Viral Structure and Multiplication Flashcards
what are the importances of viruses?
major cause of disease
important members of the aquatic world
important in evolution due to transfer of genes between bacteria
important model systems in molecular biology
what are bacteriophages?
bacterial viruses
what are the main types of viruses?
eukaryotic (plants, animals, protists, and fungi)
how are viruses classified into families?
genome structure, life cycle, morphology, and genetic relatedness
what is a virion?
mature viruses
what do all virions contain?
a nucleocapsid composed of nucleic acid (DNA or RNA) and a protein coat
what are capsids?
large macromolecular structures which serve as a protein coat of a virus
what is the function of capsids?
protect viral genetic material and aid in its transfer between host cells
what are capsids made up of?
protein subunits called protomers
what are the three shapes of capsids?
helical
icosahedral
complex
helical capsid characteristics
shaped like hollow tubes with protein walls
icosahedral capsid
regular polyhedron with 20 equilateral faces and 12 vertices
capsomers are ring or knob shaped units made of 5 or 6 protomers
complex capsids
do not fit into helical or icosahedral categories
examples of complex capsids
poxviruses
large bacteriophages with final symmetry
where do animal virus envelopes arise from?
lipids and carbohydrates from the host cell plasma or nuclear membranes
how are viral envelope spike proteins made and what is their purpose?
encoded by the virus
involved in viral attachment to the host cell
used for identification of the virus
may have enzymatic activity
may play a role in nucleic acid replication
how are viral genomes diverse?
viruses can be made of single stranded RNA, single stranded DNA, double stranded RNA, or double stranded DNA
the length varies from virus to virus
genomes can be linear or circular
list the steps of the viral multiplication process
depends on viral structure and genome, but steps are similar:
1. Attachment to host cell
2. Entry and uncoating of genome
3. Synthesis
4. Assembly
5. Release
what occurs during attachment (adsorption)
receptor determines host preference
-may be specific tissue
-may be more than one host
-may be more than one receptor
what occurs during entry into the host?
the entire genome or nucleocapsid enters
varies between naked and enveloped viruses
what are the three methods of host entry?
- Fusion of the viral envelope with the host membrane; nucleocapsid enters
- Endocytosis in vesicle; endosome aids in viral uncaring
- Injection of nucleic acid (non enveloped)
what occurs during the synthesis stage?
all viral components are produced
genome dictates the events
-RNA viruses must carry in or synthesize the proteins necessary to complete synthesis
genes and proteins may be referred to as early, middle, or late based on when they are synthesized
what occurs during assembly?
putting together all components to make complete virions
assembly process is complex
baseplate, tail fibers, and components of bacteriophage T4 are assembled separately
what occurs during virion release?
Non-enveloped viruses lyse the host cell
viral proteins may attack peptidoglycan or membrane
how does virion release occur in enveloped viruses?
viral proteins are incorporated into the host membrane
nucleocapsid may bind to viral proteins
envelope derived from host plasma membrane, but may be Golgi, ER, etc.
Virus may use host actin tails to propel through host membrane
what are the two types of phages and their reproductive choices?
virulent phage- multiplies immediately upon entry and lyses bacterial host cell
temperate phage- reproduce lytically as virulent phages do or remain in the host cell without destroying it
what is lysogeny?
a relationship in which many temperate phages integrate their genome into host genome, making them a prophage in a lysogenic bacterium
what is an advantage of a virus having a lytic cycle?
faster production, therefore there is faster infection
what is an advantage of a virus having a lysogenic cycle?
if the environment is bad without other host cell options, the virus can stay and wait for another choice
how does infection in eukaryotes work?
cytocidal infection results in cell death through lysis
persistent infections can last for years- chronic
the cell can transform into a malignant cell and cause tumor formation
what are viroids?
infectious agents comprised only of RNA
typically targets plants
lacks an outer protein layer/capsid
what are satellites (virusoids)?
infectious agents comprised of nucleic acids and a protein coat
needs a co-infector to replicate
can infect both plants and animals
what are prions?
protein viruses that cause a variety of neurodegenerative diseases in humans and animals
what are diseases caused by prions called and give examples
transmissible spongiform encephalopathies
-scrapies in sheep
-mad cow disease
-chronic wasting disease in deer
-kuru and Creutzfeldt-Jakob disease in humans
