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

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

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

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)

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

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)

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

Both Cycles Together

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 https://www.youtube.com/watch?v=EqK1CYYQIug

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

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

Viruses – Human Viral Diseases

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  

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