Capsid
Protein Shell
Nucleic Acid
DNA or RNA
Envelope
Membrane around virus
Receptors
Proteins that help virus
get into proper host
Attachment
The virus
attaches to
receptors on
the host cell.
Entry into the Cell
The virus is able
to get into the
cell in one of
three ways.
Fusion of the Membranes
The viral
envelope and
cell membrane
fuse, allowing
the virus to
enter the cell.
Endocytosis
The virus is
engulfed by the
cell membrane.
Injection
E. coli
Bacteriophage T4
What happens once the genetic
material is inside the cell?
One of two different processes may occur:
1.
The virus takes over the cell and
begins to replicate right away, killing
the host cell in the process.
2.
The virus doesn’t take over or kill the
host cell right away, but does begin to
replicate.
Lytic
Cycle
There are 5
steps in the
Lytic Cycle
ATTACHMENT
RELEASE (LYSIS)
Lytic
Cycle
1.
ATTACHMENT
Attachment – the virus attaches
to receptors on the host
Lytic
Cycle
2.
Penetration – the nucleic acid
moves across the cell membrane
into the host cell
Lytic
Cycle
3.
Replication and Synthesis – the
virus degrades the host nucleic
acid and uses the host to make
new viral components.
Lytic
Cycle
4.
Assembly – viral components are
assembled into new viruses.
Lytic
Cycle
RELEASE / LYSIS
5.
Release / Lysis – fully assembled
viruses are released when the host
cell bursts
Lytic
Cycle
ATTACHMENT
RELEASE / LYSIS
Viruses that are in a lytic cycle are
described as VIRULENT
T4 Bacteriophage Lysing
E. coli Bacterium
Bacteriophage Lambda
(λ)
ATTACHMENT
REPLICATION
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Lysogenic
Cycle
PENETRATION
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INTEGRATION
There are 4 steps in the Lysogenic Cycle
ATTACHMENT
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Lysogenic
Cycle
PENETRATION
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Steps 1 and 2 are identical to the first two
steps in the lytic cycle.
Lysogenic
Cycle
3.
INTEGRATION
Integration – Phage DNA
integrates into bacterial DNA
Lysogenic
Cycle
INTEGRATION
When it becomes integrated into the
host DNA, the viral genome is
referred to as a PROPHAGE.
Lysogenic
Cycle
4.
REPLICATION
Replication – The integrated prophage
replicates when bacterial DNA replicates
NNNNNNNN
NNNNNNNN
NNNNNNNN
NNNNNNNN
Lysogenic
Cycle
nnn
nnn
nnn
nnn
nnn
nnd
Viruses that enter into a lysogenic
cycle are known as
TEMPERATE VIRUSES
What is special about
Bacteriophage λ?
ATTACHMENT
REPLICATION
RELEASE /
LYSIS
PENETRATION
ASSEMBLY
REPLICATION
AND SYNTHESIS
INTEGRATION
Bacteriophage λ attached to
an E. coli Bacterium
Replication of Viral
Nucleic Acids
DNA Viruses – replication of
viral DNA is similar to the
process that the host cell uses
to replicates its own DNA
DNA  DNA
Replication of Viral
Nucleic Acids
Most RNA Viruses – replication
of viral RNA is dependent upon
an enzyme that is encoded on the
viral genome
RNA  RNA
Replication of Viral
Nucleic Acids
Retroviruses – RNA viruses that
convert their RNA into DNA
using an enzyme that is included
with the viral genome
RNA  DNA
Replication of Viral
Nucleic Acids
Retroviruses – This DNA is inserted
into the host genome, where it can
be transcribed back into viral RNA
during normal cellular activity.
Replication of Viral
Nucleic Acids
Retroviruses – Once the DNA is
integrated into the host genome,
some retroviruses may become
LATENT, where they remain
inactive for long periods of time.
Replication of Viral
Nucleic Acids
Human Immunodeficiency Virus
(HIV), the virus that causes AIDS,
is an example of a retrovirus that
can become latent.
Lytic Cycle
Virus hijacks the host cell and keeps making more
copies of itself until the host bursts open
Lysogenic Cycle
Virus integrates into the host genome and replicates when
the host divides. When triggered it can enter the lytic cycle.
Retroviruses
RNA viruses that convert their RNA to DNA
which then integrates into the host genome.
HIV
Retrovirus that infects humans which can
remain latent for long periods of time.
HIV
Infection
When not active, the virus is
described as being latent
Lytic and Lysogenic Cycles
Lytic and Lysogenic Cycles
ATTACHMENT
REPLICATION
RELEASE /
LYSIS
PENETRATION
ASSEMBLY
REPLICATION
AND SYNTHESIS
INTEGRATION
Lytic
Cycle
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Lysogenic
Cycle
Lytic and
Lysogenic
Cycles