Lecture 31 Viruses Viruses Viruses are tiny non cellular parasites that infect virtually every type of cell known They cannot perform metabolism on their own meaning outside a parasitized cell and are not considered to be alive Different types of viruses are specialized for infecting particular species and types of cells Smaller than bacteria and bigger than ribosomes Why study viruses Viruses are ubiquitous and abundant Everything gets infected by viruses sometimes even viruses Viruses are very small relative to eukaryotic or even bacterial cells 1um Viruses are ubiquitous in natural environments greatly exceeding cellular life forms in abundance In a sample of seawater treated to nucleic acids fluoresce the smallest lights are viruses Viruses Organisms Hereditary material DNA or RNA can be single stranded or double stranded DNA always double stranded Plasma membrane present No Can carry out transcription independently No even if a viral polymerase is present transcription of viral genome requires use of ATP and nucleotides provided by host cell replicates with the host s machinery Yes Yes Yes Can carry out translation independently No Metabolic capabilities Virtually none Extensive synthesis of ATP reduced carbon compounds vitamins lipids nucleic acids etc Discovery of viruses Virus poisonous fluid In 1892 a Russian scientist named Dmitry Ivanovsky was studying tobacco mosaic disease which destroys the leaves of tobacco plants The disease was clearly infectious plants that came into contact with the sap from diseased plants were damaged as well This ability to reproduce itself eliminated the possibility that the damaging agent might be a simple toxin Ivanovsky ran an extract of diseased leaves through a very fine filter with pores small enough to trap any known bacteria But he found that whatever caused the disease went right through his filter By the 1930s filters could be finally be manufactured with pores tiny enough to prove that viruses are particulate after all rather than being fluid in nature The earliest electron microscopes also appeared in 1930s and viruses could at last be seen Recent Viral Epidemics in Humans Viruses have caused the most devastating epidemics in recent human history An epidemic is a disease that infects a large number of people in a local geographic area Smallpox A pandemic is a disease that affects people worldwide Of all human infectious diseases smallpox caused by the Variola virus is believed to have resulted in more human deaths throughout history than from any other single pathogen In 18th century Europe 400 000 people died annually of smallpox Case fatalities ranged from 20 to 60 adults and up to 80 90 for infants Unknown in the New World smallpox devastated Native American populations and was instrumental in the fall of the Aztec and Inca empires and in the European settlement of NOrth America During the French Indian War 1754 1767 Sir Jeffrey Amherst the commander of the British forces suggested the deliberate use of smallpox to diminish the Native American population hostile to the British Historical Attempts to Induce Immunity Against Smallpox Variolation In China 16th century physicians developed the technique of variolation the deliberate infection with smallpox Dried smallpox scabs were blown into the nose of an individual who then contracted a mild form of the disease Upon recovery the individual was immune to smallpox Between 1 to 2 of those violated died as compared to the 30 who died when they contracted the disease naturally By 1700 variolation had spread to Africa India and Ottoman Empire Vaccination Edward Jenner an English Physician used folk knowledge to find an alternative to variolation Key observation dairymaids infected with cowpox were immune to smallpox Test Jenner deliberately infected James Phipps an 8 year old boy with cowpox in 1796 He then exposed Phipps to smallpox which Phipps failed to contract After repeating the experiment on other children including his own son Jenner concluded that vaccination provided immunity to smallpox without the risks of variolation Jenner s findings were published in 1798 How Vaccination Works 1 Viral antigens are introduced into body 2 Viral antigens bind to receptors on certain immune system cells macrophages 3 These cells stimulate other immune system cells D Cells to produce antibodies to the virus 4 Later if the host organism is exposed to actual virus particles the antibody producing cells are activated The virus particles become coated with antibodies 5 Viruses that are coated with antibodies are destroyed by immune system cells Antigen any foreign substance usually a protein that stimulates the body s immune system to produce Antibodies specialized proteins that tag destroy or neutralize bacteria viruses or other harmful invaders Inoculation have provide long term immunity against reinfection booster shot It takes approximately 2 weeks to mount an effective immune response to a new antigen after that the response time is much quicker leading to a period of effective immunity enhanced response also retained for many years and remain effective unless the pathogen changes its antigenic profile it evolves In 1996 the World Health Organization WHO initiated a program to eradicate smallpox from the world The last reported case of smallpox was in Somalia in October 1977 An important factor in the success of eradicating smallpox was that humans are the only host and there are no animal reservoirs for smallpox virus anthropomorphic Currently the only official stores of Variola are in freezers of 2 research sites one in the Centers for Disease Control CDC in Atlanta and one at the VECTOR research facility in Russia Viruses are highly diverse in overall morphology and in the nature of their genetic material Nonenveloped virus Genome inside Capsid protein bursting of enveloped viruses Enveloped virus genome inside capsid protein inside envelope phospholipid bilayer budding of enveloped viruses Nature of genetic material several possibilities DNA single stranded ss or double stranded ds RNA single stranded ss or double stranded ds Enveloped viruses are released from the host cell encapsulated in the host cell s membrane why might this be advantageous makes it harder for immune cells to recognize and attack it virus bc it is disguised as in normal human cell membrane Virus persists longer in body Lytic Replication The viral infection cycle can be broken down into 5 steps 1 Entry into host