Discuss genetic recombination in bacteria.

Ans. Bacterial Genetic Recombination : Many microbiologists refused to believe that bacteria had stable hereditary systems which could undergo permanent changes. But in 1943 Max Delbruck and Salvador Luria through a classical experiment demonstrated that bacteria have stable hereditary mechanisms. The establishment of the similarity between the genetic systems of bacteria and higher forms of life resulted in the experimental use of Escherichia coli. The first demonstration of genetic recombination in bacteria was achieved by Lederberg and Tatum in 1946. They combined two different auxotrophic strains (strains that use only inorganic materials as a source of nutrients) of E. coli and allowed them to mate. They found prototrophic (nutritionally independent) colonies growing there, these must have been the result of transfer of genetic material (DNA) from cell to cell and a recombination between the auxotrophs by conjugation and is indeed a true sexual fusion. But genetic recombination in bacteria is basically different from that of higher plants and animals. In genetic recombination there is the occurrence of progeny whose combinations of genes are different from those that are present in the parents. In higher plants and animals genetic recombination is the fusion of two haploid gametic nuclei resulting in the development of a diploid nucleus. The complete genomes of both gametes are involved and recombination comes about from the independent assortment of chromosomes or by the process of crossing-over between homologous chromosomes. Whereas bacteria are free-living organisms and are not differentiated into gametic and somatic cells. Each cell is a potential gamete. Bacteria displaying sexuality have cells which behave as gametes and are differentiated into two functional types: donors (or males) and recipients (or females). As a rule, in bacteria only a fraction of the genetic constitution of the donor cell is transferred to recipient cell which contributes its entire genome and cytoplasm resulting in the development of a merozygote, a zygote that is incompletely diploid. Genetic recombination is limited to the diploid portion of the genome and occurs when a new, recombinant chromosome is formed. The recombinant chromosome is formed from DNA contributed by two different organisms.

The transfer of DNA from donor cells to recipient cells and recombinant forms can be accomplished by three distinct mechanisms: cell conjugation (mating), transformation (uptake of naked soluble DNA from the environment), or transduction (bacteriophage infection).

In conjugation there is an actual cell-to-cell contact between the mating bacteria. The double-stranded closed DNA opens and part of it is transferred through pili called sex pili which behave as bridges through which DNA is transferred from donors to recipient cells. The amount of DNA transferred is directly related with the duration of conjugation between the mating bacterial cells. In 1952 Hayes first presented evidence for sexual differentiation in E. coli. It is now known that mating types exist and all pairs are not fertile. Donors (male cells) contain a small circular piece of DNA called the fertility, sex, or F factor and labelled as F⁺. Recipient (female cells) are labelled as F^– and are devoid of F factor. Crosses between two F^– strains are sterile. Only F⁺ × F^– crosses yield recombinants. Donor strains are of two kinds: F⁺ and Hfr (high frequency of recombination). Donor F⁺ strain donates only a small portion of its genome, whereas donor Hfr strain donates large amount of genome to the recipients. Sex-factor-DNA is an episome which sometimes replicates autonomously in the bacterial cytoplasm and at other times integrated into the bacterial chromosome DNA and replicates with it. In addition to Escherichia, conjugation has also been reported to occur in the genera: Salmonella, Pseudomonas, and Serratia.

In transformation, the DNA passes between the bacteria as a naked molecule and nucleic acid can be isolated in active form from the medium separating the bacteria. This phenomenon was discovered by Fred Griffith (1928) working on Diplococcus pneumoniae. During this process, lysis (disruption) of donor cell takes place and its DNA becomes fragmented and the fragments are thrown out in the culture medium. The fragments of DNA are taken up by the recipient bacterial cells. The introduced genetic fragment, called an exogenote, undergoes recombination with the recipient bacterial chromosome replacing a homologous segment. This takes place during late log phase when the recipient bacteria are said to be competent to take up and incorporate donor DNA and its characteristics. The process of recombination which basically involves the replacement of one region in the molecule by different pieces of DNA has given rise to the exciting prospects of ‘Genetic Engineering’. In 1944, Avery, Macleod and McCarthy implicated DNA as the transforming principle.

Transduction involves the transfer of DNA from a donor bacterial cell into a recipient bacterial cell through the agency of a virus (bacteriophage) which acts as a vector. This phenomenon was discovered by N. D. Zinder and J. Lederberg in 1952 when they searched for sexual conjugation among Salmonella species. During transduction bacteriophage undergoes a rapid lytic growth cycle in its host bacterial cell in which it injects its DNA.

The DNA replicates rapidly and directs the synthesis of new phage proteins. The new DNA combines with new proteins to make phage particles which are released by the destruction of cell wall and lysis of the host bacterial cell. The new phage particles carry the genes of bacterium in which they are produced. There are some phages known as temperate phages which ordinarily do not lyse the host bacterial cell, carry DNA that can behave as a kind of episome in bacteria. These viral genomes can become integrated into the host bacterial genome. They are then known as prophages. There are several types of transduction: (i) when a temperate phage transfers any gene on a bacterial chromosome it is called generalized transduction, and (ii) when the phages transduce only those bacterial genes adjacent to the prophage in the bacterial chromosome it is known as specialized or restricted transduction. Transduction has been demonstrated in several bacterial species. This technique is used for making new bacterial strains.

Again in lysogenization, the agency is a temperate phage whose DNA on entering into the bacterial cell remains attached to the DNA of the host bacterial cell, reproduces along with it, and is transferred to the progeny without causing lysis of the host bacterial cell. The phage DNA is called the prophage. The phage DNA may be carried along several generations of the host bacteria. Sometimes, when the balance of the host bacterial cell is disturbed, the phage DNA detaches from the bacterial DNA and goes into a virulent state. Because of this latent potential for lysis, cultures containing prophage are called lysogenic and the phenomenon is Iysogeny.

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