Chapter 6 - Acellular Pathogens

viruses are

obligate intracellular parasites (can’t live without a host)

host range

each species of virus infects a particular group of host species

tissue tropism

the range of tissue types that a virus can infect

viruses contain either

DNA or RNA (not both)

viral specificity

the kind of cells which a virus can infect (e.g. papillomaviruses can only infect skin cells)

viral transmission

direct contact, indirect contact, or via a vector

zoonose

when a virus is transmitted from an animal host to a human host

reverse zoonose

when a virus is transmitted from a human host to an animal host

virion

complete fully developed viral organism

4 structures of a virion

1.) Nucleic acid

2.) Capsid

3.) Envelope

4.) Spikes

nucleic acid of virion

DNA or RNA (ss or ds); linear or circular

capsid of virion

protein coat made up of capsomeres (subunits), protects and encloses the nucleic acid

envelope of virion

lipid, protein, carbohydrate coat of some viruses, bilayer membrane, acquired when they bud from the cell

spikes of virion

glycoproteins, projections from outer surfaces and binding sites

nucleocapsid

viral genome and capsid

naked viruses

viruses without envelopes

tegument

the space between the envelope and capsid that may contain other proteins

4 general morphologies of viruses

1.) Helical viruses

2.) Polyhedral viruses

3.) Enveloped viruses

4.) Complex viruses

helical viruses

hollow cylindrical capsid that is helical, ex. rabies and ebola

polyhedral viruses

“many sided”, most are icosahedron (20 triangular facets), ex. adenovirus and poliovirus

enveloped viruses

roughly spherical, ex. HIV

complex viruses

complicated structures, ex. bacteriophages

taxonomy/classification of viruses shift

originally viruses were classified based on the type of host that was infected → shift to Baltimore Classification System based on genome type and mRNA generation

Baltimore Classification System - genus names end in ___, family names end in ___, and order names end in ___

• genus names end in -virus

• family names end in -viridae

• order names end in -ales

viral species

a group of viruses sharing the same genetic information and ecological niche host

7 realms / classes of Baltimore model

1.) Class 1: dsDNA

2.) Class 2: ssDNA+

3.) Class 3: dsRNA

4.) Class 4: ssRNA+

5.) Class 5: ssRNA-

6.) ssRNA+ with reverse transcriptase

7.) dsDNA with reverse transcriptase

class 1: dsDNA

double-stranded DNA is transcribed to mRNA, enveloped

class 2: ssDNA+

generates an intermediate (dsDNA) form to be transcribed to mRNA

class 3: dsRNA

similar to replication as class 5 (dsRNA makes mRNA by using RNA dependent RNA polymerase)

class 4: ssRNA+

genome is mRNA → ssRNA+ makes a complementary (-) strand, which is transcribed to mRNA

class 5: ssRNA-

RNA replicase makes (+) sense strand which is used as mRNA template for more (-) strand genomes

class 6: ssRNA+ with reverse transcriptase

ss(+)RNA virus, replicates through DNA intermediate → reverse transcriptase enzymes copy RNA into DNA (HIV)

class 7: dsDNA with reverse transcriptase

replicates through RNA intermediate with reverse transcriptase

virus multiplies in the host cell’s cytoplasm by using

RNA dependent RNA polymerase (enzyme)

ssRNA+ sense (cytoplasm)

viral RNA serves as mRNA for protein synthesis of the viral components; genome can act immediately as mRNA and translated into proteins

ssRNA- antisense (cytoplasm)

viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis; genome needs to be transcribed into + sense RNA (complementary)

6 steps of multiplication of animal viruses

1.) Attachment of viruses to the cell membrane

2.) Entry by receptor-mediated endocytosis fusion

3.) Uncoating (separation of nucleic acids from capsid) by either viral or host enzymes

4.) Biosynthesis (production of nucleic acids and proteins)

5.) Maturation (nucleic acids and proteins assemble into the virus)

6.) Release via budding (enveloped virus) or rupture (nonenveloped virus)

7 steps of multiplication and inheritance of retroviridae class 6 (HIV)

1.) HIV attaches/fuses to host cell

2.) HIV RNA, reverse transcriptase, integrase, and other viral proteins enter host cell

3.) Viral DNA is formed by reverse transcription via reverse transcriptase

4.) Viral DNA is transported across the nucleus and integrates into host DNA

5.) New viral RNA is used as genomic RNA and used to make viral proteins

6.) New viral RNA and proteins move to cell surface and a new immature HIV forms

7.) Virus matures when protease releases proteins that form the mature HIV

latent viral infections

may initially cause an acute infection before becoming dormant; once dormant, virus remains hidden dormant inside the cell

chronic viral infections

recurrent or persistent symptoms

viroids

short pieces of naked RNA, causes potato spindle tuber diseases, consists of circular ssRNA, exists inside cells as just particles of RNA lacking capsid or envelope, RNA does not produce proteins

virusoids

viroids enclosed in a protein coat - only cause disease when plant cell is coinfected with a virus

prions

proteinaceous infectious particles - inherited and transmissible by ingestion, transplant, and surgical instrument - Creutzfeldt-Jakob disease - normal proteins which become folded incorrectly possibly due to mutation