BIS 2C Lecture Notes - Lecture 10: Tobacco Mosaic Virus, Lytic Cycle, Virus
27 Apr 2018
School
Department
Course
Viruses
Viruses are intracellular obligate parasites with nucleic acids that are capable of directing
their own replication and are not cells
Argument against being alive
because can't replicate on their own, no metabolism, non-cellular
§
○
Argument for being alive
Able to evolve, replicate, natural selection acts upon them
§
○
•
Virus is the active form in the cell (actually infecting in the cell) and virion is the dormant
stage of the cell
Viruses create virions
○
Virions have nucleic acid genome, capsid protein coat and some animal viruses have
lipid envelope
○
•
Diversity of Viruses
Main morphological types of virion
Helical/rod shaped
○
Icosahedral (like 12 sided die)
○
Complex (combination of both)
Bacteriophages have complex structure with icosahedral head and helical body
§
○
•
Size
Viruses are smaller than bacteria
○
•
Genome Sizes
Can be very short genome or very long genome
Example: tobacco mosaic virus (first virus found) - only 4 very important genes
with short length
§
Example: mimivirus is the largest virus with 1000 genes coded for
§
○
•
Genomes Types
Can be RNA or DNA; single stranded or double stranded of both
RNA viruses can have + sense (translated right away into protein) and - sense
(must be transcribed and then translated)
§
○
Can even have some double stranded regions and some single strand
○
•
Lytic Viral Life Cycle
Virus must find host
•
Virus don't multiply like cellular organisms; they assemble virions inside the cells of host
until there are enough virion components
•
Host DNA ingested
•
Virus replication cycle
Getting in
Attach to host cell membrane (only animal cells)
§
Penetrate host cells through
Endocytosis (animal cells)
mechanical penetrations (trauma) with plant and fungal viruses
®
□
§
Some bacterial viruses inject their genome directly
§
Expose coat
§
○
Replicate
○
Assemble virions to be released from cell
○
•
Some Fungal and plant virus enter cell through wounds made by phloem feeding insects
•
Bacteriophage directly inject viral genome
•
After infection, the host cell is destroyed and virions are exposed to environment
•
Lytic vs Lysogenic Cycle
Instead of degrading the host DNA, it now adds itself to the bacterial chromosome so that
DNA replication multiplies the viral genome and then the lytic cycle begins
Example herpes, chicken pox
○
•
Some viruses have receptors that count number of signal molecules produced after lytic
cycles and these virus switch to the lysogenic so that host number is not too low; give more
time for host reproduction
Kind of like quorum sensing in bacteria
○
•
Viruses and Human Genome
Viruses carry a large amount of human
9% of genome carried by the virus
○
•
Viruses and Tree of Life
Can't sequence ribosomal DNA and place viruses on the tree because they don’t have
ribosomes
•
Different Hypotheses
Relics of pre-cellular world (least support)
○
Viruses are escaped portions of cellular orgs (like plasmids getting replication abilities)
○
Viruses are extremely derived and reduced cellular orgs
Viruses are polyphyletic
§
○
•
Viruses are most abundant biological entities on planet (1031)
•
Viruses are very abundant in the ocean (kill 40% of prokaryote biomass in ocean every day)
•
Some viruses don't cause bad effect
Thermophilic plants in some areas are infected with fungus with virus which helps
plant live in this temp
○
•
New Antibiotic without Detectable Resistance: developed from
previously unculturable bacteria
Targets the lipid membrane on multiple areas, since they're more
complex molecules, resistance would be hard
Viruses (10)
Wednesday, January 24, 2018
12:08 PM
Viruses
Viruses are intracellular obligate parasites with nucleic acids that are capable of directing
their own replication and are not cells
Argument against being alive
because can't replicate on their own, no metabolism, non-cellular
§
○
Argument for being alive
Able to evolve, replicate, natural selection acts upon them
§
○
•
Virus is the active form in the cell (actually infecting in the cell) and virion is the dormant
stage of the cell
Viruses create virions
○
Virions have nucleic acid genome, capsid protein coat and some animal viruses have
lipid envelope
○
•
Diversity of Viruses
Main morphological types of virion
Helical/rod shaped
○
Icosahedral (like 12 sided die)
○
Complex (combination of both)
Bacteriophages have complex structure with icosahedral head and helical body
§
○
•
Size
Viruses are smaller than bacteria
○
•
Genome Sizes
Can be very short genome or very long genome
Example: tobacco mosaic virus (first virus found) - only 4 very important genes
with short length
§
Example: mimivirus is the largest virus with 1000 genes coded for
§
○
•
Genomes Types
Can be RNA or DNA; single stranded or double stranded of both
RNA viruses can have + sense (translated right away into protein) and - sense
(must be transcribed and then translated)
§
○
Can even have some double stranded regions and some single strand
○
•
Lytic Viral Life Cycle
Virus must find host
•
Virus don't multiply like cellular organisms; they assemble virions inside the cells of host
until there are enough virion components
•
Host DNA ingested
•
Virus replication cycle
Getting in
Attach to host cell membrane (only animal cells)
§
Penetrate host cells through
Endocytosis (animal cells)
mechanical penetrations (trauma) with plant and fungal viruses
®
□
§
Some bacterial viruses inject their genome directly
§
Expose coat
§
○
Replicate
○
Assemble virions to be released from cell
○
•
Some Fungal and plant virus enter cell through wounds made by phloem feeding insects
•
Bacteriophage directly inject viral genome
•
After infection, the host cell is destroyed and virions are exposed to environment
•
Lytic vs Lysogenic Cycle
Instead of degrading the host DNA, it now adds itself to the bacterial chromosome so that
DNA replication multiplies the viral genome and then the lytic cycle begins
Example herpes, chicken pox
○
•
Some viruses have receptors that count number of signal molecules produced after lytic
cycles and these virus switch to the lysogenic so that host number is not too low; give more
time for host reproduction
Kind of like quorum sensing in bacteria
○
•
Viruses and Human Genome
Viruses carry a large amount of human
9% of genome carried by the virus
○
•
Viruses and Tree of Life
Can't sequence ribosomal DNA and place viruses on the tree because they don’t have
ribosomes
•
Different Hypotheses
Relics of pre-cellular world (least support)
○
Viruses are escaped portions of cellular orgs (like plasmids getting replication abilities)
○
Viruses are extremely derived and reduced cellular orgs
Viruses are polyphyletic
§
○
•
Viruses are most abundant biological entities on planet (1031)
•
Viruses are very abundant in the ocean (kill 40% of prokaryote biomass in ocean every day)
•
Some viruses don't cause bad effect
Thermophilic plants in some areas are infected with fungus with virus which helps
plant live in this temp
○
•
New Antibiotic without Detectable Resistance: developed from
previously unculturable bacteria
Targets the lipid membrane on multiple areas, since they're more
complex molecules, resistance would be hard
Viruses (10)
Wednesday, January 24, 2018
12:08 PM
