Viruses (200nm) Structure Consists of a core containing genetic material DNA or RNA This is surrounded by a protective coat of protein called capsid (subunits: capsomeres) The capsid is (sometimes) surrounded by an envelope of lipoprotein Antigens, glycoproteins on its surface recognize receptors on T-lymphocytes They cause damage by taking over the host cell for multiplication Do not have a cellular structure / don't respire or need food Transmitted via sexual contact; infected woman passing it to her baby through the placenta Also by receiving blood from an infected person Human Immunodeficiency Virus HIV AIDS (Acquired Immune Deficiency Syndrome) All T-helper cells infected (and destroyed) Number of T lymphocytes decrease dramatically / sign for the disease People highly susceptible to infections, diseases and cancer Retrovirus: core contains reverse transcriptase and its genetic material as RNA HIV can change its surface proteins and evade the immune system / vaccination is difficult Cycle of infection HIV enters body from HIV +ve persons via body fluids such as blood or semen Viral glycoprotein attaches to receptors on cell membrane of T-helper cells HIV enters cell by endocytosis, releasing its RNA and reserve transcriptase into the cytoplasm Reverse transcriptase copies viral RNA strand This forms a double stranded viral DNA in the nucleus of T-helper cell / now called "provirus" Viral DNA is integrated into the host DNA / host cell replicates with provirus Latency period (variable period of time) → Infection of more cells, but no symptoms Outbreak: host DNA is transcribed to make new viral RNA. Proteins necessary for the capsid and for the envelope are synthesised by the infected host cell New viruses assembled with RNA and proteins leave the cell by exocytosis - viral envelope is constructed from the cell membrane of the host cell