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What is Genetic Recombination?

Organisms evolve because of changes to their genomes, the DNA sequences that code for proteins and RNAs.
Mutations to DNA can occur at any time and might change the structure of the proteins produced.
Prokaryotes have additional ways to evolve their genomes besides relying on relatively infrequent mutations. Through genetic recombination, individual prokaryotic cells can share DNA with other individual cells, not necessarily belonging to the same species.
This can help spread a beneficial gene that produces healthier organisms.
For example,
the appearance of a gene that confers antibiotic resistance might create a virulent strain of bacteria.
The cells may spread the beneficial gene through genetic recombination, helping to ensure the survival of the species.

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Transformation

It is a kind of genetic recombination where only the carrier of genes, i.e., the DNA molecules of donor cell, pass into the recipient cell through the liquid medium.
Certain species of bacteria can ingest DNA segments, known as plasmids, from their surroundings and incorporate the plasmids into their own chromosomes.
The bacterium must first enter a special state, called competence, that allows transformation to occur. To achieve competence, the bacterium must activate a number of genes that express the required proteins.
Scientists use transformation to introduce foreign DNA into prokaryotic cells by incorporating the DNA in the growth medium. In this way, researchers can gauge the effects of different DNA segments and even create designer microorganisms with desired traits.

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Transformation

It was described by Frederick Griffith (1928), an English bacteriologist.
He had done his experiment with laboratory mice and two types of Diplococcus pneumoniae, the pneumonia causing organism.
One type has rough (R) non-capsulated cells and another one with smooth (S) capsulated cells.
The R-type is non-pathogenic, while the S-type is pathogenic.

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The process of transformation is mentioned below

(i) When live non-pathogenic (R-type) cells are injected in mice, the mice remain alive.

(ii) When dead pathogenic (S-type) cells are injected in mice, the mice also remain alive.’

(iii) When pathogenic (S-type) cells are injected in mice, they suffer from pneumonia and died.

(iv) When live non-pathogenic (R-type) cells are mixed with dead pathogenic (S-type) cells and are injected in mice, they also suffered from pneumonia and died. On isolation of dead tissue of mice, the smooth (S) encapsulated cells are found on agar. The above experiment indicates the conversion of R-type to S-type, called transformation.

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Later, James L. Alloway (1932), transformed the rough type cells to smooth type, by using the fragments from dead smooth-type cells and confirmed Griffith’s work.
Further, Oswald T. Avery, Colin M. MacLeod and Maclyn N. McCarty (1944) also found that DNA isolated from the fragments could induce the transformation. Their experimental result was the first proof of DNA as the genetic material in living organism.

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Transformation

A cell that is able to take up a molecule of DNA and be transferred is called competent cell.
Bacteria differ in the form in which DNA is taken up. In Gram negative bacteria (eg. Haemophilus) only DS DNA is taken up into the cell, however only SS – DNA segment is incorporated into the genome.
In Gram positive bacteria (Streptococcus sp. and Bacillus) only SS – DNA is taken up.

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Transformation

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Transformation

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Transformation

In transformation, a bacterium takes in DNA from its environment, often DNA that's been shed by other bacteria. In a laboratory, the DNA may be introduced by scientists. If the DNA is in the form of a circular DNA called a plasmid, it can be copied in the receiving cell and passed on to its descendants.

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Thus the transformation takes place by horizontal gene transfer through uptake of free DNA by other bacteria. This transformation takes place either spontaneously by taking DNA from the environment, i.e., Natural, or by forced uptake under laboratory condition i.e., Artificial process.

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A. Natural Transformation:

During natural transformation, free naked fragments of double stranded DNA of donor cell become attached to the surface of the recipient cell. The free double stranded DNA molecules may be available in the medium by lysis or natural decay of bacteria.
After attachment of donor double stranded DNA with the surface of recipient bacterium, one strand is digested by the bacterial nuclease and the remaining one strand is then taken in by an energy-requiring transport system. This uptake of DNA takes place during late logarithmic phase of growth.
During this process, Rec A type of protein plays an important role. The Rec A protein binds with the single stranded DNA and forms a coating around the DNA. The coated single stranded DNA and DNA of recipient cell then move close to each other to get homologous sequence.
After reaching at proper place, the Rec A protein actively displaces one strand of chromosomal DNA of recipient cell. The process requires hydrolysis of ATP to get energy. The incoming DNA strand is then integrated with one strand of bacterial DNA by base pairing and ligation takes place by DNA ligase.
The displaced DNA strand of recipient cell is then digested by cellular DNase activity. Any mismatch between the two strands of new region is corrected by them. Thus the transformation is completed. If the introduced single stranded DNA fails to recombine with the recipient DNA, it is digested by cellular DNase and gets lost.

