Unit 6 – Organismal BiologyPart 1: Bacteria and Viruses

Are these viruses alive?

A “borrowed” life

• Cannot reproduce

• Cannot carry out metabolic activity outside of a host

• Exist as “entities” between life-forms and chemical.

Virus Structure

• Nucleic acid arranged as single linear or circular molecules.– Double-stranded DNA– Single-stranded NA– Double-stranded RNA– Single-stranded RNA

Virus Structure

• Capsid – protein coat that covers the viral genome.– Varies in shape – Built from capsomere protein subunits

Virus Structure

• Envelope– Accessory structure (not found in all)– Membrane w/proteins that surrounds envelop– Derived from host cell

Virus Reproduction

• Limited by the host range– Prokaryotic cells

(bacteria)– Eukaryotic cells

(plants, animals)

• Lock-and-key recognition mechanism (specificity)

General Features

• Virus binds to host cell and viral genome is inserted– Mechanism varies:• Inject DNA • Endocytosis (taken in)• Fusion of viral envelop with host membrane

General Features

• Once genome is inside, viral proteins take over control

• Host provides raw materials and energy to make more viruses

• New viruses exit the cell, damaging or killing it

• This all varies…based on which type of host the virus infect!

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Reproductive Cycles of Bacteriophages

• Phages infect only bacteria

• Two alternative mechanisms:– Lytic Cycle Ends in host death– Lysogenic Cycle Replication

without host death

Lytic CycleIf phage only reproduces this way, it is considered virulent phages. 1

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Lysogenic Cycle Phages that use both lytic and lysogenic are considered temperate phages.

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Classes of Animal VirusesDouble-stranded DNA Envelope? Examples/disease

Papovavirus

Herpesvirus

Poxvirus

No

Yes

Yes

HPV (human papillomavirus)

Herpes, shingles, chickenpox

Smallpox, cowpox

Classes of Animal VirusesSingle-stranded DNA Envelope? Examples/disease

Parvovirus No B19 virus

Classes of Animal VirusesDouble-stranded RNA Envelope? Examples/disease

Reovirus No Rotavirus (diarrhea; infants)

Classes of Animal VirusesSingle-stranded RNA Envelope? Examples/diseasePicornavirus

Coronavirus

Flavivirus

No

Yes

Yes

Rhinovirus (common cold); poliovirus; hepatitis A

SARS (Sever Acute Respiratory Syndrome

Yellow fever; West Nile; hepatitis C

Classes of Animal VirusesSingle-stranded RNA – Template for mRNA

Envelope? Examples/disease

Filovirus

Orthomyxovirus

Rhabdovirus

Yes

Yes

Yes

Ebola

Influenza

Rabies

Classes of Animal VirusesSingle-stranded RNA –Template for DNA (*backwards)

Envelope? Examples/disease

Retrovirus Yes HIV (human immunodeficiency virus), AIDS

Retroviruses

• Highly complex reproductive cycle

• Have an enzyme (reverse transcriptase) transcribes RNA template into DNA (opposite direction!)

• HIV causes AIDS (acquired immunodeficiency syndrome)

Viral Disease in Animals

• Some cause host to produce toxins

• Some have toxic molecular components (envelop proteins)

• Difficult to treat once infection occurs– Minimal success with antiviral drugs

Vaccines

• Tool for preventing infection– Harmless variant or derivative of a pathogen– Stimulates immune system response

• Main types– Killed Previously virulent micro-organisms– Attenuated Live micro-orgs w/virus that has

been cultivated to disable virulent properties

Emerging Viruses

• Appear suddenly, new to medical science– Examples: HIV (1980’s); Ebola

• Contributing factors:1. Mutation of existing viruses2. Spread from small, isolated population

(globalization, travel, technology)3. Spread of existing animal viruses to humans

Example: Spanish Flu pandemic (birds humans)

Viroids and Prions

• Simple infectious agents

• Viroids Circular RNA molecules that infect plants– Abnormal growth patterns

• Prions Infection proteins that infect animals– Degenerative brain diseases (scrapie in sheep, mad

cow, Creutzfeld-Jakob in humans)– Transmitted in food

Viroids and Prions

• Prions Infection proteins that infect animals– Degenerative brain diseases • Scrapie (sheep)• Mad cow• Creutzfeld-Jakob (humans)• Kuru (humans)

– Transmitted in food• Example: Beef to human; cannibalisms (New Guinea)