1. Chapter 16 Evolution of Microbial Life
I. Viruses
General information
General structures
Host specificity
Replication inside bacteria & animal host cells
Diseases & treatments

2. Viruses
Non-cellular - therefore are non-living (acellular or particles – not in Domain or Kingdom)
Smaller than bacteria: approximately 0.2 – 2 µm (1 µm = 1/1,000,000 m)
µm is a micrometer which is a millionth of a meter.
Diverse in structure; however, they ALL are composed of TWO distinct parts:
Outer protein capsid
Inner nucleic acid core (DNA or RNA)
(OPTIONAL: Some viruses that infect animals may have an envelope covering capsid.)
Are categorized by:
Type of nucleic acid – DNA or RNA – Single or Double stranded
Size and shape
Presence or absence of outer envelope

5. Have specific host range:
Bacterial viruses = bacteriophage aka phages
Plant viruses
Animal viruses (may have envelope)

6. Not classified using Linnaean system: Have capability to mutate (adapt/evolve) & multiply (replicate)
Because they replicate ONLY within living cells, viruses are considered obligate intracellular parasites (commandeers host cell’s machinery for replication)

7. Replication processes within bacteria: lytic & lysogenic cycles (pg
Lytic: shorter
Attachment, penetration, biosynthesis, maturation, release
Lysogenic: longer (dormancy or latency period)
Attachment, penetration, integration (prophage); biosynthesis, maturation, release
Lock and key receptors give “specificity”
DNA/RNA only
Virus not actively being replicated.
Lysis of the host cell

8. How do Viruses Replicate?
The lytic cycle:
1- absorption/attachment
2- injection/entry/penetration
3- replication of viral parts/biosynthesis
4- assembly/maturation
5- release by lysis
Results in death of host cell
Virulent virus (only lytic cycle)

9. Lysogenic Cycle First steps just like lytic!!
Does NOT destroy the host cell
Nucleic acid joins the cell’s DNA. Viral DNA becomes a part of hosts cell DNA
(prophage)
Could go on for years
Temperate virus (capable of using the lytic and lysogenic cycles)
i.e. HIV, cold sores, shingles

10. Both Cycles Together

11. Replication process within animal cells
If DNA based:
Attachment, penetration, biosynthesis, maturation, release
If RNA based:
Attachment, penetration, reverse transcription, integration (provirus), biosynthesis, maturation, release
Similar to lysogentic cycle – e.g. Retrovirus live HIV
Buds out to gain an envelope coat from the membrane of the host
Removes coat – NA and protein capsid enter cell

12. Examples of Diseases
Plant Viruses: Tobacco mosaic virus (TMV), Banana streak virus, Carrot thin leaf virus
Animal viruses: Rabies, Polio, Mumps, Chicken pox, Small pox, and Influenza.
Emerging – Ebola, Avian Influenza, SAR, Hantavirus, West Nile

13. Viruses – Human Viral Diseases
Human viruses causing disease vary in nucleic acid type, structure & vectors

14. Viruses – Human Viral Diseases

15. Treatment/Prevention:
Tx: anti-virals (not antibiotics)
Treat symptoms and try to prevent the virus from replicating
Prev: vaccines, public awareness, vector control
Viruses and Cancer
HPV – human papilloma virus – ↑ cervical cancer
Hep C - ↑ liver cancer  

16. Viruses – Prions & Viroids
Prions are “proteinaceous infectious particles”
infectious agents for transmissible spongiform encephalopathies (TSEs)
Mad Cow Disease – Bovine Spongiform Encephalopathy
similar sheep disease – scrapie
Creutzfeldt-Jacob disease - humans
normal animals have normal prions
misfolded prion proteins cause disease
Viroids are tiny “naked” molecules of RNA
plant pathogens
highly complementary, circular, single-stranded RNA
smallest discovered = 220 nb scRNA (small cytoplasmic RNA)
associated w/ rice yellow mottle sobemovirus (RYMV)
cause diseases in other plants - coconuts
mechanism of pathogenicity unclear