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B. Artificial Transformation:

The E. coli, an ideal material for research is not transformed naturally. Later, it has been discovered that the transformation in E. coli can be done by special physical and chemical treatments. This can be done by exposure of E. coli to high voltage electric field and also by high concentration of CaCI₂. Under such condition, the bacterial cells are forced to take up foreign DNA. This type of transformation is called artificial.
During this process, the recipient bacterial cells are able to take up double stranded DNA fragments.
Physical or chemical treatment forces the recipient bacterial cell to receive exogenous DNA. The foreign DNA is then integrated with the chromosome by homologous recombination, mediated by Rec A protein. The Rec A protein catalyses the annealing of two DNA segments and exchange of homologous region.
This involves nick i.e., small cut of DNA strands and rejoining of exchanged parts i.e., breakage and reunion.

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Transduction

It is a special method of genetic recombination where genetic material is transferred from the donor to the recipient cell through a non- replicating bacteriophage — temperate bacteriophage. This was discovered by Joshua Leaderberg and Nortor Zinder (1952) during their research with Salmonella typhimurium.
In this process, a small fragment of bacterial DNA is incorporated into an attacking bacteriophage (i.e., virus which infect bacteria) and when this bacteriophage infects a new bacterial cell, it transfers the genetic material into it, and thus genetic recombination takes place.

Transduction are of two types:

A. Specialized transduction, and
B. Generalized transduction.

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Transduction

Not all phages can transduce and not all bacteria are transducible.
In generalized transduction host DNA derived from any portion of host genome becomes a part of the DNA of the mature virus particle in place of the virus genome, which gets integrated into another cell upon entry.
In specialized transduction, when a lysogenized cell reverts to lytic cycle, a part of host DNA is exchanged for phage DNA, which replicates and forms phage, which when trasnduced, the new gene gets into another cell.

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Bacterial Transduction

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Transduction

In transduction, viruses that infect bacteria move short pieces of chromosomal DNA from one bacterium to another "by accident."
Yep, even bacteria can get a virus! The viruses that infect bacteria are called bacteriophages. Bacteriophages, like other viruses, are the pirates of the biological world—they commandeer a cell's resources and use them to make more bacteriophages.
However, this process can be a little sloppy. Sometimes, chunks of host cell DNA get caught inside the new bacteriophage as they are made. When one of these "defective" bacteriophages infects a cell, it transfers the DNA. Some bacteriophages chop the DNA of their host cell into pieces, making this transfer process more likely

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Conjugation

In this process, the exchange of genetic material takes place through a conjugation tube between the two cells of bacteria.
The process was first postulated by Joshua Lederberg and Edward Tatum (1946) in Escherichia coli.
They were awarded the Nobel Prize in 1958 for their work on bacterial genetics.
Later on, it has also been demonstrated in Salmonella, Vibrio and Pseudomonas.

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Conjugation

There are two mating types of bacteria, one is male type or F⁺ or donor cell, which donates some DNA. The other one is female type or F^– or recipient cell, which receives DNA.
Later, after receiving DNA, the recipient cell may behave as donor cell i.e., F⁺ type. The F-factor is the fertility factor, sex-factor or F-plasmid present in the cell of F⁺ i.e., donor cell or male type. The plasmid takes part in conjugation is called episome.
In this process, two cells of opposite mating type i.e., F⁺ and F^– become temporarily attached with each other by sex pilus (Fig. 2.26). The sex pilus has a hole of 2.5 pm diameter through which DNA can pass from donor to recipient cell.
The F-factor or F-plasmid is a double stranded DNA loop, present in the cytoplasm; apart from the nucleoid. The F-factor contains about 20 genes.

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After the establishment of conjugation tube, the F-factor prepares for replication by the rolling circular mechanism. The two strands of F- factor begin to separate from each other and one of them passes to the recipient i.e., F^– cell.
After reaching in F^– cell, enzymes synthesise a complementary strand that forms a double helix, which bends into a loop. The conversion process is thus completed. In the donor cell i.e., in F⁺, a new DNA strand also forms to complement the left over DNA strand of the F-factor.
There is another type of conjugation where passage of nucleoid DNA takes place through conjugation tube. Strains of bacteria are known as Hfr (high frequency of recombination) strain. William Hayes discovered such strains of E. coli in 1950s. The Hfr factor is also called episome. In Hfr strain, the F-factor is attached with the nucleoid DNA i.e., the bacterial chromosome.
In this process, Hfr and F^– cells become attached with each other by sex pilus (Fig. 2.27). At the point of attachment of F-factor, the bacterial chromosome opens and a copy of one strand is formed by the rolling circular mechanism.
A portion of single stranded DNA then passes into the recipient cell through pilus. Due to agitation in medium, the conjugation tube may not survive for long time because of broken pilus. Thereby, the total length of transfer DNA may not be able to take entry to the recipient cell.