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REPLICATION OF THE VIRUS
Claude Muvunyi M.D.; Ph.D.
Learning outcomes
At the end of this lecture you should be able to:
* List different steps of viral replication
* Know the difference between DNA and RNA viral replication
* Define and identify some common enzymes critical for viral replication
Comparing the size of a virus, a bacterium, and an
animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
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)
Viral structure
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
DNA
Tail
fiber
Glycoprotein
18  250 mm
20 nm
(a) Tobacco mosaic virus
Glycoprotein
70–90 nm (diameter)
80–200 nm (diameter)
50 nm
50 nm
(b) Adenoviruses
(c) Influenza viruses
80  225 nm
50 nm
(d) Bacteriophage T4
Families of Human Viruses
Major steps
The major steps in viral replication are the same for all
viruses. These steps are:
1. Recognition of the target cell
2. Attachement
3. Penetration
4. Uncoating
Major steps
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
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OVERVIEW OF VIRUS
LIFE CYCLE
Adsorption
(cell Surface)
Penetration
Uncoating
(Cyto. or Nuc.)
Eclipse
Begins
Biosynthetic
Period
(Cyto. and/or Nuc.)
Genome
Replication
Assembly
(Cyto. or Nuc.)
Eclipse
Ends
Release
From
Cell
Infection of
Other Cells
ATTACHMENT
HOST
FUNCTIONS
PENETRATION
UNCOATING
Transcription
Translation
REPLICATION
VIRAL LIFE
CYCLE
ASSEMBLY
(MATURATION)
RELEASE
Viral Replication
 viruses have specifically shaped attachment
proteins
 each virus infects only certain types of cells
– most are species specific
• Smallpox, polio, measles—affects only humans
– although some are not
• West Nile virus—mosquitoes, birds, humans, horses
– some are cell-type specific
• polio—affects intestine & nerve cells
The viral replication cycle can be separated
into several phases.
• Early phase of infection, the virus must recognize an
appropriate target cell; attache to the cell; penetrate the
plasma membrane and be taken up by the cell; then
release (uncoat) its genome into the cytoplasm, and if
necessary, deliver the genome to the host nucleus
• Late phase begins with the start of genome replication
and viral macromolecular synthesis and proceeds
through viral assembly and release.
Lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5 Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
2 Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4 Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
Head
Tails
Tail fibers
3 Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
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
 Replication of Animal Viruses
– Same basic replication pathway as bacteriophages
– Differences result from
• Presence of envelope around some viruses
• Eukaryotic nature of animal cells
• Lack of cell wall in animal cells
© 2012 Pearson Education Inc.
Multiplication of Animal viruses
• Attachment
Viruses attaches to cell membrane
proteins and glycoproteins
• Penetration
By endocytosis or fusion
• Uncoating
By viral or host enzymes
• Biosynthesis
Production of nucleic acid and
proteins (In nucleus (DNA viruses) or
cytoplasm (R.NA viruses)
• Maturation
Nucleic acid and capsid proteins
assemble
• Release
By budding (enveloped viruses) or
rupture
Viral Replication
Replication of Animal Viruses
– Attachment of animal viruses
• Chemical attraction
• Animal viruses do not have tails or tail fibers
• Have glycoprotein spikes or other attachment molecules
that mediate attachment
© 2012 Pearson Education Inc.
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
• Must consider
– How mRNA is synthesized
– What serves as template for nucleic acid replication
© 2012 Pearson Education Inc.
How does the virus enter the
cell?
Fusion
Endocytosis
Pinocytosis (Viropexis)
Figure 13.12 Three mechanisms of entry of animal viruses-overview
Attachment, Penetration, and Uncoating
Attachment
Endocytosis
Penetration
Uncoating
Figure 13.14
Release of an enveloped virus by budding
Figure 13.20
Multiplication of DNA Virus
Papovavirus
1 Virion attaches to host cell
7 Virions are released
Capsid
DNA
DNA
Host cell
2 Virion penetrates
cell and its DNA is
uncoated
Cytoplasm
6 Virions mature
Capsid proteins
mRNA
5 Late translation;
capsid proteins
are synthesized
3
4
Late transcription;
DNA is replicated
Early transcription and
translation; enzymes are
synthesized
Figure 13.15
Multiplication of RNA Virus
Virus replication:
variations on the theme
•
•
•
•
•
•
•
dsDNA
ssDNA
(+)ssRNA
(-)ssRNA
dsRNA
RNA retro
DNA retro
Viral Replication Schemes
 Class I = dsDNA: production of mRNA occurs as it
would in host genome.
 Class II = ssDNA: have to make complementary
strand of DNA  dsDNA  transcription into
mRNA.
 Class III-VII: special virus situations in which viral
polymerases or other special viral enzymes are
often required to replicate the virus
Genome Replication and Gene Expression
Class I: Double-stranded DNA:
Principles of Molecular
Virology
Class II: Single-stranded DNA
Principles of Molecular
Virology
Class III: Double-stranded RNA
Principles of Molecular
Virology
Class IV: Single-stranded (+)sense
RNA
Principles of Molecular
Virology
Single-stranded (–)sense RNA
Principles of Molecular
Virology
Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses-overview
Viral Replication
[INSERT TABLE 13.3]
Assembly | Maturation
Principles of Molecular
Virology
Release
Principles of Molecular
Virology
Viral Replication Comparison
[INSERT TABLE 13.4]
Summary
Virus replication involves three broad stages:
• Initiation of infection
• Replication and expression of the genome
• Release of mature virions from the infected cell
Principles of Molecular
Virology