Lecture 7 Flashcards
LAST TIME WE DISCUSSED
Environmental influences on growth
• Temperature, pressure, pH, oxygen
Extremophiles
• Mesophiles, psychrophiles, thermophiles, barophiles
Antimicrobial control measures
• Sterilization, disinfection, antisepsis, and sanitation
Microbes that grow in ____ are called?
• Temperatures 0-20 C
(Psychropiles)
• High salt 2-4M NaCl
(Halophiles)
REVIEW FROM LAST TIME
Which of the following statements about microbes and temperature is FALSE?
ANSWER : D
A. Within a specific range, the microbial growth rate roughly doubles for every 10oC rise in temperature.
B. Changes in temperature affect membrane fluidity, and thus influence nutrient transport.
C. Thermophiles are microbes that grow at temperatures between 40oC and 80oC.
D. Microbes have mechanisms to control their temperature.
VIRUSES ARE EVERYWHERE
Viruses act as a ______ consumer of marine microbes
Dominant
When marine algae overgrow they generate a _____
Bloom
Bloom
is dissipated by viruses (lyse algae as they grow)
Human virome
• Most of our own viruses go unnoticed • Some contribute to our health
We hear about the viruses that cause epidemics
(influenza, HIV)
_______ have provided tools and model systems that led to discovery of fundamental principles of molecular biology
Viruses
WHAT IS A VIRUS ?
Non-cellular (non-living) particle that infect a host cell and use the cell’s machinery to replicate
Virion (virus particle) -
consists of a nucleic acid genome (DNA or RNA, single or double stranded)
Has a protein coat
capsid
Viruses may kill their host or copy their _____ into the host’s genome
Genes
All kinds of cells (bacteria, eukaryotes, archaea) can be infected by viruses
• Estimated 10^32 viruses on earth
EVEN SMALLER THAN A VIRUS
Vitriols
Don’t have a capsid
RNA genome is itself the infectious particle
• Infect plants
• RNA structure prevents degradation by host Rnases
[pic]
Prions-
Can make other misfold as well?
protein only (no nucleic acid)
• Abnormal protein structure
• Cause of “mad cow” disease
• Creutzfeldt-Jakob disease
VIRUS STRUCTURE- ICOSAHEDRAL VIRUSES
\_\_\_\_\_\_\_\_\_\_\_ are a type of symmetrical virus that packages their genome in an icosahedral capsid
Icosahedral viruses
Capsid
polyhedron with 20 identical triangular faces
Each triangle can be composed of three identical but asymmetrical
protein units
Each triangular face is determined by the same genes encoding the same protein subunit
Rotational symmetry
no matter what the pattern of the subunits
[pic]
VIRUS STRUCTURE- FILAMENTOUS VIRUSES
Filamentous viruses
are a type of symmetrical virus with helical symmetry
Pattern of capsid molecules forms a __________ around the genome, which is coiled inside
Helical tube
Genome can be _______ stranded DNA or RNA
Single
Examples
M13 phage, Ebola, tobacco mosaic virus
VIRUS STRUCTURE- TAILED VIRUSES
Tailed viruses
Multipart structure
Icosahedral capsid or “head” containing the genome attached to a helical “neck” that channels the ___________
into the host
Nucleic acid
Example
T4 bacteriophage
[pic]
VIRUS STRUCTURE - ASYMMETRICAL VIRIONS
Asymmetrical viruses –
lack capsid symmetry
• Examples: Influenza, poxviruses
Vaccinia poxvirus (cowpox)
• Core envelope encloses the nucleo capsid-coated DNA and
accessory proteins
• Proteins can be found inside the capsid or between the core envelop and outer membrane
[PIC]
VIRUS STRUCTURE- ENVELOPES
Viruses may have a _______
lipid envelope
Allows fusion to host cell membrane
• Occurs if host cell is not covered by cell wall
*Bacteriophages and plant viruses are NON-enveloped
Envelope lipids come from the host membrane
• Not encoded by the viral genome
Envelope proteins usually encoded by virus
Coats viral capsid as virus leaves cell or organelle
[pic]
VIRAL GENOMES
- DNA or RNA
- Single- or double-stranded
- Linear or circular
..
• Includes genes encoding viral proteins
* Capsid
* Envelope proteins if virus is enveloped
* Any polymerase not found in host cell
[PIC]
Size varies greatly
• Small – fewer than 10 genes • Large – 500-2,500 genes
CLASSIFICATION OF VIRUSES
International Committee on Taxonomy of Viruses (ICTV)
Criteria:
• Genome composition – classified by the Baltimore method
• Capsid symmetry (helical, icosahedral)
• Envelope
*Presence of host-derived envelop and structure
• Size of the virus particle
• Host range
*Closely related viruses typically infect the same or related hosts
THE BALTIMORE VIRUS CLASSIFICATION
Based on genome composition and means of mRNA production
[important pic]
[pic]
[another pic]
[another pic]
MOLECULAR EVOLUTION OF VIRUSES
How do we compare virus genomes?
