MICROBIAL GENETICS MECHANISMS OF GENETIC VARIATION
Dr. Kulsum Kondiah
VIRUSES
3/6/2012
VIRUSES & HISTORY
o Played a role in shaping our history
o Roman empire measles and smallpox o Conquest of the Aztec empire smallpox o Cancer research Rous sarcoma virus
Recent viral contributions?
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chrocodiles.blogspot.com, textbookofbacteriology.net
DISCOVERY OF VIRUSES - TMV
o Dmitri Ivanowsky used filtered sap to show that leaf sap from infected plants induced tobacco mosaic disease o Martinus Beijerinck called the infectious agent a filterable virus o Wendell Stanley crystallized TMV confirming the infectious agent is a virus
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healthyhomegardening.com, brooklyn.cuny.edu
Viruses can reproduce on their own just like humans or bacteria.
o True or False?
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DEFINITION
o Virus simple, acellular entity that can only reproduce within living cells o Virion complete virus particle with one or more molecules of DNA or RNA enclosed in a coat of protein o Capsid protein coat o Nucleocapsid core nucleic acid enclosed within the protein coat o Protomer/capsomere protein building unit of capsid
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STRUCTURE OF VIRUSES
o Virion size ranges from 10 400 nm o Capsid
o Self assemble from many copies of one or more proteins o Helical, icosahedral and complex symmetries o Envelope may surround the capsid
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micro.magnet.fsu, edu mrsec.wisc.edu
What is another name for the protein subunits that make up the viral capsid?
o 1. Amino acid o 2. Capsidomer o 3. Protomer
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STRUCTURE OF VIRUSES
o Envelope
o Membranous (derived from host cell) o Contains host cell phospholipids and membrane proteins o Contains proteins and glycoproteins that project as spikes
o Influenza has neuraminidase (release of mature virions) and hemagglutinin (attachment to host cell)
o Can be flexible or firmly attached
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abc.net.au
STRUCTURE OF VIRUSES
o Genomes
o ssDNA, dsDNA, ssRNA (+/- strand), dsRNA o Linear or circular o Smallest genomes code for 3/4 proteins o Largest genomes code for as many as several hundred to a thousand
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BALTIMORE SCHEME OF VIRAL CLASSIFICATION
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viralzone.expasy.org
GENERAL VIRAL REPRODUCTIVE CYCLE
o Limited host range
o Recognition of host cell receptors by viral surface proteins
o Reproduction begins with attachment of the virus to the host cell o Mechanism of genome entry depends on host cell
o T-even phages inject DNA into cell o Viral particles can be endocytosed o Viral envelope fuses with plasma membrane
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VIRAL REPRODUCTIVE CYCLE
1. Attachment 2. Penetration 3. Replication 4. Assembly 5. Release
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sciencetechnologyaction.com
What do you think a virus that infects bacteria is called?
o 1. Bacteriavirus o 2. Baculovirus o 3. Bacteriophage
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BACTERIOPHAGES
o Viruses that infect bacteria o Exhibit complex symmetry
o Elongated icosahedral head enclosing DNA o Protein tail piece attached to head with fiber (attachment)
o First phages studied included seven that infect Escherichia coli T1 to T7
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theartfulamoeba.com, www1.gzbio.net
REPRODUCTION LYTIC CYCLE
o Reproduction results in death of the host cell o A phage that reproduces only by a lytic cycle is called a virulent phage o T4 bacteriophage
o Tail fibers attach to specific receptors on host cell o Tail sheath contracts and injects the phage DNA into the cell o Cell DNA is destroyed and phage DNA directs replication of phage genome and production of phage proteins
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REPRODUCTION LYTIC CYCLE
o Phage proteins (head, baseplate and tail fiber) self assemble and phage DNA is packaged into head o T4 lysozyme attacks ?, fluid enters cell which swells and bursts releasing 100-200 phage particles
o Entire lytic cycle from phages first contact with host cell to cell lysis takes 20-30 minutes at 37 oC
Video: Lytic cycle
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T4 BACTERIOPHAGE ASSEMBLY
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LYTIC CYCLE OF A T4 BATERIOPHAGE
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faculty.ksu.edu.sa
WHY ARENT BACTERIA EXTINCT?
o Natural selection
o Mutants with different receptors o Viral counter mutants that bind altered receptors Constant flux
o Restriction enzymes
o Bacterial DNA is methylated to prevent restriction o Viral counter resistance Coexistence
o Lysogeny
o Bacterial host is not destroyed
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REPRODUCTION LYSOGENIC CYCLE
o DNA phages can remain in the host cell without destroying it o Phages that can reproduce lytically or by lysogeny are referred to as temperate phages o Infected bacteria are known as lysogens o Advantages
o Viable in a dormant host o Continued reproduction during high multiplicity of infection (MOI), due to survival of host cells
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REPRODUCTION LYSOGENIC CYCLE
o phage infects E. coli K12
o Noncontractile tail with a single, short, thin tail fiber at the end o Binds to the cell surface and injects its linear genome o Linear genome circularizes (complementary cohesive ends base pair with each other) o Regulatory events decide if the phage pursues a lytic or lysogenic cycle
3/6/2012
biochem.wisc.edu, genemol.org
REPRODUCTION LYSOGENIC CYCLE
o DNA integrated into the host chromosome (integrase) at attachment (att) site (prophage)
o Homologous att sites on both E. coli and phage genomes base pair with each other o Bacterial att site is located between the gal (galactose) and bio (biotin) operons
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PROPHAGE
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How do you think the prophage will replicate?
o 1. Independently of the bacterial genome o 2. With the bacterial genome o 3. It will not replicate
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REPRODUCTION LYSOGENIC CYCLE
o Prophage is replicated as the bacterial genome is replicated
o Large population of daughter cells carrying the prophage o Expression of a few prophage genes can lead to alteration of the hosts phenotype e.g. diptheria, O157:H7
o Induction (chemical mutagens, UV light) triggers a switchover from lysogenic to lytic
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REPRODUCTIVE CYCLES OF PHAGE
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bio1151.nicerweb.com
