Chapter 19 Flashcards
Virus example
bacteriophage
Bacteriophage
can infect and set in motion a genetic takeover of bacteria
-i.e. Escherichia coli (E. coli)
Model systems
E. coli and its viruses because of their freuqnct use by researchers in studies that reveal broad biological principles
Microbes
- have unique genetic mechanisms that are novel and interesting
- viruses and bacteria are this
Molecular Biology
rooted in the study of viruses and their infection and takeover of cells
- i.e. bacteria
Viruses
NOT living
NOT cells
ARE infectious particles made up of nucleotide and protein
- obligate intracellular parasites
- have a genome, but can reproduce one within a host cell (they’re parasites)
- small infectious particles consisting of nucleic acid enclosed in a capsid coat (proteins)
Tobacco Mosaic Disease
stunts the growth of tobacco plants and gives their leaves a mosaic coloration
- RNA based virus
Wendell Stanley
1935
confirmed that there is a particle smaller than bacteria that caused tobacco mosaic disease
- crystallized the infectious particle, TMV (tobacco mosaic virus)
Viral genomes
may consist of either double- or single stranded DNA or RNA
Capsid
composed of identical protein subunits and is the protein shell that encloses the viral genome
- come in various shapes and sizes (i.e. icsohedral)
DNA-based virus example
herpes
RNA-based virus example
HIV
Some viruses may also consist of
nucleic acid enclosed in a membranous envelope from the host cell
- i.e. HIV, influenza virus
- membrane bound viruses are easy to inactivate by removing the membrane
Bacteriophages (T4)
- also called phages
- have the most complex capsids found among viruses
Viruses are
obligate intracellular parasites
meaning they can reproduce only within a host cell
- each virus has a limited number (range) or host cells it can infect
Viruses use
the host cell enzymes, ribosomes, and molecules to synthesize progeny viruses
Nature of the genome
the basis for the common classification of animal viruses
DNA vs. RNA
Phages
the best understood of all viruses and go through 2 alternative reproductive mechanisms (lytic and lysogenic cycles)
2 alternative reproductive mechanisms that phages go through
1) lytic cycle
2) lysogenic cycle
Lytic Cycle
culminates in the death of the host and produces new phages and digests the host’s cell wall, releasing the progeny viruses
- TMV
- Rhinovirus
- Enterovirus (self-limiting)
Lysogenic Cycle
replicates the phage genome without destroying the host
- Phage
- HIV
- VZV
Temperate Phages
capable of using both the lytic and lysogenic cycles of reproduction
- phage lambda
Latent Viruses
- Herpes (DNA based), integrated into genome (measles is a herpes-based virus)
- Varicella Zoster Virus
- HIV (RNA based)
- don’t kill the host cell (hanging around)
- produces new progeny
Varicella Zoster Virus (VZV)
chicken pox => shingles attach to spinal ganglia integrated into genome will be reintegrated lysogenic
All viruses have
1) genome
2) capsid
3) membranous envelope (not all have)
T4 lytic cycle
1) Attachment (to host cell surface)
The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell
- hast to have the right protein/capsid
2) Entry of phage DNA and degradation of host DNA
The sheath of the tail contracts, injecting the phage DNA into the cell and leaving the empty capsid outside. The cell’s DNA is hydrolyzed
3) Synthesis of viral genome and proteins
The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell
4) Assembly
Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms
5) Release
The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles
Viruses’ membrane envelopes
many have glycoproteins on their surface that bind to specific receptor molecules on the surface of a host cell
Animal viruses utilize host cell machinery to make
new viruses
Some viral envelopes are formed from the host cell’s
plasma membrane as the viral capsids exit
The broadest variety of RNA genome is found among the viruses that
infect animals
Retroviruses
use reverse transcriptase to copy their RNA genome into DNA
HIV (human immunodeficiency virus
- a retrovirus that causes AIDS (acquired immunodeficiency syndrome)
- attacks and destroys immune cells called white blood cells
- can convert its RNA genome into DNA, which can then be integrated into the host genome as a PROVIRUS
Provirus
when an animal virus integrates its genome into the host cell DNA
Diseases caused by viral infections affect …
- humans (i.e. SARS and Herpes)
- agricultural crops
- livestock worldwide (>2000 types of plant viruses)
Viruses may damage or kill cells by
causing the release of hydrolytic enzymes from lysosomes or they may cause infected cells to produce toxins that lead to disease symptoms
Vaccines
derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen and can prevent certain viral illnesses
What kinds of problems do viruses create for animal cells?
- a number of cytopathic or damaging effects
1) Changes in size and shape
2) Cytoplasmic and nuclear inclusion bodies (protein aggregates): “clumping” precipitates or clots
3) Cell fusion to form multinucleate cells
4) Inappropriate cell lysis (destruction)
5) Transform cells into cancerous (oncogenic) cells by altering DNA
Normal cells are converted to cancer cells by the accumulation
multiple mutations affecting porto-oncogenes and tumor-suppressor genes
Certain viruses promote cancer by
integration of viral DNA into a cell’ s genome
-i.e. herpes
individuals who get infected with certain viruses may be at an increased risk of developing certain types of cancer
-i.e. cervical cancer (papilloma virus)
Some infectious agents behave like viruses but
do not infect cells although they cause disease
Prions
misfiled proteins, contain no nucleic acid and cause spongiform encephalopathies or holes in the brain
- cause scope in sheep and goats, boven spongiform encephalopahties (BSE) (aka mad cow disease) and Creutzfeldt-Jakob Disease
Viroids
short pieces of RNA, no protein coat and have only been identifies in plants
Enterovirus
small RNA viruses that belong to the group that includes the poliovirus coxsackieviruses, echoviruses, and other enteroviruses
Non-polio-enteroviruses
the 2nd most common to the “common cold” viruses and cause an estimated 10-15 million or more symptomatic infectious a year in USA
Enteroviruses can be found
in the respiratory secretions (saliva, sputum, nasal mucus) and stool of infected person
Persons may be come infected by
direct contact with secretions or contaminated surfaces or objects, such as a drinking glass, door knobs, telephone
In USA, infections caused by enteroviruses are most likely to occur during
the summer or fall
Symptoms of enteroviruses include
mild upper respiratory symptoms, a flu-like illness with fever and muscle aches
less commonly, some persons have “aseptic” or viral meningitis. Few infections result in myocarditis or death
