Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 16.1 DNA Is the Genetic Material
Today, DNA is universally recognized as the molecule of heredity, but this was not always the case. Early 20th-century scientists debated whether DNA or protein carried genetic information. A series of key experiments helped establish DNA as the genetic material.
The Search for Genetic Material
- After it was shown that genes are part of chromosomes, DNA and protein were considered as possible carriers of genetic information
- Proteins were favored early on due to their complexity and variety
- DNA was thought too simple and uniform to account for inherited diversity
- This view changed with studies in bacteria and viruses—organisms simpler than fruit flies or humans
Griffith’s Transformation Experiment (1928)
- Frederick Griffith studied Streptococcus pneumoniae in mice
- Two strains:
- S strain (smooth): pathogenic, with protective capsule
- R strain (rough): nonpathogenic, lacks capsule
- Heat-killed S strain mixed with live R strain transformed R into pathogenic S
- The trait was inherited by subsequent generations of bacteria
- Conclusion: a heritable substance—later shown to be DNA—was transferred
- This phenomenon was called transformation
Avery, McCarty, and MacLeod (1944)
- Identified DNA as the "transforming principle" responsible for Griffith’s results
- However, skepticism remained because many believed only proteins could carry complex information
Hershey-Chase Experiment (1952)
- Used T2 bacteriophages (viruses that infect E. coli)
- Labeled DNA with radioactive phosphorus (³²P) and protein with radioactive sulfur (³⁵S)
- Only phage DNA entered bacterial cells—not the protein
- Cells with radioactive DNA produced new viruses containing radioactive phosphorus
- Conclusion: DNA, not protein, is the genetic material
Chargaff’s Rules (1950)
- DNA composition varies by species, supporting its role as genetic material
- Discovered regularities in base pairing:
- A = T
- G = C
- These findings suggested DNA had a specific structure related to its function
Watson, Crick, and the Double Helix (1953)
- Built the first accurate model of DNA’s structure as a double helix
- Based on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins
- Key features:
- Sugar-phosphate backbones on the outside
- Nitrogenous bases paired in the center: A with T, G with C
- Strands run antiparallel
- Held together by hydrogen bonds between base pairs
- Explained how DNA could replicate and store genetic information
- Confirmed and explained Chargaff’s rules
In a Nutshell
Experiments by Griffith, Avery, Hershey and Chase, and Chargaff laid the foundation for DNA as the molecule of inheritance. Watson and Crick's model revealed DNA’s structure—a double helix of complementary base pairs—which explained both heredity and variation, transforming biology forever.