Vivacious VirusesVivacious Viruses

Chapter 19Chapter 19

I. VirusI. Virus

A.A. CharacteristicsCharacteristics1.1. Smaller than a ribosomeSmaller than a ribosome2.2. Can form into regular crystals (cells won’t do Can form into regular crystals (cells won’t do

this)this)3.3. Made of Nucleic Acid - Genome is made of Made of Nucleic Acid - Genome is made of

one of the following:one of the following: DNA - double strandedDNA - double stranded DNA - single strandedDNA - single stranded RNA - double strandedRNA - double stranded RNA - single strandedRNA - single stranded

A. Characteristics (cont)A. Characteristics (cont)

1.1. Range from 4 genes to several hundredRange from 4 genes to several hundred2.2. Protein coat surrounds nucleic acidProtein coat surrounds nucleic acid *Capsid - protein coat*Capsid - protein coat -can be rod shaped (helical), polyhedral -can be rod shaped (helical), polyhedral

or more complex or more complex -capsids are made form large numbers -capsids are made form large numbers

of protein subunits called capsomeres of protein subunits called capsomeres

A. Characteristics (cont)A. Characteristics (cont)

1.1. Accessory Structures (found in some Accessory Structures (found in some viruses)viruses)

Viral Envelopes - membranes surrounding capsidViral Envelopes - membranes surrounding capsid come from membrane of their host cell and contain the come from membrane of their host cell and contain the

same proteins, glycoproteins and phospholipids used by same proteins, glycoproteins and phospholipids used by hosthost

These envelopes help viruses to infect host These envelopes help viruses to infect host without being detectedwithout being detected

Bacteriophages - have most complex capsidsBacteriophages - have most complex capsids

B. Reproduction of B. Reproduction of Viruses Viruses

Happens only in a host Happens only in a host

cell because they have nocell because they have no

ribosomes or enzymes toribosomes or enzymes to

reproduce on their ownreproduce on their own

B. Reproduction of B. Reproduction of VirusesViruses

1.1. Host Range - limited amount of host cells that Host Range - limited amount of host cells that a virus can infect (can be one host or a few a virus can infect (can be one host or a few related species)related species)

Host identified by “lock and key” fit between Host identified by “lock and key” fit between proteins on capsid and receptors on outside of proteins on capsid and receptors on outside of host cellhost cell

Can be Tissue Specific - eukaryotic viruses may Can be Tissue Specific - eukaryotic viruses may infect only certain tissues of their hostinfect only certain tissues of their host

ex. cold - respiratory tissue, HIV - white blood ex. cold - respiratory tissue, HIV - white blood cells onlycells only

B. Reproduction of B. Reproduction of VirusesViruses

1.1. Methods of Reproduction - virus infects Methods of Reproduction - virus infects host first and overtakes host cell to make host first and overtakes host cell to make viral nucleic acids and proteinsviral nucleic acids and proteins Host provides the following:Host provides the following:

nucleotides for synthesizing viruses nucleic acidnucleotides for synthesizing viruses nucleic acid EnzymesEnzymes RibosomesRibosomes tRNA’s, amino acids, ATP and other parts tRNA’s, amino acids, ATP and other parts

needed by virus to go through transcription and needed by virus to go through transcription and translation (to make viral proteins)translation (to make viral proteins)

A.A. Reproduction of VirusesReproduction of VirusesBacterial Viruses Bacterial Viruses

1.1. Lytic CycleLytic Cycle - viral reproductive cycle - viral reproductive cycle that ends in the death of the host cell that ends in the death of the host cell (also called virulent virus) (also called virulent virus)

ex. T4 bacteriophageex. T4 bacteriophage

3. Lytic Cycle (cont)3. Lytic Cycle (cont)

a.a. Phage attaches to cell - uses tail fiber to Phage attaches to cell - uses tail fiber to stick to receptor site on host (e. coli)stick to receptor site on host (e. coli)

b.b. DNA is injected through cell wall and DNA is injected through cell wall and membrane into the host cellmembrane into the host cell

a. Empty capsid remains outside the cell and DNA of the host is hydrolyzed (broken up)

b. Phage overtakes cell using it’s parts to manufacture viral nucleic acids and proteins. New phages are reassembled inside host cell

a.a. Phage makes host cell produce Phage makes host cell produce lysozyme - an enzyme that digests the lysozyme - an enzyme that digests the host’s cell wall. Osmosis causes the cell host’s cell wall. Osmosis causes the cell to swell and burst releasing the new to swell and burst releasing the new phages (may be 100-200 new ones) phages (may be 100-200 new ones) Phages go find new host cell to infectPhages go find new host cell to infect

