For many years, scientists believed that proteins—with
their complexity and variety—were the likely candidates for carrying genetic
information. The idea that DNA, a seemingly simple molecule, could serve
such a fundamental role in heredity faced skepticism across the scientific
community.
That skepticism slowly began to fade, thanks to the
visionary work of scientists across continents and decades.
Early Insights: Koltsov’s Vision of a
Hereditary Molecule
In 1927, Russian biologist Nikolai Koltsov proposed
a groundbreaking idea: that hereditary traits were passed through a “giant
hereditary molecule” composed of two strands capable of self-replication.
Each strand, he suggested, could serve as a template for creating its
counterpart—a concept strikingly similar to what we now know about DNA’s double
helix.
Tragically, Koltsov never saw his theory validated. He died
in 1940 under Soviet repression. Yet, his vision was finally confirmed a
quarter-century later when James Watson and Francis Crick unveiled the
double-helix structure of DNA in 1953—a discovery that changed biology forever.
Griffith’s Pioneering Experiment in
Bacterial Transformation
While Koltsov was theorizing in Russia, Frederick
Griffith, a British bacteriologist, was making discoveries of his own. In
the 1920s, while studying pneumonia-causing bacteria (pneumococci),
Griffith identified two strains:
- A
smooth (S) strain, which was virulent and caused death in
mice.
- A
rough (R) strain, which was non-virulent and did not cause
illness.
In a key experiment, Griffith injected mice with a mixture
of heat-killed S-strain bacteria and live R-strain bacteria.
Surprisingly, the mice developed pneumonia and died. The dead mice's tissues
contained live S-strain bacteria. Griffith concluded that some
"transforming factor" had turned the harmless R-strain into a deadly
form—though he did not yet know what that factor was.
Avery, MacLeod, and McCarty:
Identifying DNA as the Genetic Material
In the 1930s and early 1940s, Oswald Avery, a
leading expert on pneumococcus at Rockefeller University, set out to
identify Griffith’s mysterious transforming factor. Along with Colin MacLeod
and Maclyn McCarty, Avery recreated and refined Griffith’s experiment.
Rather than using heat to kill the S-strain bacteria, they
applied chemical treatments to selectively destroy key biological molecules—proteins,
lipids, carbohydrates, and RNA. Yet the transformation still occurred.
Only when they added deoxyribonuclease (DNase)—an
enzyme that breaks down DNA—did the transformation stop. This critical finding,
published in 1944, provided the first solid evidence that DNA is the
molecule responsible for carrying genetic information.
Key Points to Remember
- DNA
was not immediately accepted as the hereditary material—proteins
were long thought to be more likely candidates due to their complexity.
- Nikolai
Koltsov envisioned a double-stranded
hereditary molecule decades before DNA’s structure was discovered.
- Frederick
Griffith’s bacterial experiments laid the
foundation for understanding transformation, even before DNA was
identified.
- Avery,
MacLeod, and McCarty’s work pinpointed DNA
as the “transforming factor,” marking a turning point in molecular
biology.
- This
discovery paved the way for Watson and Crick’s breakthrough,
forever changing our understanding of life at the molecular level.
 |
| In the 1940s, the groundbreaking Avery-MacLeod-McCarty experiment delivered decisive proof that DNA, not protein, is the true carrier of genetic information. |
