History of Discovery: From Mice to Molecules
For a long time, science did not know which specific molecule stored the genetic code. F. Griffith's 1928 experiment shed light on this mystery.
The scientist worked with pneumococci (Streptococcus pneumoniae) and used two strains:
- R-strain (Rough): non-capsulated cells that do not cause disease (avirulent).
- S-strain (Smooth): capsulated cells that cause death in animals (virulent).
Griffith injected mice with a mixture of completely safe live R-cells and heat-killed S-cells. Contrary to expectations, the laboratory animals died. Moreover, live capsulated pneumococci were isolated from their blood.
To prove that genetic transfer had occurred rather than a spontaneous reversion of bacteria, the scientist used R-cells derived from type S II and heat-killed microorganisms of type S III. The bacteria isolated from the dead mice possessed the type S III capsule. This undeniably confirmed the fact of transformation.
Later, in 1944, researchers O. Avery, C. MacLeod, and M. McCarty established the nature of this phenomenon. They proved that pure DNA extracted from the S-strain causes the transformation of non-capsulated forms into capsulated ones. This became the first fundamental proof that DNA acts as the carrier of genetic information.
Conditions for Successful Transformation
In nature, bacteria (especially Gram-positive ones) are capable of spontaneously taking up the genetic material of dead relatives. However, for transformation to be successful, two key factors must coincide:
- Competence of the recipient cell. This is the microorganism's ability to uptake foreign DNA from the external environment. The molecular basis of this state is the presence of specific proteins in the cell membrane. These proteins possess a specific affinity for nucleic acids. In Gram-positive bacteria, competence is not constant—it arises strictly during specific phases of the population growth curve.
- Properties of the donor DNA. The molecule in the external environment must be double-stranded and highly supercoiled. In addition, the high molecular weight of the fragment is critically important.
Mechanism of Penetration and Recombination
The process of capturing and integrating foreign genomes is a complex physiological mechanism.
- Penetration: Despite the fact that donor DNA initially consists of two strands, only a single strand enters the recipient cytoplasm. The second strand is retained at the cell membrane and undergoes degradation (hydrolysis). Specifically, the energy released during the cleavage of the second strand drives the transport of the first strand into the cell.
- Protection: Transforming DNA must have a high molecular weight. A long strand is much better equipped to resist intracellular enzymes—endonucleases—which attempt to degrade foreign genetic material.
- Integration: Successful integration requires the presence of homologous (structurally similar) regions between the recipient chromosome and the donor fragment. Recombination affects only a single strand, resulting in the formation of a heteroduplex molecule within the chromosome. In this structure, one strand is original (belonging to the recipient), and the second is recombinant (integrated from the donor).
Trait Fixation and Practical Significance
Immediately after integration, the cell does not yet become a fully-fledged transformant. Fixation of the new genetic trait occurs only after the completion of the bacterium's replication (division) cycle. It is at this moment that the heteroduplex is resolved into two complete, proper double-stranded DNA molecules, each of which is inherited by a daughter cell.
Understanding this mechanism revolutionized biology. Today, transformation is a core methodology of genetic engineering, enabling the artificial creation of microbial strains with predefined, useful properties.
Briefly: answers to topical questions
Which enzymes degrade the second DNA strand at the cell membrane?
The exact names of the enzymes that degrade the second DNA strand at the cell membrane are not specified. Only the mere fact of degradation and hydrolysis of the second strand on the cell surface to provide energy for the transport of the first strand is described. However, the text mentions enzymes that cleave DNA inside the cell:
- Endonucleases are intracellular enzymes that cleave DNA, which the transforming strand successfully resists due to its high molecular weight.
To easily remember the essence of a heteroduplex, imagine a jacket zipper where one half comes from an old jacket (your own) and the second from a new one (foreign). To produce two perfect zippers, the cell must "copy" them during division.
Class and exam questions
Do both strands of donor DNA enter the recipient cell?
No. Only one strand enters the cytoplasm, while the second is degraded at the membrane, providing the energy for the entry of the first.
Why is high-molecular-weight DNA important for transformation?
The longer the DNA strand, the higher the chances that it will not be completely degraded by intracellular defense enzymes (endonucleases) prior to integration.
What is bacterial competence?
This is a physiological state of the cell in which special proteins appear in its membrane, capable of binding and taking up DNA from the external environment.
What else to study on this topic
- Genetic recombination in DNA and RNA viruses
- Mechanism of genetic reactivation upon encounters between defective viruses
- Processes without gene exchange: protein complementation
- Phenotypic mixing in microorganisms
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