No rRNA
Comparison is based on _________ – genes of common ancestry in two genomes that share the same function
Orthologs
[pic]
_______ classification is useful for viruses because their small genomes encode few proteins
Proteomic
Statistical comparison of phage proteins predicted major evolutionary categories of phage species that share common host _________
Bactria
[pic]
VIRAL LIFE CYCLES
All viruses require a host for reproduction and require:
- Host recognition and attachment
- Cell surface receptors
- Genome entry
- Enter and gain access to machinery
- Replicate genome
- Make viral proteins
Continued…
- Assemble capsids
- Exit and transmission
- Release progeny viruses from host cell
BACTERIOPHAGE LIFE CYCLE
Host recognition and attachment
*Cell- surface receptors
proteins on the host cell surface that are specific to the host species and bind to a specific viral componen
Typically a protein with an important function for the host cell
.[pic]
• Genome _____
Entry
Most phages inject only their genome into the cell through the ________
Cell envelope
The phage capsid remains outside, attached to the cell surface
• “ghost” – empty capsid
[pic]
Lytic cycle
• Phage injects its genome into a cell and immediately
reproduces as many progeny phage particles as possible
• Expression of phage genes
• “Early genes” – to produce new phage capsids etc
• “Late gene” – expressed once capsids are assembled; produces enzyme that lyses the host cell – killing the host and releasing new phage particles
• Lysis is also referred to as a burst and the number of particles released is called the burst size
Lysogenic cycle
- Temperate phage
- Integrates its genome into the host genome (prophage)
- Presence of prophage prevents further infection of host by other virions of the same type
- Prophage DNA is replicated along with the host
Continue…
• Prophage can reactivate to become lytic by excising from the
host genome
* Can occur at random
*Triggered by environmental stress(senses when host is about to die)
• During exit from lysogeny the phage can acquire host genes that it then passes on to another cell (transduction)
Lysis and Lysogeny
[PIC]
[VIDEO]
Slow-release cycle
- Phage particles reproduce without killing the host cell
- Phage M13
- Slow generation of progeny genomes and assembly of phage particles
- Phage particles extrude through the envelope without lysing the cell
DISCUSSION: SLOW RELEASE VS. TEMPERATURE PHAGE
What are the advantages and disadvantages (from virus perspective) of slow-release vs alternating between lysis and lysogeny?
Slow release- take long - host destroyed by something else
-don’t need big population of host cells
Lysogeny-host reused
Slow release- environment might not conclusive so it waits
*uses a lot of host resources
Lyric- more viruses
Lysogeny- uses little/ no resources —> many …
The slow release actually uses a lot of cellular resources
BACTERIAL HOST DEFENSES
Bacteria have evolved several forms of defense against ______________
Bacterialphage infection
Genetic resistance
Altered host receptor proteins (phage can’t bind)
Restriction endonucleases
Cleaves viral DNA which lacks methylation
CRISPR
• Clustered Regularly Interspaced Short Palindromic Repeats
• Bacterial immune system
• Copy a tiny piece of the phage – “remembers” the phage
*Detects this phage DNA in the next infection and cleave sit
ANIMAL VIRUSES
Host recognition and attachment
- Cell surface receptors
- Tropism – ability to infect a particular tissue type within a host
- Broad tropism – infect many kinds of host tissues (e.g. Ebola)
Entry into cell
• Inject genome directly (similar to bacteriophage) • Most animal viruses enter the cell as virions • Taken up via endocytosis *Brought into the cell as an endosome *Requiresuncoating • Viral envelope fuses to host membrane * Releases capsid into cell
Genome replication
DNA viruses must get DNA into the nucleus to use the host
polymerase (e.g. HPV)
• ORreplicateincytoplasmwithaviralpolymerase
Genome replication
RNA viruses encode a viral polymerase
- RNA to RNA :RNA-dependent RNA polymerase
- RNA to DNA:Reverse transcriptase (RNA-dependent DNA polymerase) – carried within the virion (Retroviruses)
(+) ssRNA viruses – translated directly by host ribosomes
• Produces RNA-dependent RNA polymerase ,which generates the
(-) RNA strand that is used as a template for progeny genomes
(-) ssRNA viruses
used as a template to create mRNA and progeny genomes
Exit and transmission
• Lysis of cell * Similar to bacteria • Budding *Viruspassesthrough membrane * Membrane lipids surround capsid * All enveloped viruses bud from a membrane * Plasma membrane or organelle membrane [pic]
