Retroviruses

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Retroviruses

	Retroviruses - RNA tumour viruses are called oncornaviruses because they are Oncogenic (cancer-causing) and contain RNA. They are classified as retroviruses because the RNA viral genome forms a DNA copy, a reversal of the normal DNA-->RNA synthesis. They have thus properties of both DNA and RNA viruses. The retroviruses exist in two distinct forms. The DNA genes are integrated into the chromosomal DNA of the host, and the infectious virus particles contain RNA genes. Retroviruses have been isolated from many vertebrates, including fish, reptiles, birds and mammals (including primates). They cause leukemia and sarcoma in chickens, and lymphoma and mammary, carcinoma in mice. It is, however, not clear whether they occur as natural infections of humans.
The better known retroviruses are the Rous sarcoma virus (RSV) and the leukemia viruses of chicken and mice. All retroviruses do not, cause cancer. Some of them are nonpathogenic, while others are associated with autoimmune disease. The terms retrovirus and oncornavirus are therefore not synonymous. Among the RNA viruses only the oncornaviruses are known to cause cancer. Oncornaviruses show alternation of generations and exist in two forms, as RNA particles and host integrated DNA genes. The infectious particle contains RNA genes. The provirus form exists as DNA genes integrated into the host chromosomal DNA (the DNA provirus hypothesis of Temin, 1964).
The viral genes behave as host Mendelian traits. Complementary RNA is synthesized from RNA by the enzyme reverse transcriptase. The proviral oncornavirus may remain latent throughout the life of the host and even for several host generations. Even if activated, the provirus may not harm the host, if the latter has developed resistance to reinfection by its own inherited viruses. The viruses on activation are infectious for other host species.
Viral proteins: The oncornavirus particle (virion) consists of a nucleoprotein core surrounded by a core shell. The virion is enclosed in an envelope derived from the plasma membrane of the host cell. The core and the core shell appear as an electron, dense nucleoid in thin sections examined by the electron microscope. In the freshly budded virus the core and the core shell appear as a ring structure. The nucleoid may be central (C-type viruses, e. g. leukemia virus) or excentric (B-type viruses. e.g. mammary tumour virus. The envelope has spikes or knobs consisting of virus coded glycoproteins. The glycoproteins recognize specific receptor sites on the host cells and thus help adsorption of the virus during infection.
	Retroviruses have 5-8 virion polypeptides. The core shell is composed of the major internal protein p30 (MW 27,300-31,000). Within it are smaller proteins, the reverse transcriptase, along with the RNA. The MW of glycoproteins is 52,000 for murine mammary tumour virus, 70,000 (gp70) for murine leukemia virus (MuLV) and 85,000 for avian leukemia sarcoma viruses. The structural proteins are designated according to their molecular weights. Thus, the major internal protein of murine leukemia virus is called p30 and the major external glycoprotein gp70. The viral proteins function as antigens and generate antibodies in the host. These antibodies are type specific and recognize only particular virus strains with closely related envelope glycoproteins.

RNA: The RNA complex has a sedimentation value of 60-708 and is known as 70S RNA. It consists of two closely related or identical 35s subunits linked at their 5' ends, plus smaller RNA species (7S,5S': contaminants 7) derived from the host cell.
The genome of the virus particle is therefore diploid and consists of two linear 35S ssRNA 5trands.
The 35S genome subunits have poly(A) at their 3 ends and an m7G cap at their 5 ends. The smaller RNA species includes 4S tRNA, (derived from the host cell) .
which acts a primer for reverse transcriptase during RNA-->DNA synthesis.
For chicken leukemia virus the primer RNA is tRNAtrp (tryptophan tRNA) and for mouse leukemia virus it is tRNApro (proline tRNA)

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List of Some Papovaviruses Polyoma and SV 40 Viruses Replication of DNA Protein Synthesis Retroviruses Proteins of the Murine Leukemia Virus Oncornavirus Replicative Cycle Types of Tumor Virus Infection Abortive Infection Defective Infection Nonpermissive Infection Inactive Infection Defective and Helper Viruses Transmission of Oncornaviruses

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