-
viruses
obligate intracellular parasites
-
capsid
outer protein coat surrounding nuceleic acid
-
purpose of capsid
- protects nucleic acid from attack
- attaches to a specific receptor site of a cell membrane
-
structure of the virion
- capsomeres
- nucleocapsid: capsid and nucleic acid
-
capsid designs
- polyhedral/ icosohedral: 20 faces, each is an equilateral triangle, nucelic acid within capsid
- helical: nucleic acid attached directly to the capsid
- complex structure: ex. binal form in bacteriophages
-
envelope
- cover capsids
- derived from portions of host cell membranes
- help viruses enter host cells
- viruses without envelopes (naked viruses) - have better survival outside of cell
-
polymerase
enzyme viruses use for replication of nucleic acid
-
four classes of polymerases
- RNA dependent RNA polymerase (replicase)
- RNA dependent DNA polymerase (reverse transcriptase)
- DNA dependent DNA polymerase
- DNA dependent RNA polymerase
-
viral attachment proteins of non-envelope viruses
it is one of the surface proteins that forms the capsid
-
viral attachment proteins of envelope viruses
protrudes from viral envelope
-
purpose of penetration and uncoating
- must penetrate so virus can replicate within cells
- uncoating so that genome is exposed
-
methods of penetration
- injection of nucleic acids by nonenveloped bacteriophages
- translocation- transit of nucleic acid across cell membrane by nonenvelope viruses
- membrane fusion by fusion factors by enveloped viruses
- endocytosis/phagocytosis - virus is engulfed by cell by enveloped and nonenveloped viruses
-
replication of viral proteins
- dna -> mRNA-> proteins
- transcription translation
-
replication of genome
replication strategy depends on type of genomic material
-
Group I: (±) dsDNA
use (-) DNA strand as template for mRNA
-
Group II: (+) ssDNA
- makes dsDNA
- use (-) DNA strand as template for mRNA
-
Group III: (±) dsRNA
use (-) DNA strand as template for mRNA
-
Group IV: (+) RNA
- make (-) RNA
- use (-) RNA strand as template for mRNA
-
Group V: (-) RNA
use (-) RNA strand as template for mRNA
-
Group VI: (+) RNA
- make (-) DNA copy using reverse transcriptase
- makes dsDNA
- use (-) DNA strand as template for mRNA
-
Group VII: (±) dsDNA
- make RNA use (-) DNA strand as template for mRNA
- make (-) DNA copy using reverse transcriptase
- make dsDNA
- use (-) DNA strand as template for mRNA
-
assembly
- replicated viral nucleic acid and viral capsid find each other within the host cell and combine, facilitated by - localization, chemical attraction
- location of assembly of nucleocapsids
- insertion of peplomers
- enzymes
-
release
- budding through plasma membrane or some other membrane - does not kill host cell
- lysis of cell (lytic infection) - kills host cell
-
virus can't infect cell or non-productive infection
- no virus receptor on cell surface
- no suitable replication enzymes and protein synthesizing machinery - abortive infection, non-permissive infection
- latent infection - virus incorporates itself into host genome and becomes provirus/prophage and enters lysogenic state which lies dormant until lytic state
-
host range
- kingdom level (animal, plant, bacteria)
- species level
- tissue level
- viruses that infect other microorganisms
-
classification of viruses based on
- type of nucleic acid
- mode of mRNA production (baltimore scheme)
- type of capsid
- enveloped or naked
- type of enzymes
- replication pattern
- nucleic acid sequence
-
viroids
- free, short single stranded RNA lacking protein coat
- causes numerous plant diseases
- causes hepatitis in humans
-
prion
- protein in a misfolded form
- causes scrapie, kuru, Creuztfeldt-Jacod disease (CJD), transmissible (bovine) spongiform encephalopathy
|
|
