Genetics Part 1: DNA Structure and Replication

Discovery of DNA as the Genetic Material

• Chromosomal Foundation of Inheritance:

  • Early in the 20th century, identifying the molecular carrier of genetic information was a paramount biological challenge.
  • Research led by T. H. Morgan established that genes are physically located on chromosomes.
  • Because eukaryotic chromosomes consist of both DNA and protein, these two molecular components became the primary candidates for genetic material.
  • Until the 1940s, the scientific consensus leaned strongly toward protein, owing to its greater structural complexity and variety of amino acid subunits relative to nucleic acids.
  • The role of DNA in heredity was elucidated by studying bacteria and the viruses that infect them.

• Bacteriophage Structure and Viral Infection:

  • Viruses that infect bacteria are designated as bacteriophages (or phages) and serve as foundational model systems in molecular genetics.
  • A virus consists of a genome composed of nucleic acid (either DNA or RNA) enclosed within a protective protein envelope or capsid.
  • Viruses are obligate intracellular parasites; they lack independent metabolic machinery and must infect host cells to hijack cellular translation and replication systems to reproduce.
  • The T2 phage possesses a head structure housing DNA, a flexible tail sheath, and tail fibers that anchor to host cellular walls during infection.

• The Hershey-Chase Experiment (1952):
  • Alfred Hershey and Martha Chase conducted a landmark experiment to determine whether protein or DNA directed the viral reprogramming of host cells using the T2 bacteriophage and Escherichia coli (E. coli).
  • They utilized distinct radioactive isotopes to specifically label protein versus nucleic acid components:
  • Batch 1 (Protein Labeling): Phages were grown in a medium containing radioactive sulfur ($^{\text{35}}\text{S}$). Because sulfur is present in amino acids (such as methionine and cysteine) but absent in DNA, the radioactive label selectively incorporated into the viral protein coat.
  • Batch 2 (DNA Labeling): Phages were grown in a medium containing radioactive phosphorus ($^{\text{32}}\text{P}$). Because phosphorus is abundant in the sugar-phosphate backbone of nucleic acids but absent in proteins, the radioactive label selectively incorporated into viral DNA.
  • Experimental Protocol:
  1. Infection: Radioactive phages infected non-radioactive E. coli host cells.
  2. Agitation: Cultures were agitated in a blender to shear off the empty viral protein coats (ghosts) remaining on the outer surface of bacterial cell walls without lysing the host cells.
  3. Centrifugation: The mixture was centrifuged to separate the heavier bacterial cells from the liquid supernatant. The bacterial cells formed a solid pellet at the bottom of the tube, while extracellular liquid retained dislodged phage parts.
  4. Measurement: Radioactivity levels were independently measured in both the pellet (bacterial cells and contents) and the liquid supernatant.
  • Results and Conclusions:
  • In Batch 1 ($^{\text{35}}\text{S}$ protein label), radioactivity was found almost exclusively in the liquid supernatant containing empty protein shells.
  • In Batch 2 ($^{\text{32}}\text{P}$ DNA label), radioactivity accumulated overwhelmingly within the bacterial pellet.
  • Hershey and Chase concluded that viral DNA—not protein—enters host cells during infection and carries the hereditary instructions necessary to program host cells to produce new phages.

Chargaff's Rules and Base Composition

• Discovery of Species-Specific Base Variations:

  • In 1950, Erwin Chargaff published analyses demonstrating that the nucleotide base composition of DNA varies significantly from one biological species to another.
  • This evidence of structural and molecular diversity elevated DNA's credibility as the candidate for genetic material, replacing the prior view that DNA was a repetitive, low-complexity polymer.

• Formulation of Chargaff's Rules:

  • Chargaff established two primary empirical rules governing DNA composition:
  1. The base composition of DNA varies between species.
  2. Within the DNA of any given species, the percentages of adenine ($A$) and thymine ($T$) bases are equal, and the percentages of guanine ($G$) and cytosine ($C$) bases are equal.
  • Mathematically expressed as:     $\%A = \%T$$\%G = \%C$
  • The structural mechanism underlying these precise mathematical equalities remained unexplained until the discovery of the double helix.

Structural Architecture of the DNA Double Helix

• Contributions of X-Ray Crystallography:
  • During the early 1950s, Maurice Wilkins and Rosalind Franklin utilized X-ray crystallography to analyze the molecular structure of DNA.
  • Rosalind Franklin produced a high-resolution X-ray diffraction photograph of DNA.

• Analysis of Franklin's X-ray crystallographic images enabled James Watson to deduce key physical features of the molecule:

  • DNA forms a helical structure.

  • The physical dimensions derived from diffraction patterns indicated a double-stranded double helix.

  • The diffraction spots revealed the overall width of the helix and the precise spacing of the nitrogenous bases.

  • Chemical Structure of the Polynucleotide Strand:

• A single DNA strand is a polymer constructed from repeating nucleotide units.

• Each nucleotide consists of three structural sub-units:

  • A nitrogenous base (Adenine, Thymine, Guanine, or Cytosine).
  • A five-carbon pentose sugar called deoxyribose.
  • A phosphate group bound to the $5'$ carbon of deoxyribose.

• Nucleotides link together via covalent phosphodiester linkages connecting the phosphate group of one nucleotide to the $3'$ carbon hydroxyl group of the adjacent sugar, forming a repeating sugar-phosphate backbone.

• The single strand exhibits distinct chemical directionality: a free phosphate group attached to a sugar carbon at the $5'$ end, and a free hydroxyl group ($\text{OH}$) attached to a sugar carbon at the $3'$ end.

