chapter nineteen

Question 1

viruses

Answer 1

tiny, infectious particles that contain genes packaged in a protein coat

Question 2

what are viruses also known as?

Answer 2

obligate intracellular parasites
- can replicated only within host cell

Question 3

how were viruses discovered?

Answer 3

TMV (tobacco mosaic virus) in 1930s
- filtrable agents - small enough to pass through filter
- non-cellular
- crystallizable

Question 4

structure of virus

Answer 4

nucleic acid genome, capsid, sometimes envelope

Question 5

nucleic acid genome of virus

Answer 5

DNA or RNA, double stranded or single stranded
- 4 to hundreds of genes

Question 6

capsid

Answer 6

protein coat or shell enclosing viral genome
- built from subunits (capsomeres)

Question 7

envelope

Answer 7

derived from host plasma membrane, contains lots of glycoprotein spikes to bind to receptors on host

Question 8

naked virus

Answer 8

doesn’t have envelope, just genome and capsid

Question 9

for which cells does the host plasma membrane not form the virus envelope?

Answer 9

fungal and plant cells

Question 10

shapes examples

Answer 10

1. spiral - TMV
2. polyhedron - adenovirus (common cold)
3. complex - bacteriophages (lunar lander)

Question 11

host ranges

Answer 11

limited number of host species that each particular virus can infect
- hosts identified by “handshake” between receptor molecules and glycoproteins

Question 12

broad host examples

Answer 12

West Nile virus, influenza, equine encephalitis
- mosquitos, horses, birds, humans

Question 13

restricted host examples

Answer 13

HIV, smallpox, measles
- humans, particular tissues

Question 14

overview of virus entering host

Answer 14

1. virus enters cell and releases viral DNA/capsid proteins
2. host enzymes replicate viral genome
3. host enzymes transcribe viral genome, make virus parts

Question 15

lytic cycle

Answer 15

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

Question 16

how can viruses enter hosts?

Answer 16

injection through tail, endocytosis/fusion of envelope with PM

Question 17

lysogenic cycle

Answer 17

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)

Question 18

lysogenic viruses that affect humans

Answer 18

1. HIV
2. herpesviruses
   - herpes simplex I and II
   - chicken pox

Question 19

bacterial anti-viral defense

Answer 19

1. restriction modification system - restriction enzymes identify/cut up foreign DNA
2. CRISPR-Cas9 system - identify/cut DNA

Question 20

what does CRISPR stand for?

Answer 20

Clustered Regularly Interspaced Short Palindromic Repeats

Question 21

animal viruses

Answer 21

1. attachment
2. fusion/endocytosis
3. uncaring - capsid removed by host
4. biosynthesis
5. assembly/maturation
6. release - budding/rupture

Question 22

how are viruses classified?

Answer 22

by nucleic acid type and number of strands

Question 23

herpes viruses

Answer 23

1. herpes simplex I (cold sores) and II (genital sores)
2. VZV (shingles, chicken pox)

Question 24

full name HIV

Answer 24

human immunodeficiency virus

Question 25

full name AIDS

Answer 25

acquired immunodeficiency syndrome

Question 26

retroviruses

Answer 26

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

Question 27

how many plant viruses are currently known?

Answer 27

over 2,000 - most have RNA genome and helical/icosahedral capsid

Question 28

vector for plant viruses

Answer 28

pollinators, humans, insects, herbivores

Question 29

how to plant viruses spread?

Answer 29

plasmodesmata

Question 30

symptoms of plant viruses

Answer 30

blight, wilt, tumor

Question 31

viroids

Answer 31

small, circular, infectious RNA molecules

Question 32

prions

Answer 32

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

Question 33

protein coding RNAs

Answer 33

mRNA

Question 34

noncoding RNAs

Answer 34

tRNA, rRNA, miRNA, siRNA, piRNA, snRNA, snoRNA and more - many (except tRNA and rRNA are encoded in introns of protein-coding genes)

Question 35

snRNA

Answer 35

small nuclear, helps form spliceosomes and some are catalytic ribozymes

Question 36

snoRNA

Answer 36

small nucleolar, especially help assemble ribosomal subunits