Viruses Ch 18 Big Idea 3: Living systems store, retrieve, transmit, and respond to info essential to life processes.

Essential Knowledge 3C3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts.

Structure of Viruses Viruses are not cells  small infectious particles consisting of nucleic acid in a protein coat or a membranous envelope

Viral Genomes Made of either: Double- or single-stranded DNA, or Double- or single-stranded RNA

RNA Viruses A lot of arv drugs target reverse transcriptase Need Reverse Transcriptase: enzyme that makes double stranded DNA to match w/host cell Lack replication error-checking mechanisms  higher rates of mutation (HIV!)

Herpes Virus-DNA

Ebola Virus-RNA

Capsids and Envelopes A protein shell that encloses viral genome Built from protein subunits called capsomeres

RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath Fig. 19-3 RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath Capsomere of capsid Tail fiber Glycoprotein Glycoproteins 18  250 nm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm Figure 19.3 Viral structure 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Viral Replication Highly efficient  allow for rapid evolution and acquisition of new phenotypes!

Bacteriophage AKA: phages, viruses that infect bacteria Have a long capsid head that encloses DNA A protein tail attaches phage to host and injects DNA inside

Fig. 19-1 Figure 19.1 Are the tiny viruses infecting this E. coli cell alive? 0.5 µm

Viruses Need Hosts! Intracellular parasites that have a host range (a limited number of host cells that it can infect)

Viral Reproductive Cycles Virus VC

VIRUS Entry and uncoating DNA Capsid Transcription and manufacture Fig. 19-4 VIRUS Entry and uncoating 1 DNA Capsid Transcription and manufacture of capsid proteins 3 2 Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Figure 19.4 A simplified viral reproductive cycle Self-assembly of new virus particles and their exit from the cell 4

Reproductive Cycles of Phages 1st Rep Cycle: lytic cycle  Death of host cell Virulent phage: virus that only reproduces via lytic cycle

Fig. 19-5-1 1 Attachment Figure 19.5 The lytic cycle of phage T4, a virulent phage

Attachment Entry of phage DNA and degradation of host DNA 1 2 Fig. 19-5-2 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5 The lytic cycle of phage T4, a virulent phage

Attachment Entry of phage DNA and degradation of host DNA Fig. 19-5-3 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5 The lytic cycle of phage T4, a virulent phage 3 Synthesis of viral genomes and proteins

Attachment Entry of phage DNA and degradation of host DNA Fig. 19-5-4 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

Attachment Entry of phage DNA and degradation of host DNA Release Fig. 19-5-5 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

Reproductive Cycles 2nd reprod. Cycle: latent/lysogenic cycle No destroying host cell viral DNA is mixed into host cell’s chromosome  prophage Gives host cell properties like increased pathogenicity in bacteria

Temperate Phages Phages that use both lytic and lysogenic cycles Lambda Phage  Attacks E. Coli Phages that use both lytic and lysogenic cycles Env. signal can trigger switch

The phage injects its DNA. Fig. 19-6 Daughter cell with prophage Phage DNA The phage injects its DNA. Cell divisions produce population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle Lysogenic cycle The bacterium reproduces, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Prophage Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

Evolution of Viruses Maybe plasmids?? (circular DNA in bacteria and yeasts) and transposons, small mobile DNA segments

Mimivirus (double-stranded DNA) is 2nd largest virus discovered Virus evolved before or after cells? Causes pneumonia

Megavirus (2 strand DNA) Oct. 2011, largest virus Hosts are amoeba Megavirus (2 strand DNA) Oct. 2011, largest virus

Viral infections cannot be treated by antibiotics Vaccines Harmless derivatives of pathogenic microbes that stimulate immune system to mount defenses against actual pathogen Viral infections cannot be treated by antibiotics

With your partner, make a list of vaccines you have received!

Epidemics vs Pandemics Epidemic: New cases of a disease appear in a local area  aka outbreak If epidemic spreads to another locale  pandemic

Plant Viruses Most RNA genome Horizontal transmission, enter in damaged cell walls Vertical transmission, inherit from a parent Cucumber mosaic disease

Viroids and Prions: Simple Viroids: circular RNA molecules that infect plants and disrupt growth Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

Ex: Prion Mad Cow Disease Bovine spongiform encephalopathy, high levels of protein in nervous system tissue

Ex: Prion Creutzfeldt Jakob Syndrome

Understanding Viruses Movie http://www. schooltube