Viruses and Prions

 

Notes for students:

Go easy on section 10.1 (p. 184-185)

 

v    PRIONS

Ø     cause diseases of brain and nervous system

§       “spongy” brain texture

§       symptoms

·       memory failure, behavior change, coordination problems, visual impairment early on

·       progresses to dementia, blindness, involuntary movements

·       coma, death at end

Ø     examples:

§       scrapie of sheep:  named due to sheep’s tendency to scrape faces or body raw from rubbing on fences, etc.

§       mad cow disease (BSE = bovine spongiform encephalitis)

·       probably due to sheep material (bones, etc) ground up and added to cattle feed as protein supplement

·       also transmitted among cattle when cow blood or other materials added to cattle feed

·       can contaminate meat during slaughter if nervous tissue debris remains on cutting tools

§       variant Creutzfeldt-Jakob disease (vCJD):  human version; obtained from eating beef with BSE

§       kuru:  disease first seen in Fore people of New Guinea; resulted from consumption or contact with brains or spinal cords during “ritual cannibalism” (ritual of respect for recently dead family members)

§       “Creutzfeldt-Jakob” disease (CJD) can be kuru, or an inherited form

Ø     cause:  non-living agent

§       “infectious” protein (PRION)

§       causes other proteins to “go bad” and turn into a non-functional form

§       up until the past 5-10 years, scientists would have said this was “impossible”—this is a completely new kind of disease!!!  Bizarre stuff.

v    VIRUSES

Ø     size ≤ 1/100th of a prokaryotic cell

Ø     composition: “genes in a protein wrapper”

§       protein + nucleic acid (DNA or RNA), or…

§       membrane + protein + nucleic acid (DNA or RNA)

Ø     not alive

§       living characteristics:

·       have DNA

·       evolve

§       non-living characteristics:

·       cannot reproduce on their own

·       not made of cells

·       no metabolism by themselves

Ø     reproduction:

·       infects host cell(s)

Ø     docking

§       proteins on virus surface must match receptors on cell surface

§       host-selectivity (both cell-type and species) depends on viral proteins

Ø     entry

§       non-enveloped viruses:

·       protein remains outside host

·       nucleic acid genome enters

§       enveloped viruses:

·       envelope fuses with cell plasma membrane

·       enzymes in cell remove protein coat from nucleic acid

·       virus takes one of two routes

¨     immediately reproduces, OR

¨     becomes quiet for a while and reproduces later

·       immediate reproduction:

¨     lytic cycle:  virus infects host, reproduces, and destroys cell immediately

Ø     virus genome is copied

Ø     virus genome is used to make proteins

Ø     proteins and genes pack together, and viruses leave cell

§       either by bursting, OR

§       by budding from cell surface (enveloped viruses only); doesn’t always destroy cell

·       delayed reproduction

¨     lysogenic cycle:  virus integrates into host genome and replicates with host; doesn’t replicate or damage cell for a long time

Ø     some trigger causes virus to change to lytic cycle

Ø     trigger may be stress or illness

·       if virus destroys cell and exits = dead cells, e.g. sores/ulcers from herpes

Ø     Types of viruses (host, structure)

§       size

§       shape

·       complex:  (e.g. bacteriophages like lunar-landing craft)

·       helical/rod shaped (e.g. TMV)

·       regular polygon (e.g. 12-sided, 20 sided)

·       some asymmetrical (e.g. HIV)

§       “envelope” (membrane wrapper outside protein coat) sometimes present; proteins stick out through it

§       genetic material:  DNA or RNA, one or more pieces

§       host

·       plants (require damage first to become infected)

·       animals

·       bacteria

Ø     Damage done by viruses varies; depends on

§       ability of tissue to repair itself (nerve vs. other)

§       speed of immune response

§       type of immune response—sneezing, coughing, mucus production, etc are immune reactions out of proportion

Ø     Drugs are tough to develop; virus has little machinery of its own

§       most drugs block enzymes

§       viruses have few enzymes

§       most drugs would also kill host cells too by interfering w/ metabolism

§       some antivirals block virus reproduction; easier if virus has one unique enzyme

Ø     “New” viruses arise from

§       mutations of old viruses (esp in RNA viruses, e.g. HIV)

·       HIV

·       common colds

·       influenza

§       other species: virus mutates enough to infect a new type of host

·       hantavirus in rodents (deer mice)

·       influenza in birds (ducks)

 

v    Examples:

v    Viruses with latency in humans:

§       oral herpes (cold sores, usu. herpes simplex 1)
genital herpes (herpes simplex 2)

¨     Herpes outbreaks stress-induced; lies dormant in nerve-cell nuclei (integrated)

§       HIV

v    phages cause toxicity involved in diphtheria, botulism, and scarlet fever; phage genome causes bacteria to produce toxins