Transfer mechanisms of genetic information in Bacteria
The evolution of the human species is guaranteed by the meiosis of the germ cells and their subsequent union (fertilization). In this way, the new generations inherit half of the genetic heritage from the father and half from the mother.
Since bacteria reproduce asexually, by simple binary cleavage, their evolution is guaranteed by two main mechanisms that of mutations and that of recombination.
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Mutations:
Random event that occurs with alterations and substitutions at the level of the nucleotide sequences that makes up the bacterial genome.
Recombinations:
derive from gene transfer mechanisms; a donor bacterium transfers nucleotide sequences to the recipient bacterium, which integrates them into its genome according to a HOMOLOGOUS RECOMBINATION mechanism. All this leads to the acquisition of new characters, such as the capsule, the ability to produce particular toxins, antibiotic resistance factors, etc.
In the bacterium the genome is contained in the only chromosome and sometimes also in extrachromosomal environments, called PLASMIDES, which have the same super spiralized structure, but a smaller diameter. Plasmids have autonomous replication and can code, for example, for toxins, pili, bacteriocins or resistance factors; some plasmids can also integrate into the bacterial genome and subsequently, they become independent; in these cases, they are called EPISOMES. In general, therefore, in plasmids, we find the genetic information of the auxiliary characters, not indispensable for the survival of the bacterium.
Some plasmids have a narrow spectrum of potential hosts, while others wider (meaning that they can be transferred to different bacteria).
To transfer the genetic material, therefore plasmids or genomic sequences, the bacteria have developed three different mechanisms, called, transformation, conjugation and transduction. To these, a fourth can be added, called TRANSPOSITION, through which genetic material is transferred from one area of the chromosome to another or from the plasmid to the chromosome within the bacterium itself.
Transformation conjugation transduction
The passage of free DNA fragments originates from bacterial lysis to a recipient bacterium.
Gene transfers through physical contact between two bacteria, of which the donor is called F + (positive fertility) and has conjugation pili, while the recipient F-. The transfer is mediated by a bacterial virus called bacteriophage.
Transformation
The transformation process can be divided into distinct stages:
1) Link between DNA and cell
2) Entry of DNA into the cell
3) Recombination of free DNA entering the recipient bacterium
4) Phenotypic expression
To be transformative, a DNA must be:
1) Double helix
2) With a molecular weight greater than 10 6 Daltons
3) Have a high analogy with the DNA of the recipient cell
The receptor cell, for its part, must be in a physiological state called competence. A cell is competent when it is at the end of its exponential or logarithmic growth. In fact, in this phase, protein synthesis has a maximum and competence factors that are expressed (proteins that allow DNA to enter).
Conjugation
It consists in the direct transfer of genetic material through physical contact between two bacterial cells.
Some bacteria contain a plasmid, called factor F, which encodes proteins that form the conjugation pili. This autonomous replication plasmid has genes that allow it to replicate and transfer from one F + bacterium to another (F-).
Conjugation phases
F + bacterium meet an F- bacterium, and a bridge is formed. At this point, the plasmid begins to replicate with a mechanism called rolling circle (in the 5 '- 3' direction). At the end of replication and transfer, we have two F +, since the first maintains the copy of the plasmid, while the F- receives the second hemolysis, which then duplicates and forms the plasmid.
Sometimes (rarely) in F + cell, the plasmid can integrate into the chromosome. The new cells where the plasmid is integrated are called HFR (high recombination frequency). In these cells, the integrated plasmid transmits its characteristics to the chromosome, such as that of transferring from a bacterium A to a bacterium B; therefore, the genes of the former can combine with those of the latter.
If we put an HFR bacterium in contact with an F- the conjugation bridge is formed, which sends a gene transfer signal for which a nuclease cuts a helix, the chromosome begins to replicate with a rolling circle mechanism and the copy passes into cell F starting from the cutting point.
The passage of the entire chromosome lasts about 90 ', but the conjugation bridge is fragile and often breaks before the transfer is complete, therefore only the plasmid head and some genes close to it pass; the terminal part, on the other hand, containing the F factor, does not pass. Consequently, the F- cell does not become HFR, and neither does F + but acquires only some of the characteristics of the donor bacterium.
The donor DNA can recombine with the chromosome of the recipient cell giving the bacterium new genetic characteristics. Other times the DNA can be degraded, and there is no change.
In addition to the F factors, there are also the so-called R factors (which lead to antibiotic resistance); they are always plasmids that contain the sequences of F factors, with which others are associated with antibiotic resistance. Then there are COL factors, which code for proteins called colicins or bacteriocins, that is, substances with bactericidal action, with which the bacterium defends itself and attacks the other cells to occupy the colonization sites.
There are also ENT factors, which code for enterotoxins and which are typical of some Escherichia Coli jambs (normally present in the body), capable of producing enterotoxins active on the mucosa of the small intestine.
The sexual pili are typical and unique of GRAM -, however, conjugation also occurs in GRAM +, which have plasmids that synthesize particular proteins, which - secreted externally - lead to the aggregation between bacteria F + and other F- (without resorting to the pili that are not there). A conjugation is, however
, a rare event.
Author: Vicki Lezama