survey of viruses 1

obligate intracellular parasite

virus that require host cells or something else for metabolic processes and protein synthesis

what are the smallest known self- replicating organism

virus

what is RNA or DNA genome encased in

a protein shell

DNA helix diameter 

2nm

virus diameter

20-400 nm

e. coil diameter

500 nm x 2000 nm

nucleus diameter

10,000 nm

cell diameter 

> 20,000 nm

hosts 

bacteria, protozoa, fungi, algae, plants, animals

size

human viral pathogens range from 20 nm- 450 nm in diameter

what do viruses and diverse organism tend to have

conserved structure

conserved structure

major types of conserved structure are icosahedral or helical but there are alsoseen

• envel[ed vs nonenveloped 

where might viruses replicate

virus may replicate in the cytoplasm, the nucleus, or both

what does a virus need

• genome

• means to replicate its genome and express its genes 

• protection for its genome during transmission 

• mechanism to attach and enter permissive cells 

• mechanism to evade the host immune response 

genome

nucleic acid

• DNA and RNA

means to replicate its genome and express its genes

viral or host polymerase

protection for its genome during transmission

protein (capsid). memebrane

mechanism to attach and enter permissive cells

at minimum the virus needs to avoid innate immunity

nucleocapsid

nucleic acid- protein assembly packaged within the virion that is discrete substructure of the particle 

capsid

protein shell surrounding the nucleic acid genome

what is the capsid purpose

is to form a stable protective shell for the genome

what does the capsid protect against

variations in temp, pH, and chemical composition of the environment

matrix 

viral protein layer associated with virion membrane 

• no in all viruses 

envelope

membrane surrounding virus core

• not present in all viruses 

Virus structure: non-enveloped/ naked 

contains only the components necessary to invade and utilize host cell machinery 

what are the two main components of naked/ nonenveloped viruses

external protein coat= capsid

core nucleic acid (DNA or RNA)

capsid + nucleic acid core

nucleocapsid

what are capsids assembled from

protein subunits called capsomers

how many types of virus shapes does the naked virus have

depending on the arrangement of capsomers, assembly results in 2 different types

what are noneveloped/ naked virus shape

helical (rod-shape)

icosahedral (soccer ball) 

naked virus envelope properties

infected cell dies (lysis) then viruses are released

• can survive in extreme conditions (stomach/ GI tract) 

naked/ non-enveloped virus consequences

resistant to drying, detergents, stomach acid, etc

• antibodies against capsid antigents may be protective (negative)

enveloped virus structure

has additional covering external to the capsid known as the envelope

what is the envelope acquired from

the host cell membrane

viral glycoproteins

spikes

what do spikes do in enveloped viruses

they are inserted into the envelope

what are spikes essential for in envelope viruses 

attachment to host cells 

envelope virus shape

usually round but can be filamentous

what type of nucleocapsid is in envelope virus shape

helical and icosahedral nucleocapsid

envelope virus properties

released by budding and cell lysis

• is a fragile virus can not survive long outside the host 

enveloped viruses consequences

it has to stay wet, can not survive in the GI tract or any hostile environments, and cell mediated immunity is important

DNA shapes

Circular and linear

single-stranded DNA

ssDNA

double stranded DNA

dsDNA

what is apart of single stranded RNA (ssRNA)

positive and negative sense

single stranded RNA: positive sense

translatable RNA (mRNA basically)

• uses viral genome as mRNA and uses host enzymes 

• ssRNA highjacks host to do translation 

ssRNA negative sense

RNA must be copied into positive RNA prior to protein expression

• in order for this, it must make positive mRNA. these negative sense viruses have RNA polymerase to do this 

• bigger virus because carrying it in genome 

double stranded RNA

dsRNA

what are most viruses limited to

a particular host, tissue, or cell type ( usually because of the cell types)

for obligate intracellular parasite viruses what must they use

use existing cell pathways (free) or induce them (cost them something)

• cost means it requires more genetic capacity (larger genome means carrying more stuff)

host range

the cells, tissues, and species that a virus can productively infect (absolute measurement)

susceptibility

the capacity of a cell, tissue or species to support virus replication (qualitative measurement)

virus binding

virion binding to host cell.

what are cellular targets referred to as

a receptor

virus binding targets include…

cellular proteins, lipids (specific/ nonspecific), carbohydrates (specific/ nonspecific)

what may receptors limit

host-cell range and susceptibility

what is fusion used for

enveloped viruse

fusion with…

plasma membrane 

fusion

receptor binding causes conformation change in envelope protein resulting in fusion with cell membrane

endocytosis

internalization into endosome

what does pH change cause

conformation change in envelope protein and fusion with endosomal membrane and release into cytosol 

• uses cell pathway 

penetration through…

plasma/ endocytic membrane

penetration forms

pore formation and membrane perforation

what type of viral uncoating can occur

partial to complete uncoating

what type of process is viral uncoating

poorly understood process

what is viral uncoating initiated by

cellular signals

• receptor binding and pH change 

replication/ assembly is the

process of forming new particles

where is the genome typically incorporated

where it is replicated

RNA virus replication

cytoplasm

DNA virus replication

nucleus

for envelope viruses where does assembly typically occur

at site of membrane acquisition

• nucleus, ER, Golgi, plasma membrane

what type of virus is lytic mechanism of budding/release used for

non-enveloped viruses

lytic mechanism

accumulation of particles until critical mass is achieved

• viral signal induce cell death and release virions

• basically cell dies/explodes when it becomes to full

what type of virus is exocytosis used for 

enveloped viruses 

exocytosis

via cellular pathways

what type of virus is budding used for 

enveloped virus

what is budding

budding out of the plasma membrane

• either is causing a signal or following pathway