Bacterial Genetics

Introduction

Genetics is the study of the transfer of genetic traits or characters from the parent to their offspring (heredity). Bacteria is often used as a model to study the replication, transcription, translation, genetic code, and genetic variations, including mutations. E.coli is the commonly preferred model organism used because it has a generation time of 6 hours and can generate rapid results in a short period. Gene is a segment of DNA carrying the information for a particular trait. Genetic information transferred from one organism to another organism is called gene transfer. Gene transfer mechanisms involve two types − vertical and horizontal gene transfer. In vertical gene transfer, the genetic traits are passed down from the parents to the offspring, while in horizontal gene transfer, the genetic information is passed between two organisms independently.

Genetic information between bacteria is typically transferred by conjugation, transformation and transduction. These genes transfer mechanisms enable the bacteria to survive in extreme environments susceptible to changes in temperature, pressure, salinity, nutrient requirements and so on. Compared to other living organisms, bacteria can quickly adapt to new environments, constantly affected by environmental changes due to gene transfer mechanisms.

Conjugation

This mode of transfer of genetic information was discovered by Joshua Lederberg and Edward Tatum in 1946. In bacterial conjugation, genetic information transfer occurs through contact between two bacterial cells through a conjugation tube called “sex pili”. F-plasmid or fertility plasmids replicating in the bacterial cell-independent to the chromosome are transferred during conjugation. F plasmid is a small, circular piece of DNA carrying different genetic information. The process involves the donor and the recipient cell. The donor cell contains the F plasmid and is referred to as the F+ (F plus) cell, while the cell devoid of F plasmid is called the F- (F minus) or the recipient cell.

The three methods by which conjugation takes place are as follows −

• F+ X F- conjugation

• Hfr conjugation

• F´ X F- conjugation.

F+X F- conjugation

This genetic conjugation occurs between a donor (F+) and a recipient (F-). The gene transfer here is non-reciprocal.

Hfr conjugation

The donor in this type of conjugation is referred to as the Hfr strain (high frequency of recombination) as there is a very high efficiency of gene transfer compared to F+ donors.

F´ X F- conjugation

F- serves as a recipient cell, while F´ contains the F plasmid of the Hfr strain.

Applications

It is widely used to synthesize new bacterial cells, which could produce new metabolites, antibiotic resistance etc.

Transformation

Fred Griffith discovered this mode of genetic recombination in 1928. In bacterial transformation, the extracellular DNA of the donor cell is taken up by the recipient cell in a heritable form. The recipient cell takes up the DNA and is transformed. The gene transfer can happen naturally in the environment or artificially induced in the laboratory. This method is widely used for generating recombinant DNA. Transformation occurs regularly in the environment. A considerable amount of DNA is released in the environment when bacterial cells lyse, which is taken up by the competent cells.

Applications

It is widely used by scientists in DNA cloning, genetic engineering, bioremediation, insulin production, enzymes, hormones, etc.

Transduction

Transduction is a form of genetic recombination between two bacterial cells through bacteriophages. Bacteriophages serve as a vector in transferring the gene. The bacterial DNA or its fragment is transferred to another bacterium via bacteriophages. This mode of gene transfer differs from conjugation and transformation in having no physical contact between the two bacterial cells. Norton Zinder and Joshua Lederberg discovered this mode of transfer of genetic material in 1952.  The two modes of transduction are generalized and specialized.

Generalized transduction

In this type of transduction, only the bacterial DNA is carried by the bacteriophage.

It involves the following steps −

• The bacteriophage lyses the bacterial cell.

• The bacteriophage inserts its genetic material into the host bacterial cell, causing the degradation of host cell DNA.

• Synthesis of phage DNA and proteins occurs in the host cell. The phage assembles itself to form a new bacteriophage. In some instances, the partially degraded host cell DNA also gets incorporated into the newly formed phage. Such a particle is called a “transducing particle”.

• The bacteriophages then lyse the host cell and release themselves out of the host cell. The cycle thereby continues. The transducing particle invades a new bacterial cell.

• The DNA of the transducing particle integrates into the bacterial chromosome. Such recombination is called generalized transduction.

Specialized transduction

Specialized transduction, also called restricted transduction, involves carrying specific portions of the host bacterial DNA.

It involves the following steps −

• The bacteriophage invades a bacterial cell (donor cell).

• The bacteriophage inserts its genetic material into the bacterial donor cell.

• The phage DNA then integrates with the bacterial DNA.

• The phage replicates to form new cells. The newly formed phage cells consist of both phage and donor DNA.

• Once the cell is lysed, the bacteriophages attack a new bacterial cell (recipient cell) and inject the hybrid DNA into the recipient cell. The hybrid DNA then integrates with the new bacterial cell.

• The recipient cell now contains its phage DNA and donor DNA.

Applications

It is widely used in genetic engineering, gene therapy, bacterial gene mapping etc.

FAQs

Qns 1. What are Bacteriophages?

Ans. Bacteriophages are viruses which invade the bacterial cells and proliferate inside them.

Qns 2. What are competent cells?

Ans. The recipient cell which takes up the foreign DNA from a different bacterial cell during transformation is called a competent cell.

Qns 3. What is genetic recombination?

Ans. Genetic recombination is called the exchange and re-joining of two DNA molecules to form a new DNA molecule.

Qns 4. What is a conjugation tube?

Ans. Conjugation tube, often referred to as the sex pilus, is a tube that pulls or joins two bacterial cells to aid in DNA transfer.

Qns 5. Which form of transduction can carry out with temperate phage?

Ans. Specialized transduction.