viruses intro

what are viruses classified as

obligate intracellular parasites (must live in something)

abiotic biological entities

microbes but not microorganisms

what things do viruses not have

metabolism, reproduction, or motility

inert outside of cells (inside cells, direct the activities of the cell to replicate)

parts of virus

1. nucleic acid

   • DNA or RNA

   • circular or linear

   • single or double stranded

2. capsid

   • protein coat

   • composed of simple identical subunits called capsomeres

3. spikes

   • attach to receptor cells on host cells

nucleocapsid = capsid plus nucleic acids

enveloped v. non-enveloped (naked) viruses

• enveloped = surrounded by lipid bilayer obtained from host cell

  • host membrane must have spikes that the virus can take with it when it leaves

  • more vulnerable

  • animal viruses only

• non-enveloped (naked) = no membrane, protein capsid only

  • more resistant to disinfectants and other damaging agents

smallest and largest viruses

smallest = poliovirus (30nm)

largest = mimivirus (800nm)

virus size

20-400nm

too small to be seen with light microscope

what are the three morphologies of viruses

1. icosahedral

2. helical

3. complex

   • phage

   • pox

animal viruses = icosahedral or helical

enveloped = not actually a morphology, obscures underlying morphology

icosahedral morphology

• 20 identical sides

• up to 5 different capsomeres

helical morphology

• capsomeres arranged in helix

• not a lot of different ones

complex morphologies

1. phage

   • viruses that infect bacteria

   • icosahedral nucleocapsid head and helical protein tail (woot combo)

   • variable attachements

   • need tail fibre or spike to penetrate peptidoglycan wall

2. poxvirus

   • the only animal virus with a complex morphology

   • unique structure

   • cannot cause chronic infection

host range and specificity

viruses can only infect cells with correct receptor — this leads to specificity

*affects other pathogens too, but most limiting for viruses

tropism

not every cell displays the same profile of receptors (depends on location)

levels of specificity

• species level

• tissue level

types of bacteriophages

1. lytic (virulent)

2. temperate

3. filamentous

productive v. nonproductive viruses

• productive = always generate new viruses

  • as may as possible quickly with goal to kill host cell OR slow release so that host survives

• non-productive = integrates into DNA and lies dormant unless found by host, in which it kills the host cell

lytic (virulent) phages

• productive (new viral particles formed) infections

• culminate in destruction of host cell — lysis of host cell releases many new infectious particles that have been assembled in the cell

• not usually dangerous to humans bc we are multicellular

• dsDNA usually

filamentous phages

• ssDNA

• look like long fibers

• productive infections

  • host cells not killed, slow multiplication of viruses

• slow release of new viruses, extrude through bacterial membrane

• HIV and hep B = devastating chronic infections

temperate phages

• two possible outcomes

  1. lysogenic infection = incorporate their DNA into host genome

     • infected cell is lysogen

     • integrated phage DNA is prophage

     • when cell divides, prophage is replicated

  2. lytic infection = occurs if the prophage is excised from host genome

     • phage induction = bacterium realizes it is infected and excises DNA, leading to lytic form of infection

     • excision is rare

     • when it occurs, often flanked by bacterial DNA = easy integration into new host (new genes!)

lysogenic conversion

viruses released by lytic stage of temperate phage infection take some host bacterial DNA with them — can integrate into new host, introducing new genes to that bacterium

often carry nasty virulence genes

viral classification

• baltimore system

  • splits viruses by nucleic acid components

• international committee on taxonomy of viruses (ICTV)

  • considers morphologies, envelopes, genome structure, and other factors

DNA viruses

• often chronic or latent within host

• associated with cancer

DNA viruses — naked v. enveloped

dsDNA

• naked

  • adenovirus

  • papillomavirus

  • polyomavirus

• enveloped

  • hepadnavirus

  • herpesvirus

  • poxvirus

ssDNA

• naked

  • parvovirus

• no enveloped

RNA viruses

• cause acute infections — either die or recover

• classified by + or - sense

RNA viruses — naked v. enveloped

ssRNA

• + sense

  • naked

    • calicvirus

    • hepevirus

    • picornavirus

  • enveloped

    • coronavirus

    • flavivirus

    • retrovirus

    • togavirus

• – sense

  • enveloped only

    • arenavirus

    • bunyavirus

    • filovirus

    • orthomyoxovirus

    • paramyoxovirus

    • rhabdovirus

dsRNA

• naked only

  • reovirus

naked RNA viruses

• calcivirus (ss, +)

• hepevirus (ss, +)

• picornovirus (ss, +)

• reovirus (ds)

naked DNA viruses

• adenovirus (ds)

• papillovirus (ds)

• polymavirus (ds)

• parvovirus (ss)

enveloped DNA viruses

• hepadnavirus (ds)

• herpesvirus (ds)

• poxvirus (ds)

steps of viral replication (DNA or RNA the same)

1. recognition and attachment

2. penetration and uncoating

3. synthesis (new viral parts)

4. maturation (self assembly)

5. release (exit)

viral replication — attachment

• viruses bind to receptors on host cell surface

• often more than receptor attachment required

  • normal function of receptor unrelated to viral infection

• specific viruses target specific receptors

  • COVID requires ACE2 receptor

  • HIV requires CD4 and CXCR4 OR CCR5

viral replication — penetration and uncoating

• enter into host cell completely, unlike bacteriophages

• enveloped = enter via fusion or endocytosis

• naked = enter via endocytosis only

viral replication — synthesis

• expression of viral genes = to produce structural and catalytic components for virus

  • capsid protein

  • enzymes required for replication

  • host subversion proteins

• synthesis of genome copies

  • multiple copies of genome always made

  • DNA viruses = replication in nucleus

  • RNA = in cytoplasm

  • reverse transcription

viral replication — maturation

• spontaneous self-assembly occurs with nucleic acid and capsid

  • only need proximity

  • no help from host cell

• maturation = some need finishing touches after assembly

• site of assembly varies with virus type

viral replication — release

• enveloped = budding

• naked = lysis

acute v. persistent infections

acute

• rapid onset, short duration

• burst of virions released from infected host cell (lytic infections)

• die or recover

persistent

• continue for years or lifetime

• may or may not have symptoms

• chronic (continuous production of virions - HIV, hep b) or latent (viral genome remains silent until a precipitating event occurs - herpes)

viruses and cancer

viruses can lead to cancer by:

1. converting proto-oncogene to oncogene when inserting into host cell genome

   • herpes

2. chronic infection damages DNA (inflammation)

   • hepatitises

3. virus itself carries oncogene

   • never been witnessed

BUT VIRALLY INDUCED TUMORS ARE RARE