 What is a virus made of?  non cellular  DNA or RNA  protein coat capsid

 dormant unless in a host cell  reproduce only when in a host cell  all energy derived from host  smaller than a bacteria cell

 Attach to host cell  Enter host (virus or just genetic material)  Direct host to make viral genetic material and protein  Assemble viral nucleic acids and proteins  Release new viral particles

 The lytic cycle is a phage replicative cycle that culminates in the death of the host cell  The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses  A phage that reproduces only by the lytic cycle is called a virulent phage  Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

Animation: Phage T4 Lytic Cycle Right click slide / Select play

Attachment 1

1 Entry of phage DNA and degradation of host DNA 2

Attachment 1 Entry of phage DNA and degradation of host DNA 2 Synthesis of viral genomes and proteins 3

Attachment 1 Entry of phage DNA and degradation of host DNA 2 Synthesis of viral genomes and proteins 3 Assembly Phage assembly Head Tail Tail fibers 4

Attachment 1 Entry of phage DNA and degradation of host DNA 2 Synthesis of viral genomes and proteins 3 Assembly Phage assembly Head Tail Tail fibers 4 Release 5

 The lysogenic cycle replicates the phage genome without destroying the host  Phages that use both the lytic and lysogenic cycles are called temperate phages  The viral DNA molecule is incorporated into the host cell’s chromosome  This integrated viral DNA is known as a prophage

 Every time the host divides, it copies the phage DNA and passes the copies to daughter cells  A single infected cell can give rise to a large population of bacteria carrying the virus in prophage form  An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

Animation: Phage Lysogenic and Lytic Cycles Right click slide / Select play

The phage injects its DNA. Daughter cell with prophage Many cell divisions create many infected bacteria. Prophage is copied with bacterial chromosome. Phage DNA integrates into bacterial chromosome. Phage DNA and proteins are synthesized and assembled. The cell lyses, releasing phages. Lytic cycleLysogenic cycle Prophage exits chromosome. Phage DNA circularizes. Phage DNA Phage Bacterial chromosome Prophage

Phage DNA Prophage Bacterial chromosome The phage attaches to a host cell and injects its DNA. Lysogenic cycleLytic cycle Virulent or temperate phage Destruction of host DNA Production of new phages Lysis of host cell causes release of progeny phages Temperate phage only Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromosome and initiate a lytic cycle

 There are two key variables used to classify viruses that infect animals  The nature of the viral genome (single- or double- stranded DNA or RNA)  The presence or absence of an viral envelope  Video Video

Capsid RNA Envelope (with glycoproteins) HOST CELL (RNA) New virus Copy of genome Viral genome (RNA) Template mRNA ER Glycoproteins Capsid proteins

 Viruses can be transmitted in 5 ways:  coughing  sneezing

 ingesting infected food  animal bites rabies

 physical contact bodily fluids

 Mononucleosis or Herpes 2  glands in your neck  tired, headaches  swollen lymph nodes  swollen spleen  Corona virus  upper respiratory infection runny nose sore throat fever

 Mumps  causes the swelling of the salivary glands,  can affect other organs as well  Measles  usually affects the skin  can affect the respiratory system, brain

 Chicken pox  Poliomyelitis (polio)  attacks the spinal cord

 Herpes 1  common cold sore  fever blisters  Conjunctivitis  pink eye

 The broadest variety of RNA genomes is found in viruses that infect animals  Retroviruses use reverse transcriptase to copy their RNA genome into DNA  HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

 Viral DNA that is integrated into the host genome is called a provirus  Unlike a prophage, a provirus is a permanent resident of the host cell  The host’s RNA polymerase transcribes the proviral DNA into RNA molecules  The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new viruses released from the cell

Animation: HIV Replicative Cycle Right click slide / Select play

Reverse transcriptase HIV Glycoprotein Viral envelope Capsid RNA (two identical strands) HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation mRNA HIV Membrane of white blood cell HIV entering a cell 0.25  m Provirus New virus New HIV leaving a cell

Reverse transcriptase HIV Glycoprotein Viral envelope Capsid RNA (two identical strands) HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation mRNA Provirus New virus

 no known antibiotic or treatment  body’s is its own natural defense mechanism  must run its course  can only treat the symptoms  when body detects virus (antigen), body produces antibodies called plasmacytes  fight off viruses  remains in blood  a single attack of a virus usually results in lifelong immunity Can viral infections repeat themselves?

 weakened or killed virus strain  cannot cause disease  helps to produce the stimulation of antibody production  immunizations are used against  Polio  Measles  Chicken pox  Mumps  Influenza (flu)

 Contagious disease pathogens must directly contact a new host  Epidemic  Pandemic (AIDS)  Sporadic  Endemic

 Emerging Pathogens  Ebola virus  West Niles virus  SARS virus  Avian Influenza  H1N1  Drug-resistant strains  Food poisoning  E. coli  Salmonella

West Nile Virus Takes Off

 Smaller than viruses  Strands or circles of RNA  No protein-coding genes  No protein coat  Cause many plant diseases

 Small proteins  Linked to human diseases  Kuru Prion found in human brain tissue New Guinea Causes nervous and brain malfunction  Creutzfeldt-Jakob disease (CJD) Caused by a spontaneous transformation Human version of mad cow  Animal diseases  Bovine spongiform encephalopathy (mad cow disease)

 Viruses do not fit our definition of living organisms  Since viruses can replicate only within cells, they probably evolved after the first cells appeared  Candidates for the source of viral genomes are plasmids and transposons  Plasmids, transposons, and viruses are all mobile genetic elements

 Host and pathogen are coevolving  If a pathogen kills too quickly, it might disappear along with the individual host  Most dangerous if pathogen  Is overwhelming in numbers  Is in a novel host  Is a mutant strain