Recombination in Bacteria and Viruses

Recombination is the rearrangement & resulfing of genes resulting in new genotype. Bacterial recombination was discovered in E.coli by Lederberg & Tatum(1946).

According to them Bacterial recombination is brought about by transfer of genetic material from one bacterium to other.The bacterium which donates the genetic material is called Donor.which recive it is called recipient. 

In that way a bacterium can acquire genetic information i.e new segment of DNA by different Mechanisms like Conjugation,Transformation and Transduction. 

Conjugation

In this process genetic information is transform from one bacterial cell to the other and for this process it requires close contact between the donor cell and recepient cell.

(Picture)  

Sexual differentiation and behave like male and female gamete of higher forms like.(1)F+=Male or donor strain because this posses the sex factor or fertility factor(F Factor).This F Factor contains the gene for maleness.(2)F-=Female or Recipient strain which is without F factor. Conjugation is characterised by the formation of a flagellum like structure called Conjugation Tube or Sex Pilus which is devlope from the donor cell.This sex pilus make a close contact with the opposite meeting cell(Recipient Cell).Thus,a conjugation bridge is form between the two cells.Generally Conjugation is mediated by the Plasmid which is a portion of bacterial genome i.e Separate from the chromosome.Such Plasmid is not connected to main bacterial chromosome and replicates independently. 

Through the conjugation tube the sex factor(F factor) is transferred from the donor to to the recipient.During this time the two DNA strands of this F factor are separated and one of them moves across the sex pilus. As the plasmids enters the recipient cell.Complimentary  DNA Synthesis occurs in both the bacteria to generate double stranded DNA.In the process of conjugation the F- cell is transferred to become F+ cell i.e having the fertility factor.In some Bacteria this F factor before transforming to the recipient cell,it is integrated in to the original chromosome. Thus,The cell containing such integrated factor is called HFR cells(High Frequency recombination).This integrated factor now replicates and transfer to the recipient cell.So that the F- cell now acquires more and more resistance.In this way the process of conjugation can be Summarized as-1)Maleness in Bacteria(F+ Strain) is due to the presence of fertility factor.2)Usually one F factor is present per male bacterial cell .3)This F factor either replicates along with entire cell division or it may replicate independently.4)F+ Male cell can be changed to F- Female cell by loosing F factor.5)If a single F- Cell is placed in a culture of F- bacteria.The entire F- population gradually changes to F+

Transformation

In this process there is no combination of any donor cell & The recipient cell.But the genetic Material is transfered from the external medium and recombinant is produced.Vertually all bacteria have an ability to take up DNA from the environment provided.

They are competent Transfermation competent is a stage of bacterial cell during which the rigid cell wall allows the passage of DNA during certain stage of cell division.
Transfermation in bacteria was described by Griffith(1928) on the two strain of bacteria i.e  Diplococcus pneumoniae. According to Griffith there are two types of Diplococcus bacteria namely Virulent & Avirulent. The virulent strain is the Wild type having a smooth carbohydrate capsule which gives rise to smooth colonies.Thus, these are also called capsulated strain& they are virulent or pathogenic causing severe pneumoniae & death also occur.

But the avirulent strain is the mutant strain which have no capsule and therefore they are called rough or non capsullated strain.They give rise to rough colonies and non pathogenic for the disease pneumonia.

As per the experiment of Griffith when such virulent strain are injected to mice,they kill them with pneumoniae infection.But when the mice are inoculated with Avirulent bacteria their was totally no ill effect.When the mice are injected with heat killed virulent bacteria,there was also no ill effect because after heat killing these virulent type become Avirulent. 

In the final experiment Griffith found that when a mixture of pure avirulent & heat killed virulent bacteria is injected into healthy mice the non virulent bacteria also acquire the capsule,because virulent and cause pneumonia.these dead mice show that they contain virulent bacteria.Thus, the analysis show that the heat killed virulent bacteria are responsible for the transformation of avirulent into virulent capsulated bacteria.This phenomenon is known as Griffith effect or Bacterial transformation. Avery,Macleod & Mccarty(1944) experimenting proof that the transformation of Avirulent form to virulent form is due to the transmission of a chemical substance.When finely groung virulent bacterial extract is added to a culture of a virulent bacteria the culture after few hour contents both avirulent & virulent bacteria.When this extract is treated with DNase(an enzyme destroying DNA),the transforming property is loss.But when the extract is treated with protease the transforming ability is retain.Thus,it is conform that the genetic material from one cell to another cell by adding DNA extract in the culture is called Transformation.

Transduction

It is the process in which the genetic material is transferred from the Donor to the recipient bacterium by a bacteriophase. The process of such type of Genetic Recombination by Transduction was experimented in Salmonella Typhimurium(The disease Paratyphoid A & B caused by this species) by Zinder and Lederberg.(Picture) 

Here fragments of DNA are transferred from one bacterium to other with the help of a viral carrier thus transduction is phage- Mediated process of the genetic transfer in bacteria.The Process of transduction takes place in the following steps-

*At first the bacteriophase attack a bacterial cell & continue its Lytic cycle.Thus it injects its DNA into the Bacterial Cytoplasm.

*Mean while the bacterial DNA also breaks down & gets fragmented Simultaneously the viral DNA replicates with in the Bacterium.

*Several Bacteriophases are Synthesized with in bacteria cell and the new fragments of original Bacterial DNA are incorporated into the newly formed phase particle.

*Now Lysis of the Bacterial cell liberates the new phase particle which carry the Bacterial DNA.

*These newly formed phases carried Bacterial DNA now in fact other bacterial cell or now host bacterium.

*in that way the fragmented DNA of First bacterium is incorporated with new bacterial DNA & Thus,recombination occurs In some cases the injected DNA by the phase particle into a new host may be degraded by nucleases and in such cases genetic exchange does not occurs.The transducted bacterial cell now undergoes usual binary fission and produce many Progeny containing new combination of genes.

                                                       

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