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Lecture 8

BMS 212 Lecture 8: Unit 1, Section 8- Viral Replication and Prions

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Department
Biomedical Sciences
Course Code
BMS 212
Professor
Aaron Baxter

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Viral Replication and Prions
Viral Replication
- Always requires a host cell viruses are obligate intracellular parasites and
they control the hosts
- This control over the host tells us:
o Pathogenicity
o Transmission
o Level of immune response
- How they replicate could indicate how we could control the infection
Steps in viral multiplication (T-even phage; lytic cycle)
1. Adsorption
- Attachment
- Brought about by random collisions
- High specificity
o Flagella
o Pili
o Fimbriae
o Cell wall
o Receptors
o Cell membrane
2. Penetration
- Lysozyme dissolves a hole in the cell wall
- The virus plunges its tail through the layers (like a hypodermic needle)
- The DNA is injected
3. Replication
- The entry hole is sealed to re-stabilize the cell wall
- Nucleic acids of the viral genome are copied
- The virus shuts down all host cell gene expression
- Direct synthesis of proteins for the next capsid begins
- In the late stage large volumes of lysozyme are produced
4. Maturation and Assembly
- Parts of the new viruses self-assemble into mature viruses as they
become available
Transduction
- Host DNA is put into the viral heads rather than the viral
DNA
- This will result in no infection in the next host, but it can
cause evolutionary change (i.e. antibiotic resistance)
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5. Release
- Lysozyme causes the cell to lyse
- The new bacteriophages are released to find a new host
Lysogenic or Temperate Phage (Lambda phage)
- Steps 1 and 2 are the same as in the lytic phage
- In the third step, replication does not occur, the host cell is not shut down. Instead,
the viral DNA is integrated into the host genome.
- As the host continues to replicate itself, it is passing on the virus
Prophage: the host cell after the viral DNA has been integrated into the host DNA
Lysogenic conversion: the addition of new DNA results in a change in the
phenotype of the cell
Induction
- When the host cell is dying, it identifies the foreign viral DNA and cuts it
out of the sequence
- This triggers the virus to take over
- The lytic cycle begins: the virus shuts down the host cells and tries to
replicate as quickly as possible before the cell lyses
Steps in replication of animal viruses
1. Adsorption
- Specific interaction with the peplomers allows for attachment
*host range: what hosts can be recognized by the virus (i.e. rabies is able
to target any mammal)*
2. Penetration
- once inside, uncoating occurs: the breaking down of the capsid to
release the genome
3 mechanisms for penetration
Direct penetration
- Done by some naked viruses
- Most rare method
- Virus attacks, punctures cell, and inserts the genome
i.e. poliovirus
Endocytosis
- Done by most enveloped viruses and some naked
viruses
- Virus attaches to receptor molecules on the cell
- Cell is stimulated to endocytize the entire virus
Herpesvirus
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Description
Viral Replication and Prions Viral Replication - Always requires a host cell – viruses are obligate intracellular parasites and they control the hosts - This control over the host tells us: o Pathogenicity o Transmission o Level of immune response - How they replicate could indicate how we could control the infection Steps in viral multiplication (T-even phage; lytic cycle) 1. Adsorption - Attachment - Brought about by random collisions - High specificity o Flagella o Pili o Fimbriae o Cell wall o Receptors o Cell membrane 2. Penetration - Lysozyme dissolves a hole in the cell wall - The virus plunges its tail through the layers (like a hypodermic needle) - The DNA is injected 3. Replication - The entry hole is sealed to re-stabilize the cell wall - Nucleic acids of the viral genome are copied - The virus shuts down all host cell gene expression - Direct synthesis of proteins for the next capsid begins - In the late stage large volumes of lysozyme are produced 4. Maturation and Assembly - Parts of the new viruses self-assemble into mature viruses as they become available Transduction - Host DNA is put into the viral heads rather than the viral DNA - This will result in no infection in the next host, but it can cause evolutionary change (i.e. antibiotic resistance) 5. Release - Lysozyme causes the cell to lyse - The new bacteriophages are released to find a new host Lysogenic or Temperate Phage (Lambda phage) - Steps 1 and 2 are the same as in the lytic phage - In the third step, replication does not occur, the host cell is not shut down. Instead, the viral DNA is integrated into the host genome. - As the host continues to replicate itself, it is passing on the virus Prophage: the host cell after the viral DNA has been integrated into the host DNA Lysogenic conversion: the addition of new DNA results in a change in the phenotype of the cell Induction - When the host cell is dying, it identifies the foreign viral DNA and cuts it out of the sequence - This triggers the virus to take over - The lytic cycle begins: the virus shuts down the host cells and tries to replicate as quickly as possible before the cell lyses Steps in replication of animal viruses 1. Adsorption - Specific interaction with the peplomers allows for attachment *host range: what hosts can be recognized by the virus (i.e. rabies is able to target any mammal)* 2. Penetration - once inside, uncoating occurs: the breaking down of the capsid to release the genome 3 mechanisms for penetration Direct penetration - Done by some naked viruses - Most rare method - Virus attacks, punctures cell, and inserts the genome  i.e. poliovirus Endocytosis - Done by most enveloped viruses and some naked viruses - Virus attaches to receptor molecules on the cell - Cell is stimulated to endocytize the entire virus  Herpesvirus Fusion - Done by some enveloped viruses - The viral envelope fuses with the cytoplasmic membrane and releases the virus inside  Measles virus 3. Replication - DNA viruses: replication/assembly occurs in the nucleus - RNA viruses: replication/assembly occurs in the cytosol Types of Genomes dsDNA - All cellular machinery is used - Since cells normally replicate double-stranded DNA, they know what to do ssDNA - Host cellular RNA polymerase s
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