RETROVIRUSES · Endogenous retroviruses = eukaryotic genomes contain a very large number of genetic elements that are related to retroviral genomes · Retrotransposons = encode reverse transcriptase and can move around within the genome by a process that uses reverse transcription and insertion but have no independent life as viruses · Around 42% of the human genome is made up of transposons THE HISTORY . Early 1900s = Wilhelm Ellerman and Olaf Bang found that leukaemia can be transmitted from one chicken to another by injecting leukaemia cell extracts (later named avian leukosis - alpharetrovirus) · 1911 = Peyton Rous showed bacterial free filtrate from solid tumours of chickens could cause identical cancer in chickens inoculated with the filtrate (later named Rous sarcoma - alpharetrovirus) . Early 1970s = reverse transcriptase identified by David Baltimore and Howard Temin THE CLASSIFICATION · Family = Retroviridae . Uses the Baltimore method which is based upon the mechanism of mRNA production · Retroviruses are VI meaning they produce mRNA by: ssRNA-RT > DNA/RNA > dsDNA > mRNA STRUCTURE . See notes for the best diagram - learn this and information attached off by heart · The retrovirus virion is 80-120 nm in diameter · The RNA genome is diploid · Contains two copies of a 7-10 kb ssRNA capped and polyadenylated . They are connected near the 5' ends by H bonds · There are four genes contained within the RNA genome: · Gag (group-specific antigen) = creates MA, CA, NC · Pro = encodes for protease PR which is required in the processing of gag polyprotein into MA, CA and NC · Pol = encodes for enzymes reverse transcriptase, RNase H, integrase · Integrase allows retrovirus DNA to be inserted at random into a host genome - if inserted where there is an oncogene, this cell can become cancerous · Env = encodes for surface glycoproteins and transmembrane proteins REPLICATION CYCLE 1. Attachment (adsorption) 2. Penetration (entry, uncoating) · Via fusion with the PM or via endocytosis · Transfer of DNA to nucleus happens during cell division when nuclear envelop is disassembled 3. Synthesis 4. Assembly (maturation, packaging) 5. Release (budding) . The way the retrovirus acquires its envelope · SU and TM are incorporated into the virion during budding . Most retroviruses do not kill the host cell but establish a persistent infection - host cell produces low levels of virus indefinitely HIV AND AIDS · The genus for HIV is lentivirus - slowly replicating complex retrovirus · HIV1 and HIV2 originated from primates in west-central Africa and transferred to humans via zoonosis · HIV1 may have evolve from simian immunodeficiency virus (SIV) which infects wild chimpanzees · Blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids and breast milk from a person infected with HIV can transmit HIV . These fluids must also come in contact with a mucous membrane or damaged tissue or the bloodstream · The presence of accessory genes allows more vigorous replication of the retrovirus that possesses