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

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

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

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

5. 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!)

6. Herpes Virus-DNA

7. Ebola Virus-RNA

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

9. 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

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

11. 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

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

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

14. Viral Reproductive Cycles
Virus VC

15. 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

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

17. Fig 1
Attachment
Figure 19.5 The lytic cycle of phage T4, a virulent phage

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

19. Attachment Entry of phage DNA and degradation of host DNA
Fig 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

20. Attachment Entry of phage DNA and degradation of host DNA
Fig 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

21. Attachment Entry of phage DNA and degradation of host DNA Release
Fig 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

22. 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

23. 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

24. 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.

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

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

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

28. 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

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

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

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

32. 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

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

34. Ex: Prion Creutzfeldt Jakob Syndrome

35. Understanding Viruses Movie http://www. schooltube