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Genetic Recombination (Sexual Reproduction)

Sexuality in bacteria does not involve sex organs, meiosis, mitosis and gametes formation and was first demonstrated by Joshua Lederberg and Edward Tatum in 1946.

In bacteria only a portion of genetic material (DNA) is transferred (exogenote) from a donor cell to recipient cell and the process is called genetic recombination. It occurs by three ways.

Conjugation

Transformation

Transduction

 

CONJUGATION

Conjugation is a common process by which one bacterium transfers DNA (mostly plasmid) to another by conjugation tube or sex pilus.

The initial evidence for bacterial conjugation came from an elegant experiment performed by Joshua Lederberg and Edward Tatum in E. coli (1946).

Bacteria that contain transmissible plasmids are called donor cells and those that receive plasmids are called recipient cells. The donor cell possesses sex factor or F plasmid or fertility factor (F factor).

The F plasmid is present in 1 or 2 copies per cell and is about 100 kilobases. 

F plasmid carrying the genes for sex pilus which allows the donor cell to produce a thin, tube-like structure called a conjugation tube, which the donor uses to contact the recipient cell.  

Conjugation occurs in and between many species of bacteria, including gram-negative as well as gram-positive bacteria and even occurs between bacteria and plants.

A donor or male cell containing F-plasmid in autonomous state and this is also called F+ cell (Fig. 15).

A donor or male cell containing F plasmid in integrated state and this is called Hfr cell (high frequency recombination) or high frequency male cell or super male (Fig 15).

The Hfr cell have max chance to pinch out the part of plasmid and during which some part of bacterial gene also pinches out and results in the formation of F prime cell (F́́ ′ cell) (Fig 15).

The recipient or female cell lacks F plasmid and this is also called F– cell (Fig 15).

 

 TRANSFORMATION

The second way, DNA can move between bacteria is through transformation.

Transformation was first time reported by Fred Griffith in 1928.

Bacterial transformation is the uptake by a cell of a naked DNA molecule or DNA fragment from the medium (liquid) and the incorporation of this DNA into the recipient chromosome in a heritable form.

This process doesn’t require a living donor cell and only requires free DNA in the environment.

DNA regions of one to tens of kilobases are transferred in the process of transformation.

F. Griffith (1928), initially conducted the experiment on transformation (Fig. 18). He took two strains of the bacterium Streptococcus pneumoniae / Pneumococcus pneumoniae / Diplococcus pneumoniae.

 Capsulated or smooth (S) type or virulent or pathogenic forms smooth colonies on the culture medium.

 Non-capsulated or rough (R) type or avirulent or non-pathogenic forms rough colonies on the culture medium.

Griffith’s experiment showing the existence of transforming principle in mice has been summarised in Fig. 18. For detailed explanation of this experiment visit YouTube channel “Biology by Dr. Imtiyaz”.

It is obvious from Griffith’s experiment that the R type strain becomes S type when mixed with heat-killed (dead). He named this change of avirulent into virulent strain as ‘transformation’.

He said that there was transfer of some factor from heat-killed (dead) virulent strain to the avirulent strain and called it “transforming principle”.

Griffith concluded that transforming principle was the polysaccharide of capsule of heat-killed virulent strains but he was wrong.

The idea of polysaccharide as transforming principle came to an end in 1944 when Avery, MacLeod and McCarty showed that it is the DNA which works as transforming principle not the polysaccharide of the capsule. They proved for the first time that DNA is the genetic material.

The success of transformation depends on the competence of the host cell. Competence is the ability of a cell to incorporate naked DNA in the process of transformation.

 

The transformation involves following steps

When donor DNA comes into contact with the competent bacterial cell, it first binds on the cell surface with the help of DNA binding protein and is taken up inside the cell (Fig. 19).

In some of the cases, it is observed that the dsDNA enters inside the bacterial cell whereas in others only ssDNA enters the recipient bacterial cell.

An endonuclease enzyme now degrades one of the strands of dsDNA of recipient bacterial chromosome in corresponding region and this gap is filled by the donor ssDNA (recombination) with the help of ligase enzyme.

Genetic recombination obtained via conjugation is temporary while via transformation is permanent.

 

TRANSDUCTION

The third mode of bacterial gene transfer is transduction. It is a method of genetic recombination in which the genetic material is transferred from the donor to the recipient cell by means of bacteriophages (bacterial viruses).

There are two types of bacteriophages, depending upon the interaction with the bacterial cells.

Virulent Phage or Lytic phage: In this the viral DNA remains as a free-floating molecule and replicates separately from the bacterial DNA. After the number of replicated phages reaches a certain number, they cause the host to lyse, so they can be released and infect new host cells. These phages are called virulent bacteriophages and follows the lytic cycle.

Temperate Phage or Lysogenic phage: In this the viral DNA after entry get inserted into the bacterial chromosome. Once inserted, the viral genome is called a prophage. The viral DNA is passively replicated as the host cell’s genome is replicated. When the host bacterial cell faces harsh conditions, the prophage is excised from the bacterial genome and follows the lytic cycle.

Transduction is a frequent mode of horizontal gene transfer in nature and was first time reported by Zinder and Lederberg (1952) in Salmonella typhimurium.

In transduction bacterial genes are incorporated into a phage capsid because of errors made during the virus life cycle.

The virus containing these genes then injects them into another bacterium, completing the transfer. There are two different kinds of transduction: generalized and specialized transduction.

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