VIRUS PROPERTIES• Infectious – must be transmissible horizontally• Intracellular – require living cells• RNA or DNA genome, not both*• Most all have protein coat*• May of may not have lipid envelope• May have broad or narrow host range• Replication involves eclipse (breaking apart of

virus particles) and reassembly• Use host factors for to complete replication cycle

SOME CONSEQUENCES AND EFFECTS OF VIRUS INFECTION

• Like other life forms, viruses promote the propagation of their own kind

• Like other life forms, viruses evolve in response to selection pressure

• Viruses are major factors in promoting the evolution of higher organisms

• Viruses help control populations of their hosts, including humans

Virus-like agents classified and studied with viruses

• Viroids– No coat protein, no coding capacity

• Prions– No nucleic acid (?)

• Retrotransposons– No infectivity (?)

Host properties influence the virus types found in that host group

• Vertebrates have broad range of viruses

• Plants have mostly small RNA viruses

• Fungi have mostly dsRNA viruses

• Single-celled organisms have mostly large dsDNA viruses

Viruses aresubmicroscopic intracellular pathogens

Flint et al., 2004Principles of VirologyFig. 1.8

Sizes of microscopic and submicroscopic biological entities and their ability to be examined using various technologies

Viruses may be simple or complex

• Genome sizes 0.3 - 1200 kb; average genome sizes vary with host organism types

• Isometric particle sizes vary from ~24 nm to ~400 nm diameter

• May have single-stranded (ss) or double-stranded (ds) RNA or DNA genome

• If ssRNA, may be + or – sense

• May have one or many proteins in particles

• May or may not have lipid envelope

Types of viral genomes• double-stranded (ds) DNA

– Rarely segmented– Often large

• single-stranded (ss) DNA– Rarely large– Less common than dsDNA

• ssRNA, negative sense– Often found in viruses with broad host ranges

• ssRNA, positive sense– Most common overall

• dsRNA– Often segmented– Particle structure often critical

Composition of viruses infecting different hosts

• No “rules” about virus families that may or may not be present in a given kingdom

• Some types of viruses are found more commonly in some kingdoms than in others– Many plant viruses contain ssRNA genomes– Many fungal viruses contain dsRNA genomes– Many bacterial viruses contain dsDNA genomes

• Host properties determine the types of viruses that tend to be found in members of a biological “kingdom”

Virus types by nucleic acid composition

DNA RNA

ss ds ss ds

env naked env naked env naked env naked0 5 9 12 9 14 2 50 100 200 300 200 600 10 300

FamiliesSpecies

Host typeVertebrateInvertebratePlantFungusBacteria

- + ++ ++ ++ ++ - ++ - + ++ - ++ ++ - ++ - ++ - + + +++ - + - - - + + + + +++ - + + +++ - + + -

Overview of Virus Properties

• Animal– RNA – 5-30 kb– DNA: 5-350 kb

– Many enveloped– Range of complexity

– Range of morphologies– Some divided genomes

• Prokaryote– RNA – 5-8 kb

– DNA – 10-200 kb– Few enveloped

– Range of complexity– Range of morphologies– Few divided genomes

• True Fungi– RNA – 2.5-28 kb

– DNA – none – Enveloped ones have no capsid

– Little genome complexity– Little morphological complexity

– Some divided genomes

• Plant– RNA – 0.3-28 kb– DNA – 3-10 kb – Few enveloped

– Little genome complexity– Little morphological complexity

– Many divided genomes

• Lower eukaryote– RNA – 5-10 kb

– DNA – 180-1200 kb– Internal envelope

– Range of complexity– Range of morphologies

– No divided genomes

Tobacco mosaic virus – a typical small RNA virus

• 18X300 nm• Single 6400 nt RNA• 2130 copies of single

17 kDa coat protein• 3 essential genes• Simple regulatory

elements

Poxvirus – a typical large dsDNA virus

• 120X200 nm• Single 180 kb DNA• Complex coat made up

of numerous proteins• >100 essential genes• Complex regulatory

elements

Virus at the edge: Mimivirus• Mimivirus infects Acantamoeba polyphaga• 400 nm particle, 1.2 megabase genome, 1262 putative ORFs

represent the largest virus identified yet• Many genes for normal cellular functions

– central translation functions• Aminoacyl tRNA synthetases• Peptide release factor 1• Translation elongation factor EF-TU

– DNA repair enzymes– Many polysaccharide synthesis enzymes

• Lineage suggests connection with eukaryotes, not prokaryotes

Raoult et al., Science Express 10/14/04

Phylogenetic position of Mimivirus

Compared to other similar DNA viruses

Compared to other prokaryotic and eukaryotic life forms

Raoult et al., Science Express 10/14/04

Mimiviruses (arrowed) can bee seen inside their amoeba host, Acanthamoeba polyphaga using a light microscope

Raoult et al., Science Express 10/14/04

By transmission electron microscopy, isolated Mimivirus can be seen as a large icosahedral virus with fibrilar protrusions

Mimivirus (green) seen by fluorescence microscopy in amoeba

Mimivirus in ultrathin section in amoebaRaoult et al., Science Express 10/14/04

At 1.2 megabases (1.2X106 nucleotides), Mimivirus has the largest genome of any known virus, larger than many bacteria

Simple virus replication cycle1. Virus enters host cell

2. RNA released; translates

3. Replication in cytoplasm

4. New virus assembled

5. New virus released from host cell

From 7th Report of the ICTV(AcademicPress, 2000)

From 7th Report of the ICTV(AcademicPress, 2000)

From 7th Report of the ICTV(AcademicPress, 2000)

From 7th Report of the ICTV(AcademicPress, 2000)

From 7th Report of the ICTV(Academic Press, 2000)

From 7th Report of the ICTV(Academic Press, 2000)

From 7th Report of the ICTV(Academic Press, 2000)

From 7th Report of the ICTV(Academic Press, 2000)

From 7th Report of the ICTV(Academic Press, 2000)

From 7th Report of the ICTV(Academic Press, 2000)