Topic 9 -Viruses, viroids, prions & parasites

Question 1

Describe the differences b/w a virus and a bacterium

Answer 1

Viruses DO NOT: grow on artificial media, divide by binary fission, have dna AND rna, are not sensitive to antibiotics
Bacteria DO: grow on artificial media, divide by binary fission, have dna AND rna, are sensitive to antibiotics

Question 2

List the general characteristics of a virus

Answer 2

• obligatory intracellular parasites
• contain DNA OR RNA
• contain a protein coat
• some are enclosed by an envelope
• some have spikes to help w/ attachment
• most only infect specific types of cells in 1 host
• host range is determined by specific host attachment sites & cellular factors

Question 3

List the properties of an enveloped virus

Answer 3

Contain nucleic acid (either DNA or RNA NOT both)
Are surrounded by a capsid made of capsomeres
Envelope maybe derived from host cell

Question 4

List the properties of a non-enveloped (complex) virus

Answer 4

Bacteriophage (T4)
Complex virion structure:
Head (capsid)
sheath
Baseplate
Tail
See diagram.

Question 5

viral taxonomy is based on?

Answer 5

nucleic acid type, strategy for replication and morphology

Question 6

Give an example of a family, genus and common name for a virus

Answer 6

Herpesviridae, herpesvirus, human herpes virus HHV-1, HHv-2, HHV-3

Question 7

Describe how bacteriophages are cultured, and define what a plaque is

Answer 7

Must be grown on living cells
Animal viruses may be grown in: living animals, embryonated eggs, tissue culture, cell lines
See lecture for tissue culture diagram.

Form plaques on ‘lawn’ of bacteria (plaque is a clearing in lawn caused by lysis of bacterial colony where bacteriophage has been successful)

Question 8

Describe the viral growth curve

Answer 8

One step growth curve
eclipse period => virions released from host cell => acute infection.

See diagram.

Question 9

List 3 techniques used to identify viruses

Answer 9

1. by their cytopathic effects (structurally visible signs e.g. via microscope)
2. By carrying out serological tests (detecting or using antibodies)
3. By nucleic acid analysis (PCR=>Polymerase Chain Reaction -detects antibodies, fragments of viral genome)

Question 10

Describe the lytic cycle of T-even bacteriophages

Answer 10

LEARN DIAGRAM.

1. Attachment => phage attaches via tail fibres to host cell
2. Penetration => phage lysozyme opens cell wall; tail sheath contracts to force tail core & DNA into cell
3. Biosynthesis => production of phage particles
4. Maturation => assembly of phage particules
5. Release => phage lysozyme breaks cell wall

Question 11

Describe the lysogenic cycle of bacteriophage lambda

Answer 11

LEARN DIAGRAM.

Phage conversion: DNA incorporated in host DNA via recombination becoming a prophage
lysogenic bacterium reproduces normally (binary fission)
Lysogenic cells immune to infection by the same phage type
Specialised transduction

Question 12

Describe the life cycle of an animal virus

Answer 12

Attachment: Viruses attach to cell membrane
Penetration: by endocytosis or fusion
Uncoating: by viral or host enzymes
Biosynthesis: Production of nucleic acid and proteins
Maturation: nucleic acid & capsid proteins assemble
Release: by budding (enveloped viruses) or rupture

Question 13

Give an example of a latent viral infection

Answer 13

Herpes Simplex Virus types 1 & 2

Question 14

Give an example of a plant virus and viroid

Answer 14

Virus: Watermelon wilt, transmission via white flies, family: potyviridae
Viroid: infectious RNA => potato spindle tuber disease

Question 15

Describe what a viroid is

Answer 15

plant pathogen
short piece of naked RNA, 300-400 nucleotides long
Internally paired, 3D structure
RNA does not code for proteins, may have evolved from introns
Pathogenesis may be due to RNA silencing
transmission is via wounds, insects, nematodes, pollen

Question 16

How can a protein be infectious?

Answer 16

Via prions. (Proteinaceous Infectious Particle)
spongiform encephalopathies

Caused by an aberrant form of a naturally occurring protein which then induces other normal proteins to mutate

Question 17

List some transmissible forms of prion-induced disease

Answer 17

Scrapie in sheep
BSE; Bovine Spongiform Encephalopathy (mad cow disease)
Kuru (human, commonly due to cannibalism)

Question 18

List some inheritable forms of prion-induced disease

Answer 18

CJD (Creutzfeld-Jacob Disease) humans

FFI (Fatal familial Insomnia) human

Question 19

What is parasitism?

Answer 19

a symbiont either harms or lives at the expense of another organism.
The parasite is always the beneficiary and the host is always the provider

Question 20

What is an ectoparasite?

Answer 20

lives ON its host

Question 21

What is an endoparasite?

Answer 21

lives IN its host

Question 22

List the 4 major divisions into which parasites are classified

Answer 22

Protozoa (amoeba, flagellates, ciliates etc)
Nematoda (roundworms)
Platyhelminthes (flatworms)
Arthropoda (insects, spiders etc)

Question 23

List the defining characteristics of protozoans

Answer 23

Chemoheterotroph
Unicellular
gets nutrients via absorption
No embryo formation
Have motility, some form cysts
Asexual