MICR-3050 Chapter Notes - Chapter 8: Intracellular Parasite, Symmetry, Rna Virus
30 Apr 2018
School
Department
Course
Professor
Viruses: What are they?
Genetic elements that replicate independently of a cells chromosome
Requires living host
○
Obligate intracellular parasite
○
Acellular infectious agent
○
•
Contain DNA and RNA
SS or DS
○
Linear, circular or segmented
Genome in pieces
Easy to get genetic recombination
How different strains of viruses arise over time
®
□
§
○
•
Extracellular form
Virion: complete virus particle
Nucleic acid surrounded by protein capsule
This is how the virus moves from one cell to another
□
§
○
•
Virus Structure
Size range: approx. 10-1000 nm
•
Contain nucleocapsid
Nucleic acid (DNA or RNA)
○
Protein coat (capsid)
Some have additional components
§
○
•
Describing shape = look at virion or capsid
•
Virus Symmetry
Very efficient in how they package genome
Look at shape of capsid
○
•
Helical symmetry
•
Icosahedral (most efficient way to enclose space)
•
Complex symmetry
Vaccinia
○
Binal symmetry: icosahedral + helical
Ex. T4 bacteriophage
§
○
•
Viral genomes
dsDNA and ssRNA: most common in animal virus
ssRNA = think reverse transcription
○
•
dsDNA: most common in bacteria
•
Central dogma
DNA => mRNA => Protein
ssRNA has a problem with this dogma
Cant use machinery of host cell
□
Solution! Use own genome directly as mRNA
□
§
○
•
Positive sense: genome of virus and mRNA are the same
Virus uses genome as mRNA
○
ssRNA
○
•
Negative sense: genome of virus and mRNA are not the same
Virus transcribes its genome into mRNA
○
ssRNA that makes complement of its genome
○
•
Viral Envelope
Outer flexible membranous layer
Lipid bilayer
Stolen from host cell as they emerge from the host cell
§
○
•
Envelope protein project from surface
Spikes or peplomers
Viral encoded
§
○
•
Function of envelope proteins:
Viral attachment to host cell
Need something to help connect with cell membrane
Cell membrane usually has "locks" (attachment sites) on
its surface which things entering the cell have to attach to
before gaining entrance.
□
§
○
Enzymatic activity
○
Role in nucleic acid replication
○
Identification of virus
○
•
Viral Replication
Mechanism depends on viral structure and genome
•
Replication steps
Attachment
Not random, very specifica.
1.
Entry into host cell
Animal viruses: whole virion goes into cell or envelope fuses with
cell membrane
a.
Bacteriophage: release DNA into cell
Do not enter cell. Empty capsid outside of cell. i.
b.
2.
Uncoating of genome
Bacteriophage: does not go through because it already emptied
capsid into cell
a.
Animal virus: separate genome from capsid or envelope b.
3.
Synthesis 4.
Assembly 5.
Release6.
Growth of Virus during replication
Latent period: eclipse + maturation
Eclipse: synthesis of enzymes, nucleic acid, protein coats
○
Maturation: assembly and release of virus
○
•
Bacteriophages
Aka phages
•
Diverse and structurally complex
•
Common genome: dsDNA
•
Most are naked
•
Lytic (virulent) bacteriophages
Begin multiplying IMMEDIATELY after entering host
•
Result: lyses of host cell for release
•
Lytic pathway only
•
Ex. T4
•
Only infects gram negative bacteria
Don’t have to worry about pesky peptidoglycan layer
○
•
Process (in minutes)
0 minutes: infection
•
0-5 minutes: early mRNA/early proteins
T4 nucleases, DNA polymerase and new sigma factors used on host
cell RNA
○
•
5-10 minutes: middle mRNA/middle proteins
Phage T4 DNA made
○
•
10-20 minutes: late mRNA/late proteins
•
20-25 minutes: self assembly
Phage head, tail, collar, base protein plate and tail fiber proteins
○
•
Process at a glance
Attachment1.
Inject viral DNA 2.
Induction occurs 3.
Phage components needed for release are synthesized 4.
Lysis of host cell5.
Lysogenic (temperate) phages)
Remain within host cell w/o destroying cell
Integrate into host genome
Called lysogeny
§
○
•
Ex. Lambda
•
Process of lysogeny
Attachment 1.