• Molecular Modeling and Antiparallel Orientation:

  • Watson and Crick constructed structural models conforming to X-ray measurements and chemical principles.
  • Franklin had determined that the hydrophobic nitrogenous bases are oriented inward toward the central interior, while two sugar-phosphate backbones lie on the exterior.
  • Watson built a model in which the backbones were antiparallel, meaning their subunits run in opposite chemical directions relative to one another (one strand runs $5' \rightarrow 3'$ while the other runs $3' \rightarrow 5'$).

• Base Pairing Geometry and Dimensional Uniformity:

  • Initial physical models evaluated base pairing between identical bases (purine with purine, pyrimidine with pyrimidine):
  • Purine + Purine Pairing: Purines (Adenine and Guanine) possess double-ring aromatic structures; pairing two purines produces an overall width that is too wide.
  • Pyrimidine + Pyrimidine Pairing: Pyrimidines (Thymine and Cytosine) possess single-ring aromatic structures; pairing two pyrimidines produces an overall width that is too narrow.
  • Purine + Pyrimidine Pairing: Pairing a double-ring purine with a single-ring pyrimidine creates a uniform width consistent with the $2\,\text{nm}$ diameter obtained from X-ray diffraction data.

• Specific Base Pairing and Hydrogen Bonding:
  • Watson and Crick determined that chemical structures dictate specific pairing partners between purines and pyrimidines:
  • Adenine ($A$) pairs strictly with Thymine ($T$), held together by hydrogen bonds.
  • Guanine ($G$) pairs strictly with Cytosine ($C$), held together by hydrogen bonds.
  • The Watson-Crick double-helix model directly explains Chargaff's rules: because $A$ always pairs with $T$ and $G$ always pairs with $C$, any organism's DNA contains equal amounts of $A$ and $T$ as well as equal amounts of $G$ and $C$

• Quantitative Dimensions of the Double Helix:
  • Helix Diameter: $2\,\text{nm}$.
  • Spacing between consecutive nitrogenous bases: $0.34\,\text{nm}$.
  • Length of one full turn of the helix: $3.4\,\text{nm}$.
  • Number of base pairs per full helical turn: 10 base pairs ($10\,\text{bp}$).

Mechanisms of DNA Replication

• The Semi-Conservative Template Model:
  • Specific base pairing immediately suggests a copying mechanism for genetic material.
  • Because the two strands of DNA are complementary, each individual strand stores the information necessary to reconstruct the other strand.
  • During DNA replication:
  1. The double-stranded parent molecule unwinds and its complementary strands separate.
  2. Each separated parent strand functions as a template for guiding the order of incoming nucleotides.
  3. Complementary nucleotides align along template strands and connect to construct two new complementary daughter strands.

DNA Proofreading, Repair, and Evolutionary Consequences

• Fidelity and Proofreading Mechanisms:

  • DNA replication functions with high speed and accuracy, resulting in an error rate in completed DNA molecules of only one in $10^{10}$ ($1\text{ in }10\text{ billion}$) nucleotides.
  • DNA polymerases proofread newly synthesized DNA during replication, excising and replacing mispaired nucleotides immediately upon occurrence.
  • Mismatch repair systems employ specialized enzymes to identify, remove, and replace uncorrected mispaired nucleotides that escape polymerase proofreading.

• Nucleotide Excision Repair:

  • Genetic material is continually subjected to physical and chemical damage, such as ultraviolet light from sunlight.
  • Cells utilize nucleotide excision repair to correct structural damage:
  1. Excision: A nuclease enzyme detects the structural distortion, cuts the damaged DNA strand at two points flanking the lesion, and removes the damaged segment.
  2. Resynthesis: DNA polymerase synthesizes replacement nucleotides to fill the gap using the undamaged strand as a template.
  3. Ligation: DNA ligase covalently seals the free end of the new strand to the existing sugar-phosphate backbone.

• Evolutionary Significance of Altered Nucleotides:
  • The error rate after proofreading and repair processes is extremely low, but non-zero.
  • Uncorrected changes in nucleotide sequences can become permanent mutations.
  • If these permanent sequence changes occur in germ cells, they are passed on to subsequent generations.
  • Mutations represent the ultimate source of genetic variation upon which natural selection operates.

Telomeres and Chromosome Maintenance

• The End-Replication Problem in Linear Chromosomes:

  • Standard replication enzymes cannot complete synthesis at the extreme $5'$ ends of daughter strands in linear chromosomal DNA.
  • Repeated rounds of replication result in progressively shorter DNA molecules with uneven, single-stranded ends.

• Structure and Function of Telomeres:

  • Eukaryotic chromosomal DNA molecules possess non-coding nucleotide sequences at their ends called telomeres.
  • Telomeres do not contain functional genes; instead, they consist of multiple repetitions of a short nucleotide sequence (in humans, the sequence motif is $\text{TTAGGG}$).
  • Telomeres do not prevent the shortening of DNA molecules, but they postpone the erosion of vital genes located near the ends of chromosomes.
  • The progressive shortening of telomeres is connected to cellular aging processes.

• Telomerase Function and Cancer Implications:
  • If chromosomal telomeres shortened during every cell cycle in germ cells, essential genes would eventually be missing from the gametes they produce.
  • An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells, preserving full chromosome length across generations.
  • Telomerase is inactive in most human somatic cells.
  • Inappropriate reactivation of telomerase occurs in many cancer cells, allowing them to maintain telomere length and divide indefinitely.
  • Telomerase is actively studied as a key target for novel cancer therapies.