Question

Please list, explain, and compare the three forms of gene transfer in the environment in regards to microbiology and give examples.

Answer 1

The three processes of gene transfer in Micoorganisms are - Transformation, Transduction and Conjugation.

1)Transformation - Fredrick Griffith, 1927 while working on several strains of Streptococcus pneumoniae identifed two strains - Virulent (which cause pneumonia in man and mice) and Avirulent strain (which did not cause pneumionia). He noticed that the harmless form of the micorbe could be converted into disease causing form by some process which later came to be known as Transformation. Those cells that formed smooth colonies (capsulated) caused diseases while those with rough surfaced (non-capsulated) colonies did not cause the disease. When he innoculated a mixture of heat killed capsulated cells and non-capsulated cells, it caused disease in the mice. He realised that there must be some tranforming principle which could bring about this tranformation.Three process are involved in tranformation -

a) Competence - This decribes the ability of cells to take up foreign DNA through changes in cell wall. It includes activation and formation of special DNA receptor proteins with the help of Competence factors.

b) DNA binding anf uptake - This includes the interaction between the cell wall of the receptor and the donor DNA which initially is reversible but later on becomes irreversible as the interaction increases.

c) The third step is the transfer of the tranforming DNA to the recipient genetic material. After this the tranforming DNA pairs with the homologous DNA of the host cell and with the help of enzymes it displaces the homologous strand in the recipient DNA. The resultig structure having one strand of the original and one strand of tranforming DNA is referred to as Heteroduplex. Following DNA replication, two cells are formed, one with the original genetic material and one with the tranforming DNA.

2. Transduction - It is a virus mediated gene transfer from bacterial cell to another. Viruses that infect bacteria are called Bacteriophages. Once a bacteriophage infects a bacterial cell, it releases its DNA into the host cell. The host DNA starts breaking down and the viral DNA replicates. The viral DNA then directs the production of viral protein component of the pahe assemblage. Once the proteins have been assembles, the mature phages are assembled and released whcih can infect other bacterial cells. The phages assembles inside the bacterial cell have the viral DNA and few of them can also have the host bactrial cell's original and are refereed to as 'defective phage'. If this defective phage infects another bacterial cell, then it can transfer the bacterial DNA into another Bacteria.

3. Conjugation - It is the temporary fusion of two single celled organisms for the transfer of genetic material. It was first demonstrated by Lederberg and Tatum in 1946 while working on two Auxotrophic strains of K12 E.coli. It involves unidirectional transfer of genetic material from the Donor to the Recipient. The donors are represented as F+ and the recipient as F- . The donor possess Fertility factor /F factor which is a mobile genetic element, a circular double stranded DNA molecule called Episome. The process includes - formation of the conjugation tube, movemtent of one strand of F factor across the conjugation tube due endonuclease action, complementary DNA synthesis on both the single strands, completion of movement across the conjugation tube and completion of DNA synthesis. Liagase enzyme closes the circle and the conjugants are separated and known as Exconjugants.