Inject viral DNA 2.
Integrate into host DNA 3.
Become prophage 4.
Replicate w/ host DNA 5.
Lysogeny
Nonlytic relationship b/w host and phage
•
Prophage: integrated phage genome
Use integrase enzyme to integrate into host chromosome
○
Integrate b/w galactose and biotin operon attachment site
○
•
Replicate w host genome
•
Lysogen: infected bacterial host
Appears normal
○
Lysogenic conversion: prophage changes host phenotype
○
Phage may switch from lysogenic to lytic cycle via induction
○
•
Induction of Lytic Cycle
Temperate phage reproduction is initiated
•
Result in switch to lytic cycle
•
Trigger: drop in levels of lambda repressor
Factors in cell that are keeping the lambda virus at bay and away
from going to the dark side
○
Cause: exposure to UV light or chem mutagens that cause DNA
damage
○
•
Excisionase: binds integrase enzyme
Enable integrase to reverse integration process
○
•
Animal Viruses
Entire virion enters the animal cell
Replicate in nucleus
○
Many are enveloped
○
•
Effects of animal virus
Virulent
○
Latent
○
Persistent
Latent viral infection: virus stops reproducing and remains
dormant
Viruses are not detectable
Ex. HSV (cold sores in adults, HSV in children)
Varicella-zoster (chicken-pox then shingles)
◊
®
□
§
Persistent viral infection: virus almost always detectable
Clinical symptoms mild or absent for long periods
□
Ex. Hepatitis B, HIV
□
§
○
Cancer: oncovirus
○
•
Retroviruses
Retrovirus: RNA virus that replicates through DNA intermediate
Enveloped viruses
○
Reverse transcriptase
RNA to DNA
§
○
•
Has provirus stage and is released by budding
•
Ex. HIV
•
Chapter 8
Monday, April 16, 2018
1:08 PM
Viruses: What are they?
Genetic elements that replicate independently of a cells chromosome
Requires living host
○
Obligate intracellular parasite
○
Acellular infectious agent
○
•
Contain DNA and RNA
SS or DS
○
Linear, circular or segmented
Genome in pieces
Easy to get genetic recombination
How different strains of viruses arise over time
®
□
§
○
•
Extracellular form
Virion: complete virus particle
Nucleic acid surrounded by protein capsule
This is how the virus moves from one cell to another
□
§
○
•
Virus Structure
Size range: approx. 10-1000 nm
•
Contain nucleocapsid
Nucleic acid (DNA or RNA)
○
Protein coat (capsid)
Some have additional components
§
○
•
Describing shape = look at virion or capsid
•
Virus Symmetry
Very efficient in how they package genome
Look at shape of capsid
○
•
Helical symmetry
•
Icosahedral (most efficient way to enclose space)
•
Complex symmetry
Vaccinia
○
Binal symmetry: icosahedral + helical
Ex. T4 bacteriophage
§
○
•
Viral genomes
dsDNA and ssRNA: most common in animal virus
ssRNA = think reverse transcription
○
•
dsDNA: most common in bacteria
•
Central dogma
DNA => mRNA => Protein
ssRNA has a problem with this dogma
Cant use machinery of host cell
□
Solution! Use own genome directly as mRNA
□
§
○
•
Positive sense: genome of virus and mRNA are the same
Virus uses genome as mRNA
○
ssRNA
○
•
Negative sense: genome of virus and mRNA are not the same
Virus transcribes its genome into mRNA
○
ssRNA that makes complement of its genome
○
•
Viral Envelope
Outer flexible membranous layer
Lipid bilayer
Stolen from host cell as they emerge from the host cell
§
○
•
Envelope protein project from surface
Spikes or peplomers
Viral encoded
§
○
•
Function of envelope proteins:
Viral attachment to host cell
Need something to help connect with cell membrane
Cell membrane usually has "locks" (attachment sites) on
its surface which things entering the cell have to attach to
before gaining entrance.
□
§
○
Enzymatic activity
○
Role in nucleic acid replication
○
Identification of virus
○
•
Viral Replication
Mechanism depends on viral structure and genome
•
Replication steps
Attachment
Not random, very specifica.
1.
Entry into host cell
Animal viruses: whole virion goes into cell or envelope fuses with
cell membrane
a.
Bacteriophage: release DNA into cell
Do not enter cell. Empty capsid outside of cell. i.
b.
