The Discovery of DNA as the Genetic Material

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DNA as the Genetic Material

Historical Context and the Search for Genetic Material

Early in the 20th century, scientists debated whether DNA or proteins served as the genetic material. Proteins, with their structural diversity, were initially favored, but pivotal experiments with bacteria and viruses shifted consensus toward DNA.

Evidence That DNA Can Transform Bacteria

Griffith's Transformation Experiment

Frederick Griffith's 1928 experiment with Streptococcus pneumoniae demonstrated that a heritable substance could transform nonpathogenic bacteria into pathogenic forms. He worked with two strains: the smooth (S) strain, which is pathogenic due to its protective capsule, and the rough (R) strain, which is nonpathogenic.

Key Point 1: Mixing heat-killed S cells with living R cells resulted in the transformation of R cells into pathogenic S cells.
Key Point 2: The trait of pathogenicity was inherited by subsequent generations, indicating a genetic change.
Definition: Transformation is a change in genotype and phenotype due to assimilation of external DNA by a cell.
Example: Griffith's experiment is a classic demonstration of transformation in bacteria.

Griffith's transformation experiment with S and R cells and mouse outcomes Results of Griffith's transformation experiment showing mouse health outcomes

Evidence That Viral DNA Can Program Cells

Bacteriophage Experiments

Bacteriophages (phages) are viruses that infect bacteria. They consist of DNA (or RNA) enclosed in a protein coat. The Hershey-Chase experiment (1952) used phages to show that DNA, not protein, is the genetic material injected into bacteria to direct viral replication.

Key Point 1: Phages labeled with radioactive phosphorus (DNA marker) transferred radioactivity into bacteria, while those labeled with radioactive sulfur (protein marker) did not.
Key Point 2: Only DNA entered the bacterial cells and directed the production of new phages.
Definition: Bacteriophage is a virus that infects bacteria, used as a model in molecular genetics.
Example: The Hershey-Chase experiment provided strong evidence that DNA is the hereditary material.

Electron micrograph of a bacteriophage attached to a bacterial cell Diagram of the Hershey-Chase experiment with radioactive labeling of phage DNA and protein

Structure of DNA

DNA as a Polymer of Nucleotides

DNA is a polymer composed of nucleotide monomers, each consisting of a nitrogenous base, a deoxyribose sugar, and a phosphate group. The nucleotides are linked by phosphodiester bonds, forming a sugar-phosphate backbone with protruding bases.

Key Point 1: The four nitrogenous bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).
Key Point 2: The backbone has directionality, with a 5' end (phosphate group) and a 3' end (hydroxyl group).
Example: The structure of a DNA strand is essential for its replication and function.

Structure of a DNA strand showing sugar-phosphate backbone and nitrogenous bases Comparison of DNA and RNA nucleotide structure

Chargaff's Rules

Base Composition and Species Variation

Erwin Chargaff discovered that DNA composition varies between species, but the amount of adenine always equals thymine, and guanine equals cytosine. These findings, known as Chargaff's rules, were critical for understanding DNA structure.

Key Point 1: Chargaff's Rule 1: DNA base composition varies between species.
Key Point 2: Chargaff's Rule 2: In any species, the amount of A = T and G = C.
Example: The table below shows base percentages in various organisms, illustrating Chargaff's rules.

Photograph of a sea urchin, one organism studied by Chargaff

Source of DNA	Adenine	Guanine	Cytosine	Thymine
Sea urchin	32.8	17.7	17.3	32.1
Salmon	29.7	20.8	20.4	29.1
Wheat	28.1	21.8	22.7	?
E. coli	24.7	26.0	?	?
Human	30.4	?	?	30.1
Ox	29.0	?	?	?
Average %

Table of DNA base percentages in various organisms

Discovery of the Double Helix

X-ray Crystallography and Model Building

Rosalind Franklin's X-ray diffraction images revealed that DNA is helical and suggested its dimensions. Watson and Crick used these data, along with Chargaff's rules, to build a double helix model with antiparallel sugar-phosphate backbones and specific base pairing.

Key Point 1: The double helix consists of two antiparallel strands twisted around each other.
Key Point 2: The bases pair specifically: A with T (via two hydrogen bonds), and G with C (via three hydrogen bonds).
Example: The X-ray diffraction photo provided the critical evidence for the helical structure.

$Rosalind Franklin and her X-ray diffraction photograph of DNA$ Visual representations of DNA structure, including double helix and base pairing

Base Pairing and the Double Helix

Specificity of Base Pairing

The width of the double helix is consistent only if a purine (A or G) pairs with a pyrimidine (T or C). This specificity is dictated by hydrogen bonding and the molecular structure of the bases.

Key Point 1: Purine-purine pairs are too wide; pyrimidine-pyrimidine pairs are too narrow; purine-pyrimidine pairs fit the helix.
Key Point 2: The base-pairing rules explain Chargaff's findings and ensure accurate DNA replication.

Possible base pairings in the DNA double helix: purine-purine, pyrimidine-pyrimidine, purine-pyrimidine Base pairing in DNA: A-T and G-C pairs with hydrogen bonds

Summary Table: Key Experiments and Discoveries

Experiment/Scientist	Discovery
Griffith (1928)	Transformation in bacteria; evidence for a heritable substance
Avery, MacLeod, McCarty (1944)	Identified DNA as the transforming substance
Hershey & Chase (1952)	DNA, not protein, is the genetic material in phages
Chargaff (1950)	Base composition rules (A=T, G=C)
Franklin & Wilkins (1952)	X-ray diffraction evidence for helical structure
Watson & Crick (1953)	Double helix model of DNA

Conclusion

The discovery of DNA as the genetic material and the elucidation of its double helix structure were foundational to modern biology. These advances explained how genetic information is stored, replicated, and transmitted across generations, setting the stage for molecular genetics and biotechnology.