The Genetics of Viruses Viral Genetics Microbiology: The Genetics of Viruses https://www.youtube.com/watch?v=UEKS4w9bfJg

Viruses &Bacteria: simplest biological systems smaller/ simpler than bacteria Contain nucleic acid and protein ONLY Not considered to be “living” b/c they need a host and NOT make out of a cell Bacteria: larger than viruses Contain ALL marcomolecules considered living

Virus Bacterium Animal cell Animal cell nucleus 0.25 µm

Bacteria with Bacteriophages infecting the cell:

What is a virus? Has a genome (RNA or DNA) but… can reproduce only within a host cell Scientists detected viruses indirectly long before they could see them The story of how viruses were discovered begins in the late 1800s: Tobacco mosaic disease stunts growth of tobacco plants  gives their leaves a mosaic coloration In late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease In 1935, Wendell Stanley confirmed this hypothesis

Tobacco Mosaic Disease

Viruses are not (living) cells! Viral structure Viruses are not (living) cells! Virus in Latin means “poison” Made up of infectious particles consisting of a nucleic acid (RNA or DNA) in a protein coat (and sometimes another membrane) Viruses have a Capsid protein shell encloses viral genome (nucleic acid) A capsid can have various structures Viral genomes may consist of Double- or single-stranded DNA (dsDNA, ssDNA) Double- or single-stranded RNA (dsRNA, ssRNA)

Viral Structure Images

Depending on viral type of nucleic acid… A virus is called a DNA virus or an RNA virus The smallest viruses have only 4 genes, while others have 100’s

HOST CELL https://www.youtube.com/watch?v=cE0qdqoBFa8

Some viruses have membranous envelopes  help them infect hosts Surround capsids Viral envelopes (made from host cell’s membrane) contains a combo of viral and host cell molecules HELPS infect other target host cells http://www.sumanasinc.com/webcontent/animations/content/herpessimplex.html http://www.sumanasinc.com/webcontent/animations/content/influenza.html

Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA ER Capsid proteins Glyco- proteins Copy of genome (RNA) New virus

General Features of Viral Reproductive Cycles Viruses are intracellular parasites they can reproduce only within a host cell Each virus has a host range, a limited number of host cells that it can infect Viruses uses HOST molecules to make more (“offspring”) viruses http://media.pearsoncmg.com/bc/bc_campbell_biology_7/mm2/ch18/InstructorResources/medialib_tab_2/1.htm

Virus uses HOST: DNA Polymerase RNA Polymerase Ribosomes tRNA’s Entry into cell and uncoating of DNA VIRUS DNA Capsid Replication Transcription HOST CELL Virus uses HOST: DNA Polymerase RNA Polymerase Ribosomes tRNA’s Amino acids Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_1.html

Reproductive Cycles of Phages Phages = best understood of all viruses lytic cycle vs. lysogenic cycle Kill infected bacterial host cells BACTERIRAL DEFENCES INCLUDE: Some bacteria have mutant receptor sites that phage can no longer bind to 2. Some bacteria also produce restriction endonucleases

Viral reproduction: Lytic Cycle A phage reproductive cycle  ends in DEATH of host cell Produces new phages and digests the host’s cell wall, releasing the progeny (“offspring”) viruses A phage that reproduces only by the lytic cycle is called a virulent (strong) phage EXAMPLES: Flu, common cold, Rabies

A bigger perspective of the Lytic Cycle: Attachment Entry of phage DNA and degradation of host DNA Phage assembly Release Head Tails Tail fibers Assembly Synthesis of viral genomes and proteins http://media.pearsoncmg.com/bc/bc_campbell_biology_7/mm2/ch18/InstructorResources/medialib_tab_2/2.htm

Viral reproduction: Lysogenic Cycle Think “S” for “Silent” Replicates viral genome (DNA) without destroying host cell Viral DNA becomes incorporated into host cell’s DNA (called prophage) Every time host divides, phage DNA is copied/passed to daughter cells Virus spread without killing host cells Occasionally, virus switches to lytic cycle Temperate virus Viruses capable of using the lytic and lysogenic cycles. EXAMPLES: HIV and Herpes

A bigger perspective of the Lysogenic Cycle: Phage DNA Daughter cell with prophage The phage attaches to a host cell and injects its DNA. Many cell divisions produce a large population of bacteria infected with the prophage. Phage Phage DNA circularizes Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle Lysogenic cycle Certain factors determine whether Prophage The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lysogenic cycle is entered Lytic cycle is induced or Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages. http://media.pearsoncmg.com/bc/bc_campbell_biology_7/mm2/ch18/InstructorResources/medialib_tab_2/3.htm http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html

RNA viruses = Retroviruses Retroviruses: transcribe DNA from an RNA template (RNA DNA) These viruses use Reverse transcriptase (catalyzing enzyme) Ex: HIV  AIDS

Retrovirus (HIV) Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse transcriptase http://media.pearsoncmg.com/bc/bc_campbell_biology_7/mm2/ch18/InstructorResources/medialib_tab_2/4.htm http://www.sumanasinc.com/webcontent/animations/content/hiv.html

LE 18-10 HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid Membrane of white blood cell HIV HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid 0.25 µm HIV entering a cell DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New HIV leaving a cell

Retrovirus Animations http://www.sumanasinc.com/webcontent/animations/content/hiv.html http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter18/animation_quiz_3.html http://www.whfreeman.com/kuby/content/anm/kb03an01.htm

Vaccines Vaccines Harmless derivatives (ex: viral proteins) of viruses https://www.youtube.com/watch?v=rb7TVW77ZCs Vaccines Harmless derivatives (ex: viral proteins) of viruses stimulates immune system to create antibodies against actual pathogen Vaccination has eradicated smallpox Effective vaccines are available against polio, measles, rubella, mumps, hepatitis B, and more

Medical Technology cannot… Treat a viral infection that has already occurred (vaccines do not ‘kill’ viruses) Antibiotics do not treat viral infections Antiviral drugs resemble nucleoides and interfere with the viral nucleic acid synthesis Valtrex stops herpes virus reproduction by inhibiting viral DNA polymerase that synthesizes viral DNA Azidothymidine (AZT) curbs HIV reproduction by interfering with DNA synthesis by reverse transcriptase Currently the most effective treatment is a ‘cocktail’ multidrug treatment method

Small Pox

Polio Polio

Herpes Simplex

Hepatitis                           

Varicella Zoster (chicken pox)

Mumps

Measles - Rubeola