2.
Uncoating of genome
Bacteriophage: does not go through because it already emptied
capsid into cell
a.
Animal virus: separate genome from capsid or envelope b.
3.
Synthesis 4.
Assembly 5.
Release6.
Growth of Virus during replication
Latent period: eclipse + maturation
Eclipse: synthesis of enzymes, nucleic acid, protein coats
○
Maturation: assembly and release of virus
○
•
Bacteriophages
Aka phages
•
Diverse and structurally complex
•
Common genome: dsDNA
•
Most are naked
•
Lytic (virulent) bacteriophages
Begin multiplying IMMEDIATELY after entering host
•
Result: lyses of host cell for release
•
Lytic pathway only
•
Ex. T4
•
Only infects gram negative bacteria
Don’t have to worry about pesky peptidoglycan layer
○
•
Process (in minutes)
0 minutes: infection
•
0-5 minutes: early mRNA/early proteins
T4 nucleases, DNA polymerase and new sigma factors used on host
cell RNA
○
•
5-10 minutes: middle mRNA/middle proteins
Phage T4 DNA made
○
•
10-20 minutes: late mRNA/late proteins
•
20-25 minutes: self assembly
Phage head, tail, collar, base protein plate and tail fiber proteins
○
•
Process at a glance
Attachment1.
Inject viral DNA 2.
Induction occurs 3.
Phage components needed for release are synthesized 4.
Lysis of host cell5.
Lysogenic (temperate) phages)
Remain within host cell w/o destroying cell
Integrate into host genome
Called lysogeny
§
○
•
Ex. Lambda
•
Process of lysogeny
Attachment 1.
Inject viral DNA 2.
Integrate into host DNA 3.
Become prophage 4.
Replicate w/ host DNA 5.
Lysogeny
Nonlytic relationship b/w host and phage
•
Prophage: integrated phage genome
Use integrase enzyme to integrate into host chromosome
○
Integrate b/w galactose and biotin operon attachment site
○
•
Replicate w host genome
•
Lysogen: infected bacterial host
Appears normal
○
Lysogenic conversion: prophage changes host phenotype
○
Phage may switch from lysogenic to lytic cycle via induction
○
•
Induction of Lytic Cycle
Temperate phage reproduction is initiated
•
Result in switch to lytic cycle
•
Trigger: drop in levels of lambda repressor
Factors in cell that are keeping the lambda virus at bay and away
from going to the dark side
○
Cause: exposure to UV light or chem mutagens that cause DNA
damage
○
•
Excisionase: binds integrase enzyme
Enable integrase to reverse integration process
○
•
Animal Viruses
Entire virion enters the animal cell
Replicate in nucleus
○
Many are enveloped
○
•
Effects of animal virus
Virulent
○
Latent
○
Persistent
Latent viral infection: virus stops reproducing and remains
dormant
Viruses are not detectable
Ex. HSV (cold sores in adults, HSV in children)
Varicella-zoster (chicken-pox then shingles)
◊
®
□
§
Persistent viral infection: virus almost always detectable
Clinical symptoms mild or absent for long periods
□
Ex. Hepatitis B, HIV
□
§
○
Cancer: oncovirus
○
•
Retroviruses
Retrovirus: RNA virus that replicates through DNA intermediate
Enveloped viruses
○
Reverse transcriptase
RNA to DNA
§
○
•
Has provirus stage and is released by budding
•
Ex. HIV
•
Chapter 8
Monday, April 16, 2018 1:08 PM
Document Summary
Genetic elements that replicate independently of a cells chromosome. How different strains of viruses arise over time. This is how the virus moves from one cell to another. Describing shape = look at virion or capsid. Viral genomes dsdna and ssrna: most common in animal virus ssrna = think reverse transcription dsdna: most common in bacteria. Dna => mrna => protein ssrna has a problem with this dogma. Positive sense: genome of virus and mrna are the same. Negative sense: genome of virus and mrna are not the same. Virus transcribes its genome into mrna ssrna that makes complement of its genome. Stolen from host cell as they emerge from the host cell. Need something to help connect with cell membrane. Cell membrane usually has locks (attachment sites) on its surface which things entering the cell have to attach to before gaining entrance. Animal viruses: whole virion goes into cell or envelope fuses with cell membrane.
