Frederick Griffith's experiment (1928)
The first undeniable evidence of the genetic role of DNA was obtained by studying two different strains of pneumococci. These strains differed in appearance and ability to cause severe disease:
- S-strain (from English Smooth). Was virulent (pathogenic). Bacteria of this strain had a protective polysaccharide capsule that saved them from the host's immunity.
- R-strain (from English Rough). Was avirulent (non-disease-causing). The cells lacked a capsule and were easily destroyed by the immune system.
Griffith conducted a series of sequential tests on laboratory mice to trace the pathogenicity of each variant.
- Administration of live R-strain did not harm the mice in any way; they remained alive.
- Administration of live S-strain predictably led to the development of infection, and the mice died.
- Administration of heat-killed S-bacteria showed that dead pathogens were safe—the mice survived.
- Crucial stage: mice were injected with a mixture consisting of absolutely safe live R-bacteria and heat-killed S-bacteria.
The result of the last injection was striking—the mixture proved fatal, and the experimental animals died. Upon autopsy, researchers isolated live S-strain bacteria from the tissues of the dead mice.
Griffith's conclusion: Hereditary properties were transferred from the killed virulent cells to the live avirulent ones. Live "rough" type bacteria turned into the "smooth" form, restoring the ability to synthesize a protective capsule. This process of property transfer was named transformation. The acquired trait (presence of a capsule) was stably inherited by all subsequent offspring.
Proof of the nature of the transforming factor (1944)
Several years later, Oswald Avery, Colin MacLeod, and Maclyn McCarty set out to determine the exact chemical nature of the mysterious factor causing bacterial transformation. To do this, they used the method of enzymatic treatment of extracts from killed cells, selectively destroying various classes of molecules.
The researchers evaluated how the treatment affected the process of hereditary trait transfer:
- Upon exposure to proteolytic enzymes (trypsin and chymotrypsin), which specifically cleave proteins, the ability to cause transformation was completely preserved.
- Upon treatment of the culture with deoxyribonuclease—an enzyme that selectively destroys the structure of DNA—the transformation process stopped and lost its activity.
These results led to a crucial conclusion: the transforming agent is high-molecular-weight DNA. The scientists proved that nucleic acids are responsible for the transmission of traits, and proteins are not the carriers of hereditary information in this process, contrary to popular hypotheses at the time.
The role of RNA and the universality of the genetic code
As science progressed, new data accumulated. In further studies using bacteriophages and tobacco mosaic virus (TMV), scientists established that the genetic apparatus could have a different structure. It was proven that in a number of viruses, the carrier of genetic information is RNA (ribonucleic acid).
Based on experiments with pneumococci, phages, and plant viruses, biologists formulated a fundamental conclusion: nucleic acids (DNA and RNA) are the sole and universal carriers of hereditary information in all known living organisms, including bacteria, plants, animals, and acellular life forms.
Briefly: answers to topical questions
In which biological objects is RNA the carrier of hereditary information?
RNA is the carrier of hereditary information in a special group of biological objects—RNA-containing viruses. In these structures, RNA performs the function of storing genetic information. Objects with this type of genome include:
- Tobacco mosaic virus—a virus with a single strand of nucleic acid, which served as an example to prove the genetic role of RNA.
- Influenza virus—has single-stranded RNA acting as the viral genetic apparatus.
- Rabies virus—a virus containing a single strand of RNA.
- Enteroviruses—single-stranded RNA-containing viruses.
- Hepatitis A virus (Hepatovirus)—a simply organized virus with an RNA-containing genome.
- Oncogenic viruses—contain two strands of nucleic acid.
- HIV—a representative of the RNA retrovirus family whose capsid contains two copies of RNA.
To avoid confusing pneumococcus strains: S-strain (Smooth) — "Lethal" (causes death due to the capsule). R-strain (Rough) — "Tame" or safe (no capsule, the mouse lives).
Class and exam questions
What is bacterial transformation?
This is the process of transferring hereditary traits from one set of cells (even deceased ones) to another. During transformation, living cells absorb foreign material factors and acquire new genetic traits that were not previously characteristic of them and are passed on to offspring.
Why did S-strain bacteria cause the death of mice, while R-strain bacteria did not?
S-strain cells possess a protective polysaccharide capsule that blocks the action of the host's immune system, making them pathogenic (virulent). R-strain bacteria lack such a coat, so the immune system easily recognizes and destroys them.
How did Avery's experiments exclude proteins from the list of carriers of heredity?
Scientists treated the bacterial extract with proteolytic enzymes (trypsin and chymotrypsin), which destroy any proteins. Despite the absence of intact protein molecules, the transfer of hereditary properties (transformation) successfully continued, which proved the non-involvement of proteins in this process.
What else to study on this topic
- Structure and functions of the polysaccharide capsule in pneumococci
- Mechanism of free DNA penetration into a living bacterial cell
- Features of the action of the enzyme deoxyribonuclease
- Organization of the hereditary apparatus in tobacco mosaic virus (TMV)
- Differences in the structure of genetic material between eukaryotes and prokaryotes
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