Virus Transduction and Integration

Some viruses produce a small proportion of viruses that contain host DNA in addition to or instead of viral DNA. These virions are able to transfer host genes from one cell to another. In bacteriophages, during the process called transduction, these genes can be incorporated into the recipient cell’s chromosome, thus effecting genetic recombination.

Integration

Many viruses also interact with the host cell in such a way that the viral DNA, instead of replicating independently within the host cell, is integrated into the host cell DNA. It is then replicated and passed on to daughter cells, as are the host cell’s genes. Single or multiple copies of part or all of the viral DNA infecting animals may be incorporated into the host cell DNA. When the viral DNA is integrated, virions are not produced and the host cell is not killed. When a complete copy of viral DNA is present, the cell may return at some time to a lytic mode of virus replication with subsequent production of new virions.

RNA as Genetic Material

The RNA containing viruses are unique living system because they use RNA as their primary genetic material. Some RNA containing viruses produce enzymes that are capable of making new copies of RNA directly from the original molecule, thus circumventing DNA entirely. Retroviruses produce the enzyme called RNA-directed DNA polymerase, or reverse transcriptase, which can make a DNA molecule from the RNA molecule. This DNA formed from RNA then serves as the principal genetic molecule within the infected cell.

Slow Virus Infection

Slow viruses are disease agents not yet identified but assumed to exist, because the disease resembles viral diseases in their epidemiology. Slow virus diseases are degenerative nerve disorders that take years to develop. They include the human diseases called kuru and Creutzfelt-Jakob disease, a fatal sheep and goat disease called scrapie, and possibly a recently identified cattle disease called bovine spongiform enchepalophaty, among others.

Theories on the nature of slow viruses include the prion theory that they are nothing but infectious proteins; this idea is not generally accepted. The virino theory proposed that the agent do have nucleic acids, as do known viruses, but lack proteins. A third theory suggests that the agents are viruses that simply are very adept at hiding in body systems. Several known viruses can also act slowly such as the AIDS virus and hepatitis viruses.

Slow viruses diseases sometimes difficult to cure, even some of the diseases still don’t any medicine yet. Slow viruses diseases will attack slow to our body organ, but if don’t cure for long time the disease will very acute and have a damage effect to body organ. Slow virus diseases also will very slowly to cure, for certain diseases need to be cured for a year minimum such as hepatitis. This condition make many people are not patient for waiting and not continue for medication, the effect, the diseases will recurrence again and more difficult to be cured.

Viruses Characteristics

Certain type of virus exists in several forms of stages. The free virus particle, or virion, consists of a molecule of nucleic acid, either DNA or RNA, depending on the specific virus, surrounded by protein and in some viruses, also lipid and carbohydrate. The virion is inert, because its lacks many of the components that are necessary for independent life and reproduction; it does, however, provide for the transfer of the nucleic acid from host cell to host cell. Replication can occur only when the nucleic acid enters a host cell. Within the cell, the nucleic acid functions as genetic material and directs the synthesis of proteins. This results in the production of new virions, which then are released from the cell and can initiate new cycles of infection.

The virions of different viruses vary greatly in size, shape, and complexity, but those of any virus are uniform. The simplest types are rods and regular 20 sided polygons (icosahedrons); the tobacco mosaic virus, for example, consists of a single molecule of a single molecule of RNA surrounded by about 2,200 molecule of a single type of protein arranged in helical fashion to produce a rod 3,000 angstroms (1/100,000 in) long.

Many virions having a simple rod or polygonal shape are composed of several types of proteins, and some of these virions are surrounded by a less regularly shaped envelope, which often includes components of the host-cell membrane. At the other extreme of complexity, bacteriophage T4 has a virion composed of at least 25 different types of protein that make up a hollow head in which a DNA molecule is enclosed, and a tubular tail by which the virion attaches to its host cell and injects its DNA into it.