Vivacious Viruses Chapter 19

I. Virus A.Characteristics 1.Smaller than a ribosome 2.Can form into regular crystals (cells won’t do this) 3.Made of Nucleic Acid - Genome is made of one of the following:  DNA - double stranded  DNA - single stranded  RNA - double stranded  RNA - single stranded

A. Characteristics (cont) 1.Range from 4 genes to several hundred 2.Protein coat surrounds nucleic acid  *Capsid - protein coat  -can be rod shaped (helical), polyhedral or more complex  -capsids are made form large numbers of protein subunits called capsomeres

A. Characteristics (cont) 1.Accessory Structures (found in some viruses)  Viral Envelopes - membranes surrounding capsid  come from membrane of their host cell and contain the same proteins, glycoproteins and phospholipids used by host  These envelopes help viruses to infect host without being detected  Bacteriophages - have most complex capsids

B. Reproduction of Viruses  Happens only in a host cell because they have no ribosomes or enzymes to ribosomes or enzymes to reproduce on their own reproduce on their own

B. Reproduction of Viruses 1.Host Range - limited amount of host cells that a virus can infect (can be one host or a few related species)  Host identified by “lock and key” fit between proteins on capsid and receptors on outside of host cell  Can be Tissue Specific - eukaryotic viruses may infect only certain tissues of their host ex. cold - respiratory tissue, HIV - white blood cells only ex. cold - respiratory tissue, HIV - white blood cells only

B. Reproduction of Viruses 1.Methods of Reproduction - virus infects host first and overtakes host cell to make viral nucleic acids and proteins  Host provides the following:  nucleotides for synthesizing viruses nucleic acid  Enzymes  Ribosomes  tRNA’s, amino acids, ATP and other parts needed by virus to go through transcription and translation (to make viral proteins)

A.Reproduction of Viruses Bacterial Viruses 1.Lytic Cycle - viral reproductive cycle that ends in the death of the host cell (also called virulent virus)  ex. T4 bacteriophage

3. Lytic Cycle (cont) a.Phage attaches to cell - uses tail fiber to stick to receptor site on host (e. coli) b.DNA is injected through cell wall and membrane 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.Phage makes host cell produce lysozyme - an enzyme that digests the host’s cell wall. Osmosis causes the cell to swell and burst releasing the new phages (may be new ones) Phages go find new host cell to infect

Reproduction of Viruses Bacterial Viruses 1.Lysogenic Cycle - replicates viral genome without killing host cell  If they use both lytic and lysogenic - also called temperate viruses

4. Lysogenic Cycle (cont) a.Phage binds to host cell and injects DNA b.Phage DNA forms a circle inside host - it can now go through either lytic or lysogenic cycle. (If lytic - see process above) c.Viral DNA lines up with host DNA and gets incorporated into the host DNA by crossing over - it is now called a prophage - one of its genes represses the other genes so it’s basically not affecting the host at all at this point d.When the host cell replicates and divides, it also replicates the virus and passes it on to its daughter cells

4. Lysogenic Cycle (cont)/ a.At some point, phages will leave the host DNA and return to a lytic cycle. At this point they will destroy their host cell - trigger can be chemicals, radiation or other b.Some other viral genes may be expressedwhen DNA is part of host cell and produce toxins (like botulism, diptheria, scarlet fever)

C. Animal Viruses  Vary in type of nucleic acid and presence/absence of viral envelope  1. Viral Envelopes - outer membrane (outside capsid), that helps parasite enter host cell  Bind to animal cell  Envelope fuses with plasma membrane and injects virus + capsid into cell  Enzymes of host cell remove capsid, virus overtakes cell (similar to bacteria)  Cycle does not always kill the host cell

C. Animal Viruses  *Some virus envelopes come from nuclear membrane and virus is replicated inside the nucleus of the host (like herpes)  -DNA of virus becomes integrated into host DNA and becomes a provirus  -trigger will cause provirus to become active and destroy host cell

C. Animal Viruses – RNA virus  1. Classification - by strandedness and how they function in host  a. Class IV - serve directly as mRNA - can be translated into viral protein as soon as they infect  b. Class V - RNA of virus serves as template to make mRNA - must be transcribed into mRNA before translation

C. Animal Viruses – RNA virus  c. Class VI - Retroviruses - reverse flow of genetic information  *Contain enzyme - reverse transcriptase - that transcribes DNA from an RNA template (goes backward)  *DNA becomes a provirus in nucleus of host  *Viral DNA is now transcribed to RNA (can be mRNA for translation, or can be packaged and sent to new host cells)  *HIV is a retrovirus that causes AIDS

E. Viral Diseases in Animals  1. Causes  a. Virus could damage or kill cells  b. Virus may produce toxins that cause symptoms

E. Viral Diseases in Animals  2. Effects  depend on ability of affected tissue to make new cells  a. If virus affects rapidly dividing cells – can have complete recovery (throat)  b. If virus affects areas where cells have stopped dividing, damage is permanent (like nerve cells - polio virus)

3. Vaccines  Harmless variants of pathogenic microbes that stimulate the immune system to mount defense against virus  *sensitizes immune system to the virus so that it reacts vigorously if ever truly exposed to the virus

4. New Viruses  Result from a variety of causes  a. Mutation of existing virus - seen often in RNA viruses because they have no proofreading mechanism  *May lead to new varieties that individuals were already immune to (flu)  b. Spread of existing viruses from one species to another  c. Dissemination from small population to large population - due to increased travel, blood transfusions, IV drug use

6. Cancer causing viruses  Oncogenes - genes that trigger cancerous characteristics in cells  a. Proto-oncogenes - versions of oncogenes found in normal cells - code for proteins that control growth factors and cell division  b. Virus may trigger proto-oncogenes to turn on causing uncontrolled cell division - Usually only cause cancer in combo with a mutagen

F. Plant Viruses  stunt growth in plants and diminish crop yields  1. Horizontal transmission of virus - insects, high winds, injury, freezes - cause plants outer layer of epidermis to become damaged  *increases chance of virus to penetrate epidermis and infect plant  *farmers transmit from plant to plant using same pruning tools

F. Plant Viruses  2. Vertical transmission - plant inherits viral infection from a parent  *Occurs during asexual reproduction or in seeds of sexual reproduction  3. Virus spreads through plant via plasmodesmata

G. Viroids  Tiny molecules of naked RNA that infect plants only  *Make no proteins but replicate in plant cells and stunt their growth

H. Prions  infectious proteins - usually affect brain tissue (mad cow, Creutzfeldt-Jakob disease)  Act slowly, virtually indestructible