Viruses

Chapter 19

virus

a very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

they are not cells

viruses characteristics

simpler in structure than even prokaryotic cells

cannot reproduce or carry out metabolism outside of a host cell

tobacco mosaic disease

stunts the growth of tobacco plants and gives their leaves a mosaic coloration

small bacteria

in the late 1800s, researchers hypothesized that unusually ____________ might be responsible for the tobacco mosaic disease

infectious agent

later work suggested that the ___________ did not share features with bacteria (such as the ability to grow on nutrient media)

wendell stanley

confirmed this latter hypothesis by crystallizing the infectious particle,
now known as tobacco mosaic virus (TMV)

double or single-stranded DNA or

double or single-stranded RNA

viral genomes may consist of either _____________

DNA viruses or RNA viruses

Viruses are classified as _______________

single linear or circular molecule

the genome is either a ________________ of the nucleic acid

capsid

the protein shell that encloses the viral genome

bacteriophages (phages)

viruses that infect bacteria

the best understood of all viruses

bacteriophage characteristic

they have an elongated capsid head that encloses their DNA

protein tail piece

a ____________ attaches the phage to the host and injects the phage DNA inside

host cells

viruses replicate only in ___________

lytic cycle and the lysogenic cyycle

phages have two alternative reproductive mechanisms. they are the ________________

lytic cycle

a phage replicative cycle that culminates in the death of the host cell

produces new phages and lyses (breaks open) the host’s cell wall, releasing the
progeny viruses

what does the lytic cycle do?

virulent phage

a phage that reproduces only by the lytic cycle

lysogenic cycle

replicates the phage genome without destroying the host

The viral DNA molecule is incorporated into the host
cell’s chromosome

How does the lysogenic cycle work?

temperate phages

Phages that use both the lytic and lysogenic cycles

prophage

integrated viral DNA is known as ___________

the phage DNA

every time the host divides, it copies ________________ and passes the copies to daughter cells

environmental signal

an __________ can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

• an RNA or DNA genome, either single-stranded or double-stranded

• the presence or absence of a membranous envelope

2 key variables are used to classify viruses that infect animals. they are…

envelope or an RNA genome

whereas few bacteriophages have an _____________, many animal viruses have both

hydrolytic enzymes from lysosomes

viruses may damage or kill cells by causing the release of ____________

produce toxins

some viruses cause infected cells to ___________ that lead to disease symptoms

envelope proteins

other viruses have molecular components such as _________ that are toxic

vaccine

harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen

prevent certain viral illnesses

what can vaccines do?

viral infections

_________ cannot be treated by antibiotics

emerging viruses

are viruses that suddenly become apparent

ebola virus

one of several emerging viruses that cause hemorrhagic fever, an often fatal illness

chikungunya virus and zika virus

other emerging viruses include __________

• RNA viruses have an unusually high rate of mutation

• The disease can be disseminated from a small,
  isolated human population and can eventually spread
  around the world

• About three-quarters of new human diseases
  originate by spreading to humans from animals

three processes contribute to the emergence of a new viral disease. they are…

• Horizontal transmission

• Vertical transmission

Plant viruses spread disease by two major routes. they are…

horizontal transmission

enters through damaged cell walls

vertical transmission

inheriting the virus from a parent

prions

infectious proteins that appear to cause degenerative brain diseases in animals

they are incorrectly folded proteins, can be transmitted in food, act slowly, and are virtually indestructible

• Scrapie in sheep

• mad cow disease

• Creutzfeldt- Jakob disease

these diseases in humans are all caused by prions. they are…