Reproduction of VirusesReproduction of VirusesBacterial VirusesBacterial Viruses

1.1. Lysogenic Cycle - replicates viral Lysogenic Cycle - replicates viral genome without killing host cell genome without killing host cell

If they use both lytic and lysogenic - If they use both lytic and lysogenic - also called temperate virusesalso called temperate viruses

4. Lysogenic Cycle (cont)4. Lysogenic Cycle (cont)

a.a. Phage binds to host cell and injects DNAPhage binds to host cell and injects DNAb.b. Phage DNA forms a circle inside host - it can now go Phage DNA forms a circle inside host - it can now go

through either lytic or lysogenic cycle. (If lytic - see through either lytic or lysogenic cycle. (If lytic - see process above)process above)

c.c. Viral DNA lines up with host DNA and gets Viral DNA lines up with host DNA and gets incorporated into the host DNA by crossing over - it is incorporated into the host DNA by crossing over - it is now called a now called a prophageprophage - one of its genes represses - one of its genes represses the other genes so it’s basically not affecting the host the other genes so it’s basically not affecting the host at all at this pointat all at this point

d.d. When the host cell replicates and divides, it also When the host cell replicates and divides, it also replicates the virus and passes it on to its daughter replicates the virus and passes it on to its daughter cells cells

4. Lysogenic Cycle 4. Lysogenic Cycle (cont)/(cont)/

a.a. At some point, phages will leave the At some point, phages will leave the host DNA and return to a lytic cycle. At host DNA and return to a lytic cycle. At this point they will destroy their host cell this point they will destroy their host cell - trigger can be chemicals, radiation or - trigger can be chemicals, radiation or otherother

b.b. Some other viral genes may be Some other viral genes may be expressedexpressed when DNA is part of when DNA is part of host cell and produce toxins (like host cell and produce toxins (like botulism, diptheria, scarlet fever)botulism, diptheria, scarlet fever)

C. Animal Viruses C. Animal Viruses

Vary in type of nucleic acid and Vary in type of nucleic acid and presence/absence of viral envelope presence/absence of viral envelope

1. Viral Envelopes - outer membrane (outside 1. Viral Envelopes - outer membrane (outside capsid), that helps parasite enter host cellcapsid), that helps parasite enter host cell

Bind to animal cellBind to animal cell Envelope fuses with plasma membrane and Envelope fuses with plasma membrane and

injects virus + capsid into cellinjects virus + capsid into cell Enzymes of host cell remove capsid, virus Enzymes of host cell remove capsid, virus

overtakes cell (similar to bacteria)overtakes cell (similar to bacteria) Cycle does not always kill the host cellCycle does not always kill the host cell

C. Animal VirusesC. Animal Viruses

*Some virus envelopes come from *Some virus envelopes come from nuclear membrane and virus is replicated nuclear membrane and virus is replicated inside the nucleus of the host (like inside the nucleus of the host (like herpes)herpes)

-DNA of virus becomes integrated into -DNA of virus becomes integrated into host DNA and becomes a host DNA and becomes a provirusprovirus

-trigger will cause provirus to become -trigger will cause provirus to become active and destroy host cellactive and destroy host cell

C. Animal Viruses – RNA C. Animal Viruses – RNA virusvirus

1. Classification - by strandedness and 1. Classification - by strandedness and how they function in hosthow they function in host

a. Class IV - serve directly as mRNA - a. Class IV - serve directly as mRNA - can be translated into viral protein as can be translated into viral protein as soon as they infectsoon as they infect

b. Class V - RNA of virus serves as b. Class V - RNA of virus serves as template to make mRNA - must be template to make mRNA - must be transcribed into mRNA before translationtranscribed into mRNA before translation

C. Animal Viruses – RNA C. Animal Viruses – RNA virusvirus

c. Class VI - Retroviruses - reverse flow of c. Class VI - Retroviruses - reverse flow of genetic informationgenetic information

*Contain enzyme - *Contain enzyme - reverse transcriptasereverse transcriptase - - that transcribes DNA from an RNA template that transcribes DNA from an RNA template (goes backward)(goes backward)

