Microbiology

Microbiology Study Guide

to undergo genetic changes. It has been further found that large amount of DNA is released in the environment by bacteria. It is also possible that these released DNA are taken up by the competent cells. For successful transformation to take place, the bacteria must exercise logarithmic stage of growth and have a competence factor. When the competent cells take up the released DNA, one strand of the double helix is destroyed and is replaced by a non-identical fragment in the recipient. This makes the process of transformation complete. Transduction Transduction generally takes place when the transfer of DNA fragments takes place between one bacterial virus, or bacteriophage, to another. The bacterium from which the DNA fragment is transferred is known as the donor, while the bacterium that receives the new DNA fragment is the recipient. In this process, the bacteriophage enters the body of the bacterium and replicates as the bacteriophage. After this, the host bacterium undergoes lysis and releases new bacteriophage. This process is termed as lytic cycle . There is also a possibility that the integration of the DNA of the virus and bacteria takes place when the virus sticks to the chromosome of the bacteria. Staying there for a while, it can detach and continue to replicate to form new organisms. This process is called the lysogenic cycle. In this condition, the host bacteria are not killed or injured by the virus but stay in a lysogenic state with the bacteria. Such a virus is known as temperate phage or prohage. Transduction can be generalized or specialized. In a generalized transduction, the phage stays in a lysogenic relation with the host bacterium and stays with the chromosomal DNA. In the case of replication, some fragments of the DNA of the bacteria are erroneously packed by new phages. In a specialized transduction, at the time of breaking from the bacterial DNA, the phage DNA extracts about 5% of the DNA of the bacteria. During replication, the bacterial genes are also reproduced. Conjugation Propounded by Joshua Lederberg and Edward Tatum in the 1940s, bacterial conjugation deals with the process of combining two bacterial cells and transferring the genes between them. Of the two bacteria, one donor cell gives up the DNA while the recipient cell receives the DNA. At the time of this transfer, a special pilus known as the sex pilus joins both the donor and recipient. The DNA that is transferred is a replica of the F-factor plasmid, which enters the recipient organism and is replicated to produce double-stranded DNA for the purpose of integration. Gene Manipulation Plasmids A plasmid is a DNA molecule different from the chromosomal DNA of the organism. It possesses the capability to replicate without the need of the chromosomal DNA. The structure of a plasmid is generally double stranded or circular. They are common in the structure of bacteria and rarely found in the eukaryotic organisms. They have the capability to replicate autonomously in a suitable host and are considered replicons. They are commonly found in bacteria, archaea, and eukarya.

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