Tortora Chapter 13

1
Tortora Chapter 13

• Viruses, Viroids, and Prions

2
Characteristics of Viruses

• Obligate intracellular parasites-require a host
• Contain EITHER RNA or DNA but not both
• Contain a protein coat
• May have an envelope of lipids, proteins, and
  carbohydrates
• Use the hosts machinery to multiply
• Cause the synthesis of specialized structures
  than can transfer the nucleic acid to other cells
  (infect other cells)

3
Host range

• Host cells a virus can infect
• Most can only infect one type of host species
• Some can jump species and expand range (these
  are particularly bad)
• May infect plants, invertebrates, bacteria,
  fungi, humans, other vertebrates
• May now use viruses (phage therapy) to treat
  diseases

4
Sizes

• Very small
• Called filterable because they usually pass
  through filters
• 20 to 1000 nm in length
• A nanometer (nm) is 1/1,000,000th of a mm

5
Structure

• Virion is a complete and fully formed viral
  particle that hasnt attacked a host yet
• Nucleic acid may be single or double stranded RNA
  or DNA but NOT both
• Total amount of nucleotide bases varies from a
  few thousand base pairs up to 250,000

6
Capsid and envelope

• A protein capsid protects the nucleic acid
• A capsid contains pieces called capsomeres
• The capsid may be covered by an envelope which
  has lipid, carbohydrate, and protein.
• Sometimes this is actually a piece of the hosts
  plasma membrane (animal viruses)
• Some have spikes which protrude from the envelope
  
• Spikes can help some viruses clump red blood
  cells (hemagglutination)
• Viruses with no envelope are called naked viruses
  or nonenveloped

7
morphology

• Based on capsid structure
• Helical viruses look like long rods (rabies and
  Ebola)
• Polyhedral viruses are many sided with 20
  triangular faces and 12 corners (poliovirus and
  adenovirus)
• Enveloped helical-influenza
• Enveloped polyhedral-herpes simplex
• Complex viruses-a bacteriophage has other
  structures like tail sheath

8
Taxonomy

• Viruses are named by nucleic acid type, strategy
  for replication, morphology
• Once used symptomatology-virus named for the
  disease symptoms
• Use suffix virus for genus name
• Use viridae for family groups
• Use ales for orders
• (Herpesviridae, Simplexvirus, human herpesvirus 2)

9
Taxonomy continued

• A species is a group of viruses sharing the same
  genetic information and host range
• Dont use Latin names, use common names HIV
  (human immunodeficiency virus)
• See table13.2 for family names and descriptions

10
Identifying viruses

• Must use living cells so it is tricky
• Bacteriophages are easiest because they grow in
  bacteria on culture dishes
• Plaque method-as new viruses destroy bacteria it
  leaves a hole where a colony would have been
  called a plaque.
• Count number of holes-plaque forming units (PFU)

11
Growing Animal Viruses

• May use living animals but sometimes the virus
  doesnt show any symptoms in the research animal
  (HIV-1 can grow in chimps but doesnt have
  symptoms so cant really study it)
• Found a way to study viral replication in 1990
• infect mice with human HIV by using
  immunodeficient mice that were grafted to produce
  T cells and gamma globulin

12
More methods

• May use embryonated eggs
• Drill hole in egg shell and inject virus into
  fluid of egg in appropriate membrane
• Death or damage to embryo signals viral growth
• Another is cell culture technique-NOW PREFERED
  METHOD
• Cells form monolayer-viruses cause CPE
  (cytopathic effect) which is similar to plaques

13
Cell lines

• Primary cell lines-come from tissue slices and
  die out quickly after a couple of generations
• Diploid cell lines developed from human embryos
  can live 100 generations and are used to culture
  viruses that require a human host
• Continuous cell lines-immortal (HeLa cell line)
  are transformed cancer cell that never die out

14
Multiplication of bacteriophageshttp//biology.ab
out.com/gi/dynamic/offsite.htm?sitehttp//faculty
.ccbcmd.edu/courses/bio141/lecguide/unit3/viruses/
adlyt.html

