Viral Replication
Subtitle
First a Review of Where we have gotten
so far
▪ Taxonomy and Scientific Naming
▪ Viruses
– Characterizing and Grouping Viruses based on structure
▪ Nucleotides and Proteins
▪ Envelope and glycoproteins
▪ Icosohedral, spherical, complex, helical shapes
▪ Microscopy and Scientific Drawings
First a Review of Virus Structure and
Scientific Drawings
▪ Provide a Scientific Drawing of a virus using the
following labels:
– Nucleic Acid
– Capsid
– Glycoprotein
– Envelope
Viral Replication
▪ Consider what we already know/ have learned about:
▪ We know viruses cannot reproduce independently – they
need a host in order to replicate
▪ Viruses contain DNA and RNA , and are comprised of
proteins
▪ How can Viruses utilize host cells for their own replication
purposes?
The Bacteriophage
▪ A Virus which infects bacteria
▪ With the ability of scientists to
more easily study bacteria,
bacteriophages provided a window
into how Viral diseases replicate
▪ Used to help develop gene therapy
Bacteriophage Structure
• Icosohedral head
• Contains the Viral nucleic
acid
• Comprised of Protein
• Collar and Sheath (Tail)
• Comprised of Protein
• Along with head makes up
the capsid of the virus
• Act like “syringe” – inject
viral nucleic acid into the
host
• Tail fibers
• Assist in virus attachment
to cell
Viral Replication
▪ Two Basic Types of Viral Replication that have been
studied
▪ Lytic Cycle
– What makes viruses “viral” – cause diseases
▪ Lysogenic Cycle
– What makes a virus “temperate” – Remains
dormant in host cells for extended periods of time
The Lytic Cycle
▪ 5 Phases of the Lytic Cycle:
1. Attachment
2. Entry
3. Replication
4. Assembly
5. Release
1. Attachment
▪ Begins by virus attaching to host cell using tail
fibres to attach to receptor sites on cell surface
– Receptor Sites: Areas on the cell which the virus
can recognize and bind to
– Receptor sites are important, because if a cell
doesn’t have them, viruses move to the next
group of cells which do
– This is how Viruses gain specificity (e.g. Hepatitis
Virus only infects liver cells)
2. Entry
▪ Bacteriophage releases an enzyme which weakens
the cell wall, allowing for Nucleic acid to be inserted
into the cell
3. Replication
▪ How the Viral DNA is replicated
–Utilizes our DNA  Protein process
▪ Transcribe Viral DNA into Viral RNA, which
is then translated into Viral proteins
▪ Viral proteins form the capsid
4. Assembly
▪ Viral genes are enclosed in the newly
created viral capsids
–Occurs mostly in cytoplasm, can occur
in nucleus in eukaryotic organisms
5. Release
▪ Bacteriophage releases enzyme which
disintegrates host cells
▪ Cell disintegration is known as LYSIS
▪ Viruses with envelopes force themselves through
the cell and “borrow” a piece of the cell as th
envelope
To Do Now:
▪ Using the sheet in front of you to show the Lytic
Cycle in action:
– Cut and paste the different stages in order of the
cycle
– Provide a brief description of each stage
Lysogenic Cycle
▪ Allows viruses to remain dormant for days, months,
weeks, or even years.
– “Temperate Viruses”
▪ E.g. Human Immunodeficiancy Virus
▪ 4 stages
1.
2.
3.
4.
Attachment
Injection
Integration
Cell Multiplication
1. Attachment and 2. Injection
▪ Work in the same way as the Lytic Cycle – Tail fibers
find receptor sites, and inject nucleic acid into the
host cell
3. Integration
▪ After the bacteriophage has injected it’s DNA into
the host cell, it integrates its DNA into the host
cell’s DNA. The Viral DNA is now part of the host
cells genome, and can be replicated, transcribed
and translated the same way it would for the host
cell’s DNA.
▪ The Bacteriophage DNA molecule that
integrates into the host genome is known as
the PROPHAGE
4. Replication
▪ Once integrated into the host cells genome, the
Viral DNA will replicate, and become part of cell
division in the same way that our DNA does (2
copies – one per each cell in cell division)
▪ Our cells keep replicating and replicating,
containing the viral DNA each time it does.
Replication cont’d
▪ Lysogenic viruses are said to be “temperate” –
remain dormant, do not cause diseases.
▪ Radiaton, or certain chemicals however can
cause a temperate virus to break its lysogenic
cycle, and become part of the lytic cycle
(causes disease – Virulent)
▪ It will produce viral RNA/DNA and protein,
assemble into new viruses and lyse the cells,
spreading the virus throughout the host
Lysogenic Puzzle
▪ Like the Lytic Cycle, please cut and paste the
different stages of the lysogenic cycle onto a piece
of paper, and provide a brief description of each
stage.
▪ Do not worry about “e” – It is just the stage when
the Lysogenic cycle becomes the Lytic Cycle.
Where the Viral DNA no longer becomes part of
the host genome and begins the replication step
of the Lytic Cycle.
The Lysogenic Cycle
To Do Now
▪ Work on 15.2 Review