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Microbiology: An Evolving Science 4th Edition

Microbiology: An Evolving Science (4th Edition)

Book Edition4th Edition
Author(s)Slonczewski
ISBN9780393614039
PublisherW. W. Norton
SubjectBiology
End of Chapter

Chapter 9, End of Chapter, Review Questions, Exercise 1

Page 354

Here is a tip:

Horizontal gene transfer involves recombination or insertion of the newly acquired DNA.

Explanation

Genes are transferred by the following methods in the microorganisms:

 

Transformation: The process of importing a free genetic material into a bacterial cell is now known as transformation. The cell capable of receiving a foreign DNA from its environment is referred to as competent. 

  • Most of the bacteria transform with the help of a protein complex, called transformasome, embedded with their membranes. These bacteria produce a competence factor that accumulates and binds to a two-component signal transduction system. The binding results in the production of transformasome that facilitate the intake of foreign DNA.
  • Some bacteria do not become competent naturally and require artificial manipulations, such as electrical (electroporation) and chemical (calcium chloride) methods, to change the membrane permeability and allow DNA to enter the cell. 

 

Conjugation: The process where direct cell-to-cell contact is made by the means of sex pilus to faciliate gene transfer, is called conjugation. The sex pilus is a filamnt of protein monomers that protrude from the donor cell and attaches to the recipient cell. The donor cell possesses a specialized plasmid, called F (fertility) plasmid, that carries all the genes needed for the process. 

Conjugation begins when the donor cell (F+ cell) forms a protein bridge with the recipient cell (F- cell). Formation of pilus triggers the cutting of one strand of the donor plasmid at the origin of transfer (oriT). The 5 end of the strand begins moving into the recipient cell through the pilus. After the transfer of the entire strand, DNA replication in the donor cell replaces the transferred strand. Simultaneously, a complementary strand to the transferred strand is also being synthesized in the recipient cell. Eventually, the protein bridge disintegrates and the two cells separate. The recipient cell after the completion of conjugation is converted to a new F+ donor cell.

 

Transduction: The process by which a bacteriophage acts as a vector to transfer genes between bacteria is known as transduction. Bacterial transduction is of two types:

  • Generalized transduction: It is the process where the phage moves any segment of donor chromosome to a recipient cell. The transducing phage mistakenly fills its capsid with a segment of the host DNA. When this phage infects another bacterium, it releases the donor into the new recipient cell. The donor DNA replaces the homologous sequence on the recipient genome to form a recombinant DNA. 
  • Specialized transduction: During this process the phage DNA integrates within the host chromosome and multiplies along with the host cellular machinery. At some point the phage DNA leaves the host chromosome and assembles into various progenies.  Some phage particles receive the integrated phage DNA along with a part of the bacterial DNA, flanking the phage DNA insertion site. This DNA is further transferred with the phage to a new host cell.

Verified Answer

  1. Transformation: It involves the uptake of naked DNA (deoxyribonucleic acid) by the bacterial cell from the environment.
  2. Conjugation: It requires two bacterial cells to have close physical contact with each other to transfer genetic material from one bacterium to the other. 
  3. Transduction: It includes a bacteriophage (virus that infects bacteria) that carries DNA from one bacterium to another.
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