chapter nineteen Flashcards
viruses
tiny, infectious particles that contain genes packaged in a protein coat
what are viruses also known as?
obligate intracellular parasites
- can replicated only within host cell
how were viruses discovered?
TMV (tobacco mosaic virus) in 1930s
- filtrable agents - small enough to pass through filter
- non-cellular
- crystallizable
structure of virus
nucleic acid genome, capsid, sometimes envelope
nucleic acid genome of virus
DNA or RNA, double stranded or single stranded
- 4 to hundreds of genes
capsid
protein coat or shell enclosing viral genome
- built from subunits (capsomeres)
envelope
derived from host plasma membrane, contains lots of glycoprotein spikes to bind to receptors on host
naked virus
doesn’t have envelope, just genome and capsid
for which cells does the host plasma membrane not form the virus envelope?
fungal and plant cells
shapes examples
- spiral - TMV
- polyhedron - adenovirus (common cold)
- complex - bacteriophages (lunar lander)
host ranges
limited number of host species that each particular virus can infect
- hosts identified by “handshake” between receptor molecules and glycoproteins
broad host examples
West Nile virus, influenza, equine encephalitis
- mosquitos, horses, birds, humans
restricted host examples
HIV, smallpox, measles
- humans, particular tissues
overview of virus entering host
- virus enters cell and releases viral DNA/capsid proteins
- host enzymes replicate viral genome
- host enzymes transcribe viral genome, make virus parts
lytic cycle
virulent, kills host 20-40 min
1. attachment - surface proteins
2. penetration - entry of phage DNA, degradation of host DNA (hydrolyzed)
3. biosynthesis - using host’s cellular machinery to make viral genomes/proteins
4. assembly/maturation of new viruses
5. lysis - lysozymes rupture wall, 100-200 particles released
how can viruses enter hosts?
injection through tail, endocytosis/fusion of envelope with PM
lysogenic cycle
temperate phages, doesn’t kill host, latent/dormant stage
1. viral DNA inserted into host (provirus/prophage)
2. viral genome copied/divides with host
3. occasionally, prophage exits bacterial chromosome (lytic cycle)
lysogenic viruses that affect humans
- HIV
- herpesviruses
- herpes simplex I and II
- chicken pox
bacterial anti-viral defense
- restriction modification system - restriction enzymes identify/cut up foreign DNA
- CRISPR-Cas9 system - identify/cut DNA
what does CRISPR stand for?
Clustered Regularly Interspaced Short Palindromic Repeats
animal viruses
- attachment
- fusion/endocytosis
- uncaring - capsid removed by host
- biosynthesis
- assembly/maturation
- release - budding/rupture
how are viruses classified?
by nucleic acid type and number of strands
herpes viruses
- herpes simplex I (cold sores) and II (genital sores)
- VZV (shingles, chicken pox)
full name HIV
human immunodeficiency virus
full name AIDS
acquired immunodeficiency syndrome
retroviruses
ssRNA to dsDNA using reverse transcriptase
- RNA to DNA to provirus in DNnA
- provirus never leaves
- dormant/inactive = only 1 in 100 HIV + has AIDS
- active = kills T helper cells of immune system
how many plant viruses are currently known?
over 2,000
- most have RNA genome and helical/icosahedral capsid
vector for plant viruses
pollinators, humans, insects, herbivores
how to plant viruses spread?
plasmodesmata
symptoms of plant viruses
blight, wilt, tumor
viroids
small, circular, infectious RNA molecules
prions
infectious proteins, transmitted in food, slow incubation period (10 yrs)
- not destroyed by cooking food or by acidic stomach pH
- able to cross BBB
- Mad Cow disease
protein coding RNAs
mRNA
noncoding RNAs
tRNA, rRNA, miRNA, siRNA, piRNA, snRNA, snoRNA and more
- many (except tRNA and rRNA are encoded in introns of protein-coding genes)
snRNA
small nuclear, helps form spliceosomes and some are catalytic ribozymes
snoRNA
small nucleolar, especially help assemble ribosomal subunits