*DNA becomes a provirus in nucleus of host*DNA becomes a provirus in nucleus of host *Viral DNA is now transcribed to RNA (can be *Viral DNA is now transcribed to RNA (can be

mRNA for translation, or can be packaged and mRNA for translation, or can be packaged and sent to new host cells)sent to new host cells)

*HIV is a retrovirus that causes AIDS*HIV is a retrovirus that causes AIDS

E. Viral Diseases in E. Viral Diseases in AnimalsAnimals

1. Causes 1. Causes a. Virus could damage or kill cells a. Virus could damage or kill cells b. Virus may produce toxins that cause b. Virus may produce toxins that cause

symptomssymptoms

E. Viral Diseases in E. Viral Diseases in AnimalsAnimals

2. Effects 2. Effects depend on ability of affected tissue to depend on ability of affected tissue to

make new cellsmake new cells a. If virus affects rapidly dividing cells – a. If virus affects rapidly dividing cells –

can have complete recovery (throat)can have complete recovery (throat) b. If virus affects areas where cells have b. If virus affects areas where cells have

stopped dividing, damage is permanent stopped dividing, damage is permanent (like nerve cells - polio virus)(like nerve cells - polio virus)

3. Vaccines3. Vaccines

Harmless variants of pathogenic Harmless variants of pathogenic microbes that stimulate the immune microbes that stimulate the immune system to mount defense against virussystem to mount defense against virus

*sensitizes immune system to the virus *sensitizes immune system to the virus so that it reacts vigorously if ever truly so that it reacts vigorously if ever truly exposed to the virusexposed to the virus

4. New Viruses4. New Viruses

Result from a variety of causesResult from a variety of causes a. Mutation of existing virus - seen often in a. Mutation of existing virus - seen often in

RNA viruses because they have no RNA viruses because they have no proofreading mechanismproofreading mechanism

*May lead to new varieties that individuals *May lead to new varieties that individuals were already immune to (flu)were already immune to (flu)

b. Spread of existing viruses from one species b. Spread of existing viruses from one species to another to another

c. Dissemination from small population to large c. Dissemination from small population to large population - due to increased travel, blood population - due to increased travel, blood transfusions, IV drug usetransfusions, IV drug use

6. Cancer causing 6. Cancer causing viruses viruses

Oncogenes - genes that trigger cancerous Oncogenes - genes that trigger cancerous characteristics in cells characteristics in cells

a. Proto-oncogenes - versions of oncogenes a. Proto-oncogenes - versions of oncogenes found in normal cells - code for proteins that found in normal cells - code for proteins that control growth factors and cell divisioncontrol growth factors and cell division

b. Virus may trigger proto-oncogenes to turn b. Virus may trigger proto-oncogenes to turn on causing uncontrolled cell division - Usually on causing uncontrolled cell division - Usually only cause cancer in combo with a mutagenonly cause cancer in combo with a mutagen

F. Plant Viruses F. Plant Viruses

stunt growth in plants and diminish crop yieldsstunt growth in plants and diminish crop yields 1. Horizontal transmission of virus - insects, 1. Horizontal transmission of virus - insects,

high winds, injury, freezes - cause plants outer high winds, injury, freezes - cause plants outer layer of epidermis to become damagedlayer of epidermis to become damaged

*increases chance of virus to penetrate *increases chance of virus to penetrate epidermis and infect plantepidermis and infect plant

*farmers transmit from plant to plant using *farmers transmit from plant to plant using same pruning toolssame pruning tools

F. Plant VirusesF. Plant Viruses

2. Vertical transmission - plant inherits 2. Vertical transmission - plant inherits viral infection from a parentviral infection from a parent

*Occurs during asexual reproduction or in *Occurs during asexual reproduction or in seeds of sexual reproductionseeds of sexual reproduction

3. Virus spreads through plant via 3. Virus spreads through plant via plasmodesmata plasmodesmata

G. ViroidsG. Viroids

Tiny molecules of naked RNA that infect Tiny molecules of naked RNA that infect plants onlyplants only

*Make no proteins but replicate in plant *Make no proteins but replicate in plant cells and stunt their growthcells and stunt their growth

H. Prions H. Prions

infectious proteins - usually affect brain infectious proteins - usually affect brain tissue (mad cow, Creutzfeldt-Jakob tissue (mad cow, Creutzfeldt-Jakob disease)disease)

Act slowly, virtually indestructibleAct slowly, virtually indestructible