History of Discovery: From Mice to Molecules
For a long time, the biochemical nature of the genetic code was unknown. F. Griffith's 1928 experiment shed light on this mystery.
The scientist worked with pneumococci (Streptococcus pneumoniae) and used two strains:
- R-strain (Rough): unencapsulated cells, non-pathogenic (avirulent).
- S-strain (Smooth): encapsulated cells, causing fatal infection (virulent).
Griffith injected mice with a mixture of harmless live R-cells and heat-killed S-cells. Unexpectedly, the laboratory animals died. Furthermore, live virulent encapsulated pneumococci were isolated from their blood.
To prove that genetic transfer occurred rather than spontaneous reversion, the scientist used R-cells derived from type S II and heat-killed type S III microorganisms. The bacteria isolated from the dead mice possessed type S III capsules, confirming transformation.
Later, in 1944, O. Avery, C. MacLeod, and M. McCarty identified the nature of this phenomenon. They proved that pure DNA extracted from the S-strain converts unencapsulated forms into encapsulated ones. This provided the first fundamental proof that DNA acts as the carrier of genetic information.
Conditions for Successful Transformation
In nature, bacteria (especially Gram-positive species) can spontaneously take up genetic material from lysed relatives. However, successful transformation requires two key factors:
- Recipient Cell Competence: The physiological ability of a microorganism to uptake exogenous DNA from the environment. This state relies on specific membrane-bound proteins that exhibit a strong affinity for nucleic acids. In Gram-positive bacteria, competence is transient and occurs strictly during specific phases of population growth.
- Donor DNA Properties: The extracellular DNA molecule must be double-stranded, highly coiled, and possess a high molecular weight.
Penetration and Recombination Mechanism
The uptake and integration of foreign genomes represent a complex physiological process.
- Penetration: Although donor DNA is initially double-stranded, only a single strand enters the recipient's cytoplasm. The second strand is retained at the cell membrane and degraded (hydrolyzed). The energy released from the cleavage of this second strand powers the transport of the first strand into the cell.
- Protection: Transforming DNA must have a high molecular weight. A long strand is better able to resist intracellular enzymes—endonucleases—which target and degrade foreign genetic material.
- Integration: Successful integration requires homologous regions (structural similarity) between the recipient chromosome and the donor fragment. Recombination affects only a single strand, creating a heteroduplex molecule within the chromosome. In this structure, one strand is original (belonging to the recipient) and the other is recombinant (integrated from the donor).
Trait Fixation and Practical Significance
Immediately following integration, the cell does not yet become a fully transformed stable clone. Fixation of the new genetic trait occurs only after the bacterial replication (cell division) cycle is complete. At this point, the heteroduplex resolves into two proper, double-stranded DNA molecules, each segregated into a daughter cell.
Understanding this mechanism revolutionized biology. Today, transformation is a core technique in genetic engineering, enabling the artificial creation of microbial strains with predetermined, beneficial traits.