Download Viral Replication Replication of Animal Viruses

Chapter 13
Characterizing
and Classifying
Viruses and
Prions
© 2012 Pearson Education Inc.
Lecture prepared by Mindy Miller-Kittrell
North Carolina State University
Characteristics of Viruses
• Virus
– Minuscule, acellular infectious agent having
either DNA or RNA
– Causes many infections of humans, animals,
plants, and bacteria
– Causes most of the diseases that plague the
industrialized world
© 2012 Pearson Education Inc.
Characteristics of Viruses
• Cannot carry out any metabolic pathway
• Neither grow nor respond to the
environment
• Cannot reproduce independently
• Recruit the cell’s metabolic pathways to
increase their numbers
• No cytoplasmic membrane, cytosol,
organelles, but some have a membrane-like
envelope.
• Have extracellular and intracellular state
© 2012 Pearson Education Inc.
Characteristics of Viruses
• Extracellular State
–
–
–
–
Called virion
Protein coat (capsid) surrounding nucleic acid
Some have phospholipid envelope
Outermost layer provides protection and
recognition sites for host cells
• Intracellular State
– Capsid removed
– Virus exists as nucleic acid
© 2012 Pearson Education Inc.
Figure 13.1 Virions-overview
Characteristics of Viruses
• Genetic Material of Viruses
– Show more variety in nature of their genomes
than do cells
– Primary way scientists categorize and classify
viruses
– May be DNA or RNA, but never both
– Much smaller than genomes of cells
© 2012 Pearson Education Inc.
Figure 13.2 The relative sizes of genomes
Partial genome
of E. coli
Viral
genome
Characteristics of Viruses
• Hosts of Viruses
– Most viruses infect only particular host’s cells
– Affinity of viral surface proteins for proteins on
host cell
– May be so specific they infect only particular
kind of cell in a particular host
– Bacteriophages are viruses that infect
bacteria.
© 2012 Pearson Education Inc.
Figure 13.3 Hosts of viral infections-overview
Figure 13.4 Sizes of selected virions
E. coli (bacterium)
(1000 nm  3000 nm)
Red blood cell
(10,000 nm in diameter)
Bacterial
ribosomes
(25 nm)
Poliovirus
(30 nm)
Bacteriophage MS2
(24 nm)
Smallpox virus
(200 nm  300 nm)
Bacteriophage T4
(50 nm  225 nm)
Tobacco mosaic virus
(15 nm  300 nm)
Characteristics of Viruses
• Capsid Morphology
– Capsids
– Provide protection for viral nucleic acid
– Means of attachment to host’s cells
© 2012 Pearson Education Inc.
Figure 13.5 The shapes of virions-overview
Figure 13.6 Bacteriophage T4-overview
Characteristics of 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
– Envelope’s proteins often play role in host
recognition
© 2012 Pearson Education Inc.
Figure 13.7 Enveloped virion-overview
Viral Replication
• Dependent on hosts’ organelles and enzymes
to produce new virions
• Lytic replication
– Replication cycle usually results in death and lysis
of host cell
• Stages of lytic replication cycle
–
–
–
–
–
Attachment
Entry
Synthesis
Assembly
Release
© 2012 Pearson Education Inc.
Figure 13.8 The lytic replication cycle in bacteriophages-overview
Attachment
Bacteriophage
genome
Entry
Tail sheath
Outer
membrane
Peptidoglycan
Cytoplasmic
membrane
Bacterial
chromosome
Entry
Attachment
Phage
DNA
Lytic replication
cycle of bacteriophage
Bacterial
chromosome
degraded
Release
Synthesis
Phage
proteins
Assembly
Assembly
Base
Tail
Sheath
DNA
Capsid
Mature head
Tail fibers
Mature virion
Viral Replication
• Lysogeny
– Modified replication cycle
– Infected host cells grow and reproduce normally
for generations before they lyse
– Virus gets inserted into host cell DNA.
© 2012 Pearson Education Inc.
Figure 13.11 The lysogenic replication cycle in bacteriophages: phage lambda and E. coli
Attachment
Prophage
in chromosome
Entry
Lambda
phage
Lytic
cycle
Lysogeny
Synthesis
Release
Replication of
chromosome
and virus;
cell division
Assembly
Induction
Further replications and
cell divisions
Viral Replication
• Replication of Animal Viruses
– Synthesis of animal viruses
– Requires different strategy depending on its
nucleic acid
– DNA viruses often enter the nucleus
– RNA viruses often replicate in the cytoplasm
© 2012 Pearson Education Inc.
Figure 13.14 The process of budding in enveloped viruses
Enveloped
virion
Budding of
enveloped virus
Viral
glycoproteins
Viral capsid
Cytoplasmic
membrane
of host
Viral Replication
• Replication of Animal Viruses
– Latency of animal viruses
– When animal viruses remain dormant in host cells
– May be prolonged for years with no viral activity
– Some latent viruses do not become incorporated
into host chromosome
– Herpesvirus
– Incorporation of provirus into host DNA is
permanent
© 2012 Pearson Education Inc.
Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate
Bacterial lawn
Viral plaques
Are Viruses Alive?
• Some consider them complex pathogenic
chemicals
• Others consider them the least complex
living entities
– Use sophisticated methods to invade cells
– Have the ability to take control of their host cell
– Are able to replicate themselves
© 2012 Pearson Education Inc.
Other Parasitic Particles: Viroids and Prions
• Characteristics of Prions
– Proteinaceous infectious agents.
– Prion diseases
– Fatal neurological degeneration and loss of brain
matter
– Large vacuoles form in brain
– Characteristic spongy appearance
– Spongiform encephalopathies
– Prions are extremely difficult to kill. Must be
incinerated.
© 2012 Pearson Education Inc.
Figure 13.23 The brain of a sheep with the prion disease called scrapie
Vacuole