Viruses

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Viruses are...

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An infectious agent consisting of a nucleic acid molecule within a protein coat

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Viruses require what to see them?

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Most viruses require an electron microscope to see them. Some pox viruses can be seen with a light microscope.

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They require what to replicate?

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a host

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Are they the smallest living things in science?

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Yes

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Where are viruses found?

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Everywhere! ex: ice, soil, air, water

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Where in us do a vast number of viruses live?

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our lungs

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Do we contain more genomes from viruses than our own genes?

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yes

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What do viruses affect? Name an example

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Everything, even the climate!

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Viruses...

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• Move DNA between species
• Provide new genetic material for evolution
• Regulate vast populations of organisms
• Have very simple organization
• Are obligatory intracellular parasites
• Are most frequently host specific
  

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Are considered “infectious agents” because…

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• Viruses produced in one cell can invade others (spread)
• Viruses cause functional alterations of host (death)
  

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What is the biggest mystery of viruses?

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Are viruses very simple microbes or very complex chemicals?

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What are the major viral types?

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Bacteriophages – viruses that use bacteria

Plant viruses – viruses that use plants

Animal viruses – viruses that use animals

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Bacteriophages...

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• Infects and replicates within a bacteria
• Consists of proteins with either DNA or RNA genome
• Replicate after injecting their genome into the cytoplasm of the bacterial cell
• Most common and most diverse group of viruses
• Widely distributed, especially in oceans infecting most marine bacteria
• Phage Therapy
  

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Plant Viruses...

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• Most have RNA rather than DNA
• Most are rod-shaped with spirally arranged proteins on the outside
• Often infect plants through injuries
• Seed and pollen borne
  
• Transmitted through vectors

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Plant Virus Vectors

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Insect borne – e.g., leaf hoppers

Nematode borne – e.g., root feeders

Protozoan borne – e.g., parasites of roots

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Animal Viruses...

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• RNA and DNA genomes
• Often have an Outer Envelope (membrane) with projecting spikes of proteins which help virus enter and leave the cell
• RNA enveloped virus get their envelope from the host cell membrane and reproduce in the cell’s cytoplasm
• DNA enveloped virus get their envelope from the host nuclear membrane and reproduce in the cell’s nucleus
  

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Influenza

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• Killed ~80 million people in 100 years

• Responsible for ~1/3rd of death among soldiers in WWI
  

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Smallpox

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• death toll unknown
• killed millions in Europe over centuries
• devastated the Native Americans after European contact, killing an estimated 90% of the indigenous population

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Yellow fever

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• death toll unknown
• likely killed thousands in Central and North America
• Along with Malaria it nearly prevented the construction of the Panama Canal
  

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Viral Shapes

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• Helical capsid – e.g., Tobacco Mosaic virus
• Icosahedra capsid – most common type e.g., rhinovirus
• Bullet-shaped capsid – e.g., rabies virus
• Lunar Lander shape – e.g., bacteriophages
  

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Enveloped Viruses

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• Some viruses create an envelope around their capsid and nucleic acid

• The envelope is made of membrane phospholipids, and importantly glycoproteins

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How do viruses become enveloped?

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• The envelope is derived from portions of the host’s cell membrane
  
• The glycoproteins identify and bind to receptors on the host membrane
  
• The viral envelope then fuses with the host membrane and the viral capsid and genome enter the cell
  

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Naked Viruses

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• Naked viruses do not have an enclosing envelope and consist only of the capsid and nucleic acid
• Have a protein coat with sub units called capsomeres
• The capsomeres have adhesins to attaches the virus to the host cell surface
• Naked viruses are known to last a long time in the environment
  

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Examples of Enveloped Viruses

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Examples: herpes viruses, poxviruses, hepatitis, and retroviruses.

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Examples of Naked Viruses

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Examples: poliovirus and papillomavirus.

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Endocytosis is when...

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host cells absorb important proteins through a process of engulfing them because big molecules can’t pass through the cell membrane

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How do viruses exploit Endocytosis?

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• masquerading as important molecules (Trojan Horse) to get absorbed into host cell

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What does a virus do once it's inside of a cell?

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Uses the cell to replicate

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What determines the family of a virus?

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DNA or RNA, single or double stranded, with or without an envelope

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What is Papillomavirus and how is it passed?

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• skin warts, genital warts, cervical cancer
• passed via host skin and genital contact
  

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What are the Rhinoviruses and how are they passed?

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• common colds
• passed via surface contact
  

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What are the Polioviruses and how are they passed?

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• poliomyelitis
  
• passed via contact with feces contaminated food or surfaces and rarely via oral contact persons to person
  

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Mutation rates of DNA viruses

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• DNA viruses mutate more slowly than RNA viruses
  
• Enzymes used to copy DNA (DNA Polymerases) can proofread/edit, correct mistakes, make fewer mistakes, thus fewer mutations
  

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Mutation rates of RNA viruses

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• RNA viruses mutate more quickly than DNA viruses
  
• Enzymes which copy RNA (RNA Polymerases) do not proofread/edit, therefore make many mistakes that equal mutations
  

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Mutation rates of both types of viruses

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• DNA viruses (e.g., Herpes, pox virus, HPV): 1 mutation/100,000,000-10,000,000,000 nucleotides
• RNA viruses (e.g., HIV, influenza virus): potential for 1 mutation/10,000-100,000 nucleotides (at least 1,000 times more mutations than DNA viruses)
  

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Are viruses the fastest evolving thing on Earth?

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yes