Taxonomy of viruses – 2475 sp. Reported so farBaltimore system of International Committee forclassification Taxonomy of Viruses By International union of microbiological societiesBased on genetic material and its replication strategies
Retroviruses (Family Retroviridae)family of single-stranded RNAviruses having a helical envelopeand containing an enzyme (RT)that allows for a reversal ofgenetic transcription, from RNAto DNA rather than the usual DNAto RNA.the newly transcribed viral DNA isincorporated into the host cellsDNA strand for the production of Schematic cross section through anew RNA retroviruses (Integrase) retroviral particle. Transmembrane (TM) and the surface (SU) components linked together by disulfide bonds, matrix (MA) protein, capsid (CA) protein, and nucleocapsid (NC) protein. Major products of the pol-coding region are reverse transcriptase (RT) and integrase (IN). The protease (PR) is derived from the pro gene between gag and pol.
Retroviridae Taxonomy: Synonym(s): RNA tumor virus group (and related agents). Retroviruses are currently classified into 7 genera. Host: Virus infects vertebrates. Morphology: Virions enveloped; slightly pleomorphic; spherical; 80-100 nm in diameter. Surface projections of envelope small (surface appears rough), or distinct (8 nm long glycoprotein); spikes; dispersed evenly over all the surface. Capsids isometric (to spherical), or rod-shaped (or a truncated cone eg HIV). Nucleoid of spherical nucleocapsid symmetric, or asymmetric.
Retrovirus genome RNA. Single stranded. Linear; genomic nucleic acid positive sense. Genome dimeric. Total genome of one monomer 7000-11000 nucleotides long. Genome of retroviruses is flanked at each end by repeated sequences ("R") the gag gene is translated into molecules of the capsid protein the pol gene is transcribed into molecules of reverse transcriptase the env gene is translated into molecules of the envelope protein
Family Retroviridae - Taxonomy Genus Gammaretroviruses - Mouse mammary tumor virus Genus Episilonretroviruses - Reticuloendotheliosis virus Subgenus: Reptilian type C oncovirus virus group - Viper retrovirus Reticuloendotheliosis virus group - Reticuloendotheliosis virus Genus Alpharetroviruses - Avian leukosis virus Genus Betaretrovirus group - Mason-Pfizer monkey virus Genus Deltaretroviruses - human T-lymphotropic virus (HTLV) Genus Lentivirus Subgenus: bovine lentivirus group - bovine immunodeficiency virus Subgenus: Equine lentivirus group - Equine infectious anemia virus Subgenus: Feline lentivirus group - feline immunodeficiency virus Subgenus: Ovine/caprine lentivirus group - caprine arthritis encephalitis virus Subgenus: Primate lentivirus group - HIV• Genus Spumavirus Chimpanzee foamy virus
Life cycle - Retroviruses When a retrovirus infects a cell its molecules of reverse transcriptase are carried into the cell attached to the viral RNA molecules. The reverse transcriptase synthesizes DNA copies of the RNA. These enter the nucleus and are inserted into the DNA of the host. These inserts are transcribed by the hosts enzymes into fresh RNA molecules which re-enter the cytosol where some are translated by host ribosomes
Human Immunodeficiency Virus (HIV) ICTV Group: +ssRNA RT Baltimore Group( Earlier Classification): Group VI (ssRNA-RT viruses (+)sense RNA with DNA intermediate in life-cycle) Family: Retroviridae Genus: Lentivirus Species: Human Immunodeficiency Virus (HIV)
HIV An HIV particle is around 100-150 billionths of a metre in diameter. Thats about the same as: 0.1 microns one seventieth of the diameter of Host Cell- human CD4+ white blood cell. Unlike most bacteria, HIV particles are much too small to be seen through an ordinary microscope. However they can be seen clearly with an electron microscope. HIV particles surround themselves with a coat of fatty material known as the viral envelope (or membrane). Projecting from this are around 72 little spikes, which are formed from the proteins gp120 and gp41 that act as receptors.. Just below the viral envelope is a layer called the matrix, which is made from the protein p17. The viral core (or capsid) is usually bullet-shaped and is made from the protein p24. Inside the core are three enzymes required for HIV replication called reverse transcriptase, integrase and protease. Also held within the core is HIVs genetic material, which consists of two identical single strands of RNA.
Different types of HIV There are two types of HIV: HIV-1 and HIV-2. Both types are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2. Worldwide, the predominant virus is HIV-1, and generally when people refer to HIV without specifying the type of virus they will be referring to HIV-1. The relatively uncommon HIV-2 type is concentrated in West Africa and is rarely found elsewhere.
Life Cycle - HIV Virus entry into lymphocytes is receptor mediated After entry it releases the 2 RNA genome and 3 proteins viz., protease, reverse transcriptase and integrase into cytosol. Viral RNA undergoes transcription to cDNA with help of RT Viral RNA removed with viral enzymes and sscDNA undergoes duplication to form dsDNA dsDNA moves to the nucleus and integrates (with the help of integrase ) with the host cell DNA which undergoes transcription to viral RNA and translation to viral proteins – core proteins (further processed by protease) 2 viral RNA mol, integrase, RT and protease come together forms the core of the virus and acquires the envelope / lipid coat and penetrates out of the cell as infective virions.