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Lecture 8

BMS 212 Lecture Notes - Lecture 8: Reverse Transcriptase, Provirus, Exocytosis

5 pages136 viewsFall 2016

Biomedical Sciences
Course Code
BMS 212
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
- Host DNA is put into the viral heads rather than the viral
- 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
- 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
- Done by most enveloped viruses and some naked
- Virus attaches to receptor molecules on the cell
- Cell is stimulated to endocytize the entire virus
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