Wednesday, 23 October 2019

Genetic Transfer in Microorganisms

Genetic Transfer in Microorganisms

Microorganisms have transferred his Genetic materials. Like bacteria to bacteria, bacteria to there Viruses, Bacteriophage is a bacterial host-virus those transfer his genetic material to bacterial cell and form a new colony under the bacterial cell. There are given some methods to understand how to transfer genetic materials in microorganisms.

Genetic Transformation


Genetic Transformation

The genetic transformation was first observed in bacteria and was the first bacterial system for genetic transfer to be discovered. Although it occurs naturally in only certain bacteria, under laboratory conditions it seems to be possible to carry out genetic transformations with any cell type, prokaryotic or eukaryotic. In bacteria, the process begins when a bacterial cell (living or dead) releases some DNA into the surrounding medium. This DNA is, of course, vulnerable to degradation but may encounter another bacterial cell before any significant change can occur. The second cell may take up the DNA, transport it across the cell wall and cell membrane, and allow it to recombine with the homologous portion of the resident bacterial chromosome. The resulting recombinant cell is called a transformant. In theory, any piece of genetic information may be transferred by this method, although the amount of DNA transferred per event is small, on the order of 10 kb in length.

Transduction


Transduction

In transduction, a bacterial virus 'bacteriophage' is involved intimately in the genetic transfer process. Phage infections begin with adsorption of virus particles to specific receptor sites on the host cell surface. The nucleic acid contained inside the viral protein coat is then transferred to the cytoplasm of the bacterial cell, where it becomes metabolically active and undergoes replication and transcription.

Typically there are two possible outcomes of phage infection. During a lytic response, the virus produces structural components of new phage particles, packages its nucleic acid inside them, and then causes the cell to lyse and release progeny phage. During a temperate response, the virus establishes a stable relationship with a host cell in which some phage functions are expressed, but not those that lead to uncontrolled DNA replication or the production and assembly of new particles. Instead, viral DNA is replicated along with host DNA, usually as an integral part of the same molecule, and is transmitted to all progeny cells. Occasionally lysogens (cells carrying a temperate phage) undergo a metabolic shift that reactivates the viral DNA. The result is the same as for an initial lytic response. Some phages may give only lytic responses and some only temperate ones; some, however, may give either response, depending on growth conditions.

During the course of a phage infection of a bacterial cell, some or all of the viral DNA inside an individual virion (virus particle) may be replaced by bacterial DNA, and this process may occur only rarely or with great frequency. After such an altered phage particle is released into the medium, it may encounter another bacterial cell and attempt to initiate an infection. In so doing, however, it transfers the DNA fragment from the previous host’s chromosome. If the newly infected cells are not killed and the DNA fragment can either replicate or recombine, the result is the production of transductants.

The amount of DNA transferred by these means varies considerably but generally is not more than the amount of DNA normally present in a single bacteriophage particle. It may approach 200 kb in length. The actual amount of DNA recombined is significantly less in most cases and, in addition, depends on whether the transduction is generalized or specialized. During generalized transduction, the phage enzyme system that packages viral DNA attaches to the bacterial chromosome and packages some of that DNA instead. The DNA that is packaged is chosen on a more or less random basis, and as a result, it is possible for any piece of host genetic information to be transferred. Specialized transduction, on the other hand, involves a temperate phage that has physically integrated its DNA into the bacterial chromosome at a specific site. 

As mentioned earlier, such an integrated phage may be stable for long periods of time. However, it may reactivate and replicate itself independent of the bacterial chromosome. During the reactivation process, it is possible for a mistake to occur so that some bacterial DNA located adjacent to one end of the viral DNA is also excised from the chromosome instead of the appropriate DNA from the other end of the viral genome. Because the overall size of the excised DNA must be nearly constant, only certain pieces of genetic information can be transferred, and their size depends on the physical nature of the mistake that caused their production.

Conjugation


Conjugation

The conjugation term can be used in several senses in biology. For example, in yeast, the result of conjugation is a fusion of haploid cells and the formation of a diploid cell type. In a bacterium such as E. coli, instead of cell fusion, there is the unidirectional transfer of DNA from a donor cell (which carries a conjugative plasmid) to a recipient cell beginning at a definite point on the DNA molecule and proceeding in a linear fashion. The transferred DNA may be all or part of the plasmid and may include a portion of the host DNA as well. Through analogy to other bacterial transfer processes, the recombinant bacteria are called transconjugants. The amount of bacterial DNA that can be transferred by conjugation ranges from a few kb to the entire chromosome.

Protoplast Fusion


Protoplast Fusion

Protoplast fusion has been used successfully for many years with eukaryotic cells. Its use with prokaryotic cells is comparatively rare, but apparently, the technique is applicable to most cells. For protoplast fusion to work, the protoplasts (cells that have been stripped of their walls) must be prepared by various enzymatic or antibiotic treatments. The fusion of cell membranes is aided by a high concentration of polyethylene glycol. The resulting diploid cell usually segregates haploid offspring, many of which show extensive recombination of parental characters. Formation of stable, noncomplementing diploid B. subtilis cells has also been reported. 

The diploid state can be stable over many generations, as evidenced by the successful transformation of parental genes whose phenotype was not present in the diploid donor cell. Successful fusions have been reported with Brevibacterium, Actinoplanes, Mycobacterium, Bacillus,  Providencia, Staphylococcus, and Streptomyces. Experimenters often use protoplasts in a simpler way. Protoplasts are good recipients in genetic transformation and readily take up plasmid DNA such as that prepared by genetic engineering technology.

Electroporation


Electroporation

When a high voltage (as much as 2500 V) is passed from a capacitor through a solution containing living cells, significant damage occurs to cell membranes, and many cells die. Among the survivors, however, are cells that developed small holes (pores) in their cell membranes as a result of the brief passage of current. These pores are quickly sealed, but while they are open, solutes can pass into or out of the cytoplasm. What is important to geneticists is that plasmid DNA molecules can also enter a cell if the exterior concentration is sufficiently high. This technique has been very successful with Gram-negative bacteria and somewhat less successful with Gram-positive bacteria.

Bacteriophage Genetic Exchange


Bacteriophage Genetic Exchange

Viral genetics can be studied effectively by arranging the virus/cell ratio so that a cell is simultaneously infected by more than one virus particle. Assuming that the two viruses are genetically distinguishable, the selection is applied to prevent parent-type phage particles from successfully completing an infection. Under this type of condition, only cells in which phages carrying recombinant DNA have been produced yield progeny virus particles. The resulting virions are tested for phenotype, and recombination frequency is calculated in the same manner as for bacteria.

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