evidence for DNA as the genetic material (Hershey–Chase) and the chemical basis of replication (Chargaff’s rules and base-pairing geometry) underpin modern genetics.
    • The evolution of sequencing technologies from Sanger to high-throughput methods transformed genomics and personalized medicine.
    • Insights from ancient DNA (e.g., Neanderthal genome) illuminate human evolution and gene flow between populations, influencing anthropology and evolutionary biology.
  • Formulas and specific numeric details to memorize
    • Base-pair distance and geometry:
    • Base pair separation: dbp=0.34 nmd_{bp} = 0.34\ \text{nm}
    • Rise per turn: dturn=3.4 nmd_{turn} = 3.4\ \text{nm}
    • Base pairs per turn: nbp=10n_{bp} = 10
    • DNA diameter: D=2 nmD = 2\ \text{nm}
    • Hydrogen bonding in base pairs: A-T:2 H-bonds, G-C:3 H-bonds\text{A} \text{-} \text{T}: 2\text{ H-bonds},\ \text{G} \text{-} \text{C}: 3\text{ H-bonds}
    • Chargaff’s rule notation: A=T,G=CA = T, \quad G = C
    • Genome size reference (example for E. coli): ∣G∣≈4.6×106 bp|G| \approx 4.6\times 10^{6}\ \text{bp}
    • Neanderthal genome length reference: ∼4×109 bp\sim 4\times 10^{9}\ \text{bp} in the analyzed samples; divergence time estimates around tens of thousands of years ago (e.g., 30,000 years ago for disappearance events).
  • Quick recall prompts
    • What evidence supported DNA as genetic material in the Hershey–Chase experiment?
    • What are Chargaff’s rules and why are they important for DNA structure?
    • What are the key structural features that ensure DNA can be replicated and read as a code (base pairing rules, antiparallel strands, major/minor grooves)?
    • How does the Sanger method terminate DNA synthesis, and how is the sequence read from the data?