Lecture 2

• What is a virus?

  a tiny obligate parasite that cannot manage themselves nor replicate without a host cell.

  Comes from a latin name that means toxic/poison
• Viruses are made from?

  only one type of material. Either DNA or RNA
• Why do they only have one type of material?

  since they cannot replicate without a host, why should they bother in having parts they do not need?
• Main function of a virus?

  to deliver its DNA or RNA to a host cell, infect it so it can be expressed (transcribe and translate by said host cell)
• What does it infect?

  animal cells, bacteria *bacteripohages, human cells
• Similarities between bacteria and viruses

  Both extremely small, need an electron microscope to be seen with naked eye.
• Differences between bacteria and viruses

  bacteria:

  • self-replicates
  • cellular machinery like ribosomes
  • can has DNA and RNA
  • Prokaryotes

  Virus:

  • cannot self-replicate
  • does not have anything like ribosomes
  • has either RNA or DNA
  • is neither prokaryotic or eukaryotic
• How are they measured?

  nanometers
• Parts of a virus

  1. elvelope
  2. capsid
  3. Nucleic acid
• Envelope

  outermost layer of a virus. Made from glycoproteins

  (not all have one, can be a naked virus)

  • one without are more envirionmentally stable since envelopes help viruses live in the extreme conditions of the body and will weaken the virus when not inside an organism
• Capsid

  protein shell that protects nucleic acid.

  • gives structure and symmetry to virus
  • either icosahedral, helical or spherical
• How is it replicated?

  1. attachment
  2. entry and uncoating
  3. replication and assembly
  4. egress and release of the virus
• Ways of entry?

  Direct fusion

  Endocytosis

  Receptor mediate entry

  Nucleic acid translocation (rare and for non-enveloped)
• Direct fusion

  no receptor needed, can infect any cell

  the virus binds to the host's plasma membrane and the nucleic acid is released to the inside
• Endocytosis

  only specific cells

  whole virus cell is swallowed by the host cell. It becomes a vacuole then transported to an endosome then releases its nucleic acid
• Receptor mediate entry (HIV, Hep)

  receptors are on host's cells attract/bind to virus. this allows the whole cell goes into the host cell. this will change the structure of cell to further help the entry
• Nucleic acid translocation

  capsid attaches to host cell membrane, virion gets rearranged to allow the nucleic acid to be released in the host cell
• Lytic viral lifestyle?

  steps:

  1. bacteriophage attaches to host cell's cell wall
  2. bacteriophage injects its material into host
  3. the material takes over metabolism to force it to make proteins and nucleic acids that will make new bacteriophages
  4. the proteins and nucleic acids start to make new particles
  5. the bacteriophage's enzyme breaks the host cell's wall releasing the new particle that will then infect more cells to start the lytic viral particles
• Lysogenic viral lifecycle

  steps:

  1. bacteriophage material enters cell
  2. the material mixes with the host cell's DNA
  3. cell then goes through regular cell division that will spread the viral DNA to more and more daughter cells
  4. the bad DNA will stay in cells doing nothing until something triggers it to start the lytic cycle again
• Classifying viruses

  NUcleic acid, DNA/RNA, single-stranded or double stranded, how it replicates

  DNA = mos stable, RNA = less stable, more likely to mutate
• dsDNA examples

  all members of herpes, cytomegalovirus
• dsDNA viruses replication

  turned into viral mRNA by VIRAL polymerase. mRNA is then translated into proteins and enzymes to allow new viruses particle production
• RNA replication

  much simpler than dsDNA, some can act directly as mRNA and be read by ribosome without being transcripted. Some need more replication enzymes with them into host cells
• RNA viruses?

  examples:

  1. rotavirus
  2. poliovirus
  3. west nile virus
  4. influenza virus
  5. hepatitus A virus
• Latent viral infection

  viral genomes are present in cells, but not infectious versions, without the destruction of the infected cell

  Herpes simplex virus
• Integration of viral DNA

  some viral material can become one with the host's material (latent infection) these can start replicating later on and cause infection
• Negative sense RNA viruses

  must be converted into positive stranded genome before anything else can happen (before mRNA).
• positive RNA viruses

  can directly be turned into mRNA that can be turned into proteins (SARS-CoV-2 and other coronaviruses)
• Retroviruses

  RNA -> DNA -> RNA

  • reverse transcriptase enzyme makes a single stranded of viral DNA complementary to the retro viral RNA (cRNA)
  • ssDNA copied to make complementary DNA
  • dsDNA then enters host cell nucleus and later infects
• Serologic responses

  detection of the immune response by the host against the infectious agent. Finds specific immunoglobulin (host antibody response).

  Can be used when virus is at low levels soon after infection