Download General Properties of Viruses

PowerPoint® Lecture Slides for
MICROBIOLOGY
Characterizing and Classifying
Viruses, Viroids, and Prions
Prof. Manal Baddour
Viruses
Cause many infections of humans, animals, plants,
and bacteria
Cannot carry out any metabolic pathway
Neither grow nor respond to the environment
Cannot reproduce independently
Obligate intracellular parasites
Characteristics of Viruses
Cause most diseases that plague industrialized
world
Virus – miniscule, acellular, infectious agent having
one or several pieces of either DNA or RNA
No cytoplasmic membrane, cytosol, organelles (one
exception)
Have extracellular and intracellular state
Characteristics of Viruses
Extracellular state
Called virion
Protein coat (capsid) surrounding nucleic acid
Nucleic acid and capsid also called nucleocapsid
Some have phospholipid envelope
Outermost layer provides protection and
recognition sites for host cells
Intracellular state
Capsid removed
Virus exists as nucleic acid
How Viruses Are Distinguished
Type of genetic material they contain
Kinds of cells they attack
Size of virus
Nature of capsid coat
Shape of virus
Presence or absence of envelope
Viruses depend on favorable hosts in a favorable
environment.
Virus
Host
Environment
Genetic Material of Viruses
Show more variety in nature of their genomes than
do cells
May be DNA or RNA; never both
Primary way scientists categorize and classify
viruses
Can be dsDNA, ssDNA, dsRNA, ssRNA
May be linear and composed of several segments or
single and circular
Much smaller than genomes of cells
T7
E coli
Hosts of Viruses
Most only infect particular kinds of host’s cells
Due to affinity of viral surface proteins or
glycoproteins for complementary proteins or
glycoproteins on host cell surface
May only infect particular kind of cell in host
Generalists – infect many kinds of cells in many
different hosts
Viral Hosts
Tobacco mosaic virus
Viral Hosts
Viral Hosts
Influenza
Viral Hosts
CD4
Sizes of Viruses
Capsid Morphology
Capsids – protein coats that provide protection for
viral nucleic acid and means of attachment to host’s
cells
Capsid composed of proteinaceous subunits called
capsomeres
Come capsids composed of single type of
capsomere; other composed of multiple types
ٌٌHelical
Polyhedral (icosahedral), Spherical
Viral Shapes
Complex Viruses
Complex Viruses
Complex Viruses
Complex Viruses
The Viral Envelope
Acquired from host cell during viral replication or
release; envelope is portion of membrane system of
host
Composed of phospholipid bilayer and proteins;
some proteins are virally-coded glycoproteins
(spikes)
Envelope’s proteins and glycoproteins often play
role in host recognition
The Viral Envelope
Viral Replication
1- Adsorption *
2- Penetration
3- Uncoating
4- Synthesis
5- Assembly
6- Release
Replication of Bacteriophage
Bacteriophage lambda
Lysogeny
Penetration
Penetration of nonenveloped virus
Penetration by membrane fusion
*
Penetration by receptor mediated endocytosis
+
Penetration by receptor mediated endocytosis
+
Synthesis
Depends on the type of nucleic acid (DNA or RNA; single
stranded or double stranded)
DNA viruses are synthesized in the nucleus except one
exception
RNA viruses are synthesized in the cytoplasm
dsDNA replicate like normal cellular DNA
ssDNA viruses form a complementary strand
+ssRNA forms a -ssRNA which acts as a template
-ssRNA carries RNA dependant RNA polymerase
dsRNA each strand acts as a template for its opposite
Retroviruses use reverse transcriptase to form a DNA
intermediate that serves as template for synthesis of its RNA
genome (+ssRNA)
Budding of enveloped viruses
Budding of enveloped viruses
The novel Properties of Viruses
Viruses
Cells
Are inert macromolecules outside of
a cell, but become active inside a cell
Do not divide or grow
Acellular
Obligate intracellular parasites
Contain either DNA or RNA, never
both
Genome can be dsDNA, ssDNA,
dsRNA or ssRNA
Ultramicroscopic in size ranging
from 10nm to 300nm
Have a proteinaceous capsid around
the genome, some have an envelope
around the capsid
Replicate in an assembly-line manner
using the enzymes and organelles of
the host
Metabolize on their own
Divide and grow
Cellular
Most free living
Contain both DNA and RNA
Genome is dsDNA
300nm to 12cm in diameter
Surrounded by a phospholipid
membrane and often a cell wall
Self replicating by asexual and/or
sexual means
Characteristics of Viroids
Extremely small, circular pieces of RNA that are
infectious and pathogenic in plants
Similar to RNA viruses, but lack capsid
May appear linear due to H bonding
Viroids
Figure 13.19
Characteristics of Prions
Proteinaceous infectious agents
Composed of single protein PrP
All mammals contain gene that codes for primary
sequence of amino acids in PrP
Two stable tertiary structures of PrP
Normal functional structure with α-helices called
cellular PrP
Disease-causing form with β-sheets called prion PrP
Prion PrP converts cellular PrP into prion PrP by
inducing conformational change
Characteristics of Prions
Normally, nearby proteins and polysaccharides
force PrP into cellular shape
Excess PrP production or mutations in PrP gene
result in initial formation of prion PrP
When prions present, they cause newly synthesized
cellular PrP to refold into prion PrP
Prion Diseases
All involve fatal neurological degeneration,
deposition of fibrils in brain, and loss of brain
matter
Large vacuoles form in brain; characteristic
spongy appearance
Spongiform encephalopathies – Bovine spongiform
encephalopathy (BSE), Creutzfeldt-Jakob disease
(CJD), kuru
Only destroyed by incineration; not cooking or
sterilization
Scrapie in Sheep