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Viruses What is a Virus? • Particles of nucleic acid and protein • reproduce by infecting living cells What do all viruses have in common? • They enter living cells and, once inside, use the machinery of the infected cell to produce more viruses. Comparing the size of a virus, a bacterium, and an animal cell Most viruses are so small they can only be seen with the aid of a powerful electron microscope A virus is smaller than a bacterium Virus Bacterium Animal cell Animal cell nucleus 0.25 m Viral Structure A typical virus is composed of a core of either DNA or RNA surrounded by a protein coat Capsid – made of protein DNA or RNA sheath T4 Bacteriophage Viral structure Virus consists of a protein shell called a Capsid The capsid may have many shapes Capsomere of capsid RNA Capsomere DNA Membranous envelope Head Capsid Tail sheath RNA Tail fiber Glycoprotein Glycoprotein 18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses Respiratory infections DNA 80 225 nm 50 nm 50 nm (c) Influenza viruses (d) Bacteriophage T4 How does a virus get inside a cell • The capsid proteins bind to receptors on the host cell • It “tricks” the cell into allowing it inside. • Once inside the viral genes are expressed. • The host cell makes more viruses and the host cell is destroyed. Viruses are very specific to the cells they infect a particular virus can only infect a specific cell. Bacteriophage – infects bacteria Reproductive Cycles of Phages • Two types – Lytic cycle or Virulent • Host cell is lysed (bursts) and immediately destroyed – Lysogenic cycle or Temperate • The virus embeds its DNA into the host’s DNA • The virus’s DNA is replicated along with the host cell’s DNA • A host cell makes copies of the virus indefinitely. The lytic cycle of phage T4, a virulent phage 1 Bacteriophage attaches to 2 The bacterium’s cell wall Bacteriophage injects its DNA into the cell 5 The bursting of the cell Releases new bacteriophage particles Phage assembly 4 New viruses are assembled. The Head Tails Tail fibers 3 Virus wrecks the cell causing it to The cell makes mRNA from the burst Bacteriophage’s genes Lysogenic Infection • The host cell is not immediately destroyed • Viral DNA inserted into the host cell as a prophage A comparison of the lytic and lysogenic cycles of a bacteriophage Phage DNA The phage attaches to a host cell and injects its DNA. Many cell divisions produce a large population of bacteria infected with the prophage. Phage DNA circularizes Phage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle Lysogenic cycle Certain factors determine whether The cell lyses, releasing phages. Lytic cycle is induced New phage DNA and proteins are synthesized and assembled into phages. or Lysogenic cycle Prophage The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. The viral DNA is embedded in the Host’s DNA Retroviruses • Contain RNA as their genetic information • They produce a DNA copy of their RNA – RNA produces DNA – (this is the reverse of what normally happens in healthy cells) • This DNA is inserted into the DNA of the host cell – Example….AIDS Is a Virus a living thing? Some biologists do not consider viruses to be alive because they can’t reproduce independently. Viruses do have some characteristics of a living organism once it infects a host cell, so they are at the borderline of living and non-living things. Viruses and Cells Virus Cell Structure Structure DNA or RNA, core, capsid Reproduction only within a host cell Genetic Code – DNA or RNA Growth and Development – NO Obtain and Use Energy – NO Response to the Environment NO Change Over Time - YES cell membrane, cytoplams;eukaryotes also contain nucleus and organelles Reproduction Independent cell division either asexually or sexually Genetic Code – DNA Growth and Development – YES Obtain and Use Energy - YES Response to the Environment - YES Change Over Time - YES influenza Hand, foot and mouth mumps HPV Ebola Cold sore Chicken pox rubella AIDS Small pox Viral Disease • Diseases in Humans – Polio, measles, AIDS, mumps, influenza, yellow fever, smallpox, the common cold, and many others • Viral Diseases in Humans • Disrupt the bodies normal equilibrium – Symptoms caused by • Toxins produced by infected cells • Toxic components of the viruses themselves • Cells killed or damaged by the virus • Body defense mechanisms fighting the infection Viral Damage • Depends on the ability of infected tissue to repair itself – Epithelium of respiratory tract heals fast so we get over colds – Polio –damage to nerve cells is permanent Cancer causing viruses • Oncogenic viruses – Examples: • Human papilloma virus • Human herpes virus • Hepatitis B Viral Diseases in Animals • Rabies • Parvo • Hoof and mouth disease Viral Diseases in Plants • Have a difficult time invading plants due to the cell wall • Plant injuries make a plant susceptible to viral disease • Insects act as carriers The simplest infectious agents • Viroids – small infectious molecules of single stranded RNA that have no surrounding capsids. – Affect plant cells • Cause stunted plant growth • Prions – protein infectious particles – Linked to several degenerative brain diseases • Ex. Mad cow, Creutzfeldt-Jacob – Transferred in food – Prions are nearly indestructible Ways to protect against viruses • Vaccines • Variants or derivatives of pathogens – Stimulate the immune system to react against the actual disease – Few drugs treat or cure viral diseases • Antiviral drugs that are effective interfere with viral DNA or RNA synthesis • Example polio, smallpox, rubella, mumps, Hepatitis B Most viral infections cannot be cured once they occur – Antibiotics are powerless against viruses • Viruses do not contain the enzymes which antibiotics work against. How the body attacks a virus http://www.bbc.co.uk/science/0/22028517