1Viruses: intimate parasites

• Are viruses alive?– Not made of cells, in violation of Cell Theory– Do not grow (but self assemble)– Do not metabolize (but steal energy)– Cannot reproduce w/o a host cell (but other

organisms may require another species in order to reproduce)

– Can evolve over time– Some can respond to environmental stimuli– Have a complex, organized structure

2If not cells, then what?

• Viruses are particles

• Some components are essential– A genetic material with the blueprint for making more

• Could be: ds DNA, ss DNA, ss RNA, ds RNA• Space is limited, so genes are few

– A covering to protect the genetic material• Capsid, made of one or more proteins• Capsid + nucleic acid: nucleocapsid

• Viruses are obligate intracellular parasites

3Viruses may have other parts

• Envelope: piece of organelle membrane or cell membrane covering capsid– Virus is formed by budding, pushes through membrane

taking a piece.– Viral envelope usually contains viral proteins.– Envelope makes virus susceptible to some disinfectants

• Spikes (peplomers) extend from envelope– Used for attachment, escape

• Accessory enzymes– Reverse transcriptase, RNA RNA enzymes

4Viral size and shape

• Viruses range from 20 nm to 300 nm– Ribosomes are about 30 nm– The smallest known bacteria are about 200 nm

• Viral shapes: – helical, polyhedral, and complex

http://www.glencoe.com/qe/images/b136/q4323/ch18_0_a.jpg; www.blc.arizona.edu/.../ Figures/Icos_Virus.GIF; http://www.foresight.org/Updates/Update48/Images/T4Schematic.jpg

5Examples of virus shapes

http://www-cgi.cnn.com/HEALTH/9604/16/nfm/ebola.levine/ebola.reston.large.jpg; http://www.virology.net/Big_Virology/EM/Adeno-FD.jpg

Ebola Adenovirus

6Specificity• A recurring theme in biology:

– Enzymes, membrane receptors, antibodies, etc.

• Viruses are limited to certain types of host cells– Species barriers: rabies not specific, but most are– Tissue type: rabies specific to nerve, salivary tissue– Cell type: HIV infection mostly restricted to Helper T

cells, a kind of lymphocyte.

• Different characteristics of host cells involved– Attachment to cell surface often a major point

• Every type of organism has a virus that infects it?

7Viruses across kingdoms

• A densovirus newly isolated from the smoky-brown cockroachPeriplaneta fuliginosa

• Acanthamoeba castellanii Promotion of In Vitro Survival and Transmission of Coxsackie B3 Viruses

• The causal organism is the Tulip Breaking Virus (TBV). The pathogen is a potyvirus and is divided into two strains,

• Genome characterization of Botrytis virus F, a flexuous rod-shaped mycovirus

8Classification

• Microbes problematic, viruses especially so.– No sexual reproduction, no asexual reproduction, just

“assembly”. – No clear evolutionary relationships

• Classification scheme (from David Baltimore)– First, by nucleic acid type, e.g. ds DNA, + sense RNA– Next, structural characteristics (presence of envelope,

capsid shape), type of organism infected, etc.

9Life cycle of a virus

• Manner of infection and reproduction depends on whether host is prokaryotic or eukaryotic.

• Life cycle here outlined is general:– ADSORPTION: following contact,

molecules on surface of virus bind

to particular molecules on host cell.– PENETRATION: the nucleic acid

must get access to the machinery

of the cell to replicate.

http://www.tthhivclinic.com/Fig_20.gif

10Life cycle continued• SYNTHESIS/REPLICATION: once inside the

nucleic acid issues orders leading to– Replication of the nucleic acid– Transcription (usually) and translation, producing the

necessary capsid proteins.

• ASSEMBLY: a spontaneous process– Capsid proteins and nucleic acid combine to make virion.– Cheap but highly inefficient process.

• RELEASE: successful parasite must spread to others– Virus causes lysis of cell or pushes through cell

membrane. Virions may acquire an envelope.

11Bacteriophage: lytic vs. lysogenic• Most bacteriophages multiply then lyse the host cell

– This life cycle is called a lytic cycle

• Others are “temperate”, enter a lysogenic cycle.– Lysogeny is an effective way to multiply the viral DNA– Viral DNA inserts into the bacterial chromosome

• Now called a “prophage”• Bacterial replication also replicates viral DNA• Prophage may bring new genes for use by bacterium

– Damage to bacterial DNA (e.g. UV) prompts virus to begin lytic cycle; DNA excises, virus multiplies.

12Measuring numbers of virions:the Plaque Assay

http://dept.kent.edu/biology/Courses/30171/imageQBT.JPG; news.bbc.co.uk/.../ _230333_cell_culture_300.jpg;

•Virus and host cells are mixed

•Bacteria cover in a Petri dish as a “lawn”, eukaryotic cells cover bottom of a dish.

•Multiplication of virus leads to release, spread to and destruction of nearby cells.

•Visible as holes, plaques, on bacterial lawn; eukaryotic cells in culture are first stained for easier view to see plaques.

13Growing viruses

• Obligate intracellular parasites: require a host cell!!• Whole organism

– Animal models, human volunteers• Ethically, humans require consent, safety, pay

• Eggs: aseptic incubator– Various cells and membranes support growth of viruses– Shell provides protection from bacterial contamination– Used for large batches of viruses for vaccines

• Egg allergies a problem sometimes

http://www.fao.org/ag/againfo/foto/egg-facts.gif

14Growing viruses continued

• Organ/tissue/cell culture– Minced tissue, separated into cells by enzyme treatment– Grown in sterile plastic dish with nutrient solution– Cells prepared this way grow until dish is covered, stop.

• Scrape up, use some to inoculate new culture• Limited number of rounds of replication

– Transformed cells, with cancer properties, grow forever.• Must be subcultured when dish bottom is covered

• Cell culture major reason for advances in virology