Lesson 1: Introduction to the study of viruses Flashcards
are the
smallest viruses (20nm)
picornavirus
are the largest viruses
(300nm)
poxviruses
Viruses cannot be seen by light microscope because of their small size except?
poxviruses
General description of Virus
• Filterable agents
• seen only by the aid of electron microscope
• No cellular organization and do not have organelles
• Composed of nucleic acid surrounded by a protein coat
• Contain only one type of nucleic acid, either DNA or RNA
• viable host
cells are required for replication
• Viruses are unaffected by antibiotics
• Viruses multiply by a complex process involving protein synthesis and nucleic
acid production
Three categories;
✓ DNA viruses
✓ RNA viruses
✓ Viruses that utilize both DNA
and RNA for replication
the viruses that infect bacteria
Bacteriophages or phages
an infectious extracellular virus particle consists of nucleic acid (DNA or
RNA) that is covered by a protein coat called CAPSID.
Virion
a shell of subunits of proteins called CAPSOMERE that encloses the
genome of vertebrate viruses
Capsid
a shell of subunits of proteins called CAPSOMERE that encloses the
genome of vertebrate viruses
Capsid
capsid functions
• offers protection for the nucleic acid against adverse conditions
• it facilitates attachment and entry of the virus into host cell
• it
possesses antigens used for virus identification in serological tests
• it
determines the symmetry of the virus
two types of capsid symmetry
described in viruses;
Icosahedral and helical symmetries
large viruses with large genome have
complicated symmetry which is neither icosahedral nor helical such as?
Poxviruses
used to refer to the combined nucleic acid and capsid
which can either be naked or covered with a membrane termed an envelope
nucleocapsid
The viral
genome also codes for important enzymes called _______________
required for viral replication but are not incorporated in the virion.
non-structural proteins
proteins that make up the subunit of capsid
structural proteins
generally assembled in the host cell prior to
incorporation of the viral nucleic acid
icosahedral capsid
formed by the insertion of protein units between each turn of
the nucleic acid helix, incorporating the RNA in the tubular package
Helical capsids
lipid bilayer and associated glycoproteins that cover a nucleocapsid
envelope
acquired when the nucleocapsid buds through a cellular membrane,
endoplasmic reticulum, the Golgi apparatus or the nuclear membrane
envelope
are usually susceptible to detergent and are rendered noninfectious following damage to the envelope
enveloped viruses
the proteins encoded by viral nucleic acid for binding to
receptors on host cells, membrane fusion, uncoating of the virion and destruction
of receptors on host cells
glycoproteins
are knob-like projections from the envelope formed from
the oligomers of glycoproteins.
peplomers/spikes
Peplomers/spikes are present in certain viruses including;
coronaviruses, retroviruses, orthomyxoviruses, rhabdoviruses and
paramyxoviruses
a layer of protein present between the nucleocapsid and the
envelope in some enveloped viruses that provides additional rigidity to the virion
matrix protein
Some viruses are named according to the type of disease they cause (creeping lesion) Examples;
poxviruses
herpesviruses
viruses named based on acronyms of disease example
picornavirus and papovavirus
Viruses are also named based on morphology as revealed by electron microscopy example;
coronavirus, togavirus, rhabdovirus, and calicivirus
viruses are named after individual discoverer
Epstein-bar viruses
developed and expanded the universal scheme in which characteristics of
virions are used to assign them to five hierarchical levels (order, family,
subfamily, genus and species).
International Committee on Taxonomy of Viruses (ICTV) 1973
The hierarchical levels are denoted with the following suffixes:
Order: -virale
Family: -viridae
Genus: virus
Species: virus
Four orders containing viruses of animals are so far recognized:
• mononegavirale
• herpesvirales
• picornavirales
• nidovirales
are infectious particles, which can transmit a disease, composed mainly
of a protein without any detectable nucleic acid.
Prions
___________ causative agents of slow viral infections such as ___________
Prions
Subacute spongiform encelophathy
After long incubation period of years, Prions produce
a progressive disease that causes damage to the central nervous system,
leading to
subacute spongiform encephalopathy
Susceptibility to Physical and Chemical Agents
• Disinfectants
• Temperature
• pH
• lipid solvents
• radiations
initial stage of virus replication
whereby the infecting virus loses its physical identity and most or all of its infectivity
eclipse phase
stage as new viral particles are formed and released from the cell
wherein the number of viral particles increases exponentially.
productive stage
Steps in Virus Replication
Attachment
Entry
Uncoating
Biosynthesis or Replication of nucleic acid
Maturation/Assembly of virus
enveloped viruses enter the cell by an alternate method called
___________, in which the viral envelope fuses with the plasma
membrane and releases the capsid into the cell cytoplasm.
Fusion
naked viruses or non-enveloped viruses undergo a
receptor-based endocytosis also known?
viropexis
process of separation of viral nucleic acid from its protein
core for transcription to take place
uncoating
in certain viruses, transcription may
proceed without complete release of the viral genome
reovirus
Replication of DNA viruses
DNA viruses replicate their DNA in the
nucleus of the host cell by using viral enzymes.
Replication of RNA viruses
RNA viruses replicate in the cytoplasm
except orthomyxoviruses and Borna disease virus which require host DNA
transcription inside the nucleus, paramyxoviruses which have a non-obligatory
nuclear phase of replication and retroviruses which replicate via a DNA
intermediate (provirus)
Abnormal replicative cycles may occur in four ways:
incomplete viruses
pseudovirions
abortive infections
defective viruses
Due to defect during assembly of viral components, some of
the daughter virions that are produced may not be infective
incomplete viruses
viruses that occasionally enclose host cell
nucleic acid instead of viral nucleic acid, therefore, are non infective and lack the
capability to replicate.
pseudovirions
type of infection, the virus components may be
synthesized but the maturation is defective maybe due to infection of the wrong host
cells by the virus.
abortive infections
are viruses that produce fully mature virions only in the
presence of helper viruses which supplement the genetic deficiency in the defective
viruses
Defective viruses
Example of defective virus
Hepatitis D virus, replicate only in the presence of Hepatitis B virus (helper virus)
Spontaneous and random errors in the copying of viral nucleic acid
mutations
can occur during the replication of viruses. It is the most important
mechanism by which a virus can be genetically modified which results in production
of new viral strains showing properties different from parental or wild-type virus such
as inactivation of viruses, altered antigenicity and pathogenicity of the virus, and
induce drug resistance in viruses
mutations
resulting from single nucleotide substitutions, are the most common type
of mutation
point of mutations
a new area of antiviral research wherein those RNA viruses with
inherently high mutation rates are administered with mutagenic agents to drive viral
extinction through violation of the error threshold and error catastrophe.
lethal mutagenesis
is the extinction of an organism as a result of excessive
mutations
error catastrophe
