Features of bacterial recombination
The interaction of two genomes with different genotypes leads to the formation of daughter recombinant DNA. The main feature of this process in bacteria is that not the entire donor chromosome penetrates the recipient cell, but only a small DNA fragment.
Because of this, a merozygote is formed inside the recipient — an incomplete zygote representing a partially diploid cell. The result of such a cross is the formation of only one recombinant. Its genotype is based on the original recipient genotype, which incorporates the transferred donor fragment. There is no mutual exchange of segments.
Three pathways of horizontal gene transfer
Genetic recombination is only the final stage. For the donor DNA to meet the recipient DNA, the material must be transferred via one of three fundamental ways:
- Conjugation. Occurs through direct physical contact between two cells, between which a special "bridge" is formed. The process requires a conjugative plasmid. The directed transfer of genetic material proceeds from the donor (F+) to the recipient (F-), always starting from the 5’ end.
- Transformation. Represents the uptake of high-polymer free DNA. The source is lysed donor cells that release fragments of their genome into the external environment. These fragments then independently penetrate from the environment into a competent recipient cell.
- Transduction. A process in which a bacteriophage acts as an intermediary. During assembly inside a destroying (lysed) donor cell, the phage accidentally captures a piece of bacterial DNA. Emerging outside, it infects a new cell and introduces the previous host's genome fragment into it.
Molecular mechanisms of recombination
Depending on molecular features, three main mechanisms of integrating foreign DNA into the recipient's genome are distinguished.
- Homologous recombination. Requires a high degree of homology (similarity) between the exchanging segments. The mechanism involves the breakage and reunion of strands with the formation of a cross-shaped Holliday junction. Within it, complementary pairing of single-stranded segments from different parental molecules occurs. The process is strictly controlled by the REC system (recA, B, C, D genes). The products of these genes unwind DNA strands and reorient (cleave) the Holliday junction to complete the exchange.
- Site-specific recombination. Proceeds only in strictly defined regions of the genome and does not require a high degree of homology. It does not depend on the function of the recA, B, C, D genes. Examples include the integration of a plasmid into the bacterial chromosome (usually between identical IS elements), the integration of phage DNA into the E. coli chromosome, and mechanisms of gene activity regulation, such as phase variations of the flagellar H-antigen in salmonellae.
- Illegitimate (replicative) recombination. A non-homologous process that also does not depend on the REC system. Its essence is transposition (movement) of mobile genetic elements within a replicon or between different replicons. The most important feature is that such movement is always accompanied by the duplication (replication) of the transferred DNA.
Briefly: answers to topical questions
What types of transduction are distinguished in microbiology?
Microbiology distinguishes three main types of transduction.
- Generalized transduction — random incorporation of a bacterial DNA fragment into the phage genome, where the trait is fixed upon integration into a homologous region of the recipient.
- Specialized transduction — phage DNA joins strictly defined donor genes adjacent to the integration site (for example, in bacteriophage λ Escherichia coli).
- Abortive transduction — the transferred fragment does not integrate into the genome, persists autonomously in the cytoplasm, and is transmitted to only one of the two daughter cells.
What is the state of competence of a bacterial cell during transformation?
The state of competence is the ability of a bacterial cell to take up free DNA. It is induced by a special low-molecular-weight protein — the competence factor. This factor promotes the synthesis of autolysin, endonuclease, and DNA-binding protein. The maximum number of competent cells in a population is observed at the end of the logarithmic growth phase.
Which types of plasmids are capable of being transferred during conjugation?
Transmissible (conjugative) and non-transmissible (mobilizable) plasmids are capable of being transferred during conjugation.
- Transmissible plasmids — contain the tra operon, which encodes sex pili for forming a bridge with the recipient, and are transmitted independently. These include F-plasmids (fertility plasmids) and R-plasmids (resistance plasmids).
- Non-transmissible plasmids — lack tra genes and cannot be transmitted independently. They are transferred only in the presence of a transmissible plasmid in the cell that provides the conjugation apparatus.
Transfer is possible both when the plasmid is in an autonomous state and when it is integrated into the bacterial chromosome.
For quick memorization of the three transfer mechanisms, use the rule "TCT": Transformation (Tracts free DNA from the medium), Conjugation (Contact of cells directly), Transduction (Transport with the help of a phage).
Class and exam questions
What is a merozygote in microbiology?
This is an incomplete, partially diploid cell formed after only a fragment of donor DNA, rather than an entire chromosome, penetrates the recipient bacterium.
Which genes control homologous recombination?
Homologous recombination is controlled by the gene products of the REC system (recA, B, C, D), which are responsible for unwinding DNA strands and cleaving the Holliday junction.
What is the main difference between site-specific and homologous recombination?
Site-specific recombination occurs only in certain regions of the genome, does not require high homology of DNA strands, and does not depend on the work of the REC system.
What else to study on this topic
- Features of the formation and cleavage of the Holliday junction
- Functions of protein products of the recA, B, C, D genes
- Mechanism of phase variations of the flagellar H-antigen in salmonellae
- The role of IS elements in plasmid integration into the chromosome
- The process of transposition of mobile genetic elements
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