• Use one of two mechanisms
• Lytic cycle-kills host
• Lysogenic cycle-host remains alive
• Lytic cycle (as demonstrated by T-even
  bacteriophages)
• 1. attachment-(adsorption)-attachment site on
  virus attaches to complementary receptor site on
  bacteria cell wall
• 2. penetration-injects DNA into bacterium like a
  hypodermic syringe

15
Lytic cycle continued

• 3. biosynthesis-host proteins synthesis stops
  and viral protein synthesis begins
• An eclipse period where viral proteins and DNA
  are present but no complete viral particles are
  found occurs for several minutes following
  infection
• 4. maturation-assemble the virions
• 5. release-lysis occurs due to lysozyme and
  virions are released

16
Lysogeny (lysogenic cycle)

• Lysogenic phages or temperate phages do not cause
  the death of the host
• Instead, they penetrate into host
• The phage DNA forms a circle that can multiply
  and be transcribed leading to lysis eventually OR
• The phage DNA circle can recombine with the host
  DNA (call this a prophage)
• It hides in host DNA and repressor genes turn
  off the directions for synthesis of new virions
• As the host DNA replicates, so does the virus
• Eventually the prophage pops out and lytic
  cycle occurs

17
3 results of lysogeny

• Lysogenic cells are immune to reinfection by the
  SAME phage
• Phage conversion may occur where the host cell
  gets new properties
• Corynebacterium diptheriae produces toxin only
  when it is infected by a phage
• Specialized transduction may occur where the
  lysogenic phage packages bacterial DNA along with
  its own and when the prophage leaves, bacterial
  genes remain attached to the phage DNA

18
Multiplication of Animal Viruses

• Differ on how they get in a host
• 1. Attachment-complementary receptor sites but
  are proteins and glycoproteins in the plasma
  membrane
• These receptors are inherited characteristics of
  the host, so receptor sites vary from person to
  person
• 2. Entry-enter by pinocytosis or enveloped
  viruses may enter by fusion (HIV enters this way)
• See figure 13.14 or click here http//faculty.ccbc
  md.edu/courses/bio141/lecguide/unit3/viruses/penfu
  sion_ev_fl.html

19
Multiplication continued

• 3. uncoating-separating capsid from nucleic acid
  (lysosomal enzymes help this)
• 4. Biosynthesis of DNA viruses-usually replicate
  DNA in nucleus of host by using viral enzymes,
  then make the capsid and other components in the
  cytoplasm
• Biosynthesis of RNA viruses-RNA viruses multiply
  in host cytoplasm, a sense strand (the RNA) is
  translated into 2 proteins, another strand of RNA
  is made (antisense), which serves as a template
  for more sense strands

20
Continued

• 5. maturation and release
• assembly of capsid
• add envelope by budding when viral particle
  pushes out and is folded around the virus
  http//faculty.ccbcmd.edu/courses/bio141/lecguide/
  unit3/viruses/release_bud_fl.html
• Nonenveloped are released thorough ruptures in
  cell membrane and usually kills cell

21
Viruses and Cancer

• Some cancers are caused by viruses (10)
• A human cancer causing virus was isolated in 1972
  by Sarah Stewart
• Oncogenes are turned on by cancer inducing
  viruses or other mutations
• Tumor cells undergo transformation and change
  properties
• Often have tumor-specific transplantation antigen
  (TSTA) on surface of cells

22
Latent Viral infections

• Remain in equilibrium for years and not cause
  infections
• Oncogenic viruses, herpesviruses are examples
• Chickenpox (varicella) is a skin disease of
  childhood. Occasionally the virus may enter
  nerves and go into the latent phase
• They reappear years later as shingles (zoster)

23
Other weird and strange

• Chronic viral infections-some people develop SSPE
  (subacute sclerosing panencephalitis) years after
  measles.
• Scrapie in sheep
• Prions-proteinaceous infectious particles
• animal diseases including mad cow disease
• Human disease includes kuru, Creutzfeldt-Jakob
  disease (CJD)
• Viroids-naked RNA that causes plant disease

24
misc

• Burst time-how long it takes from attachment to
  lysis
• Burst size-number of new viral particles
  released
• Damage to host includes CPE, inclusion bodies
  (masses of viruses or damaged organelles), and
  negri bodies in rabies
• Chronic latent state in animal viruses (virus
  reactivates-herpes) This is lysogeny in animal
  virus and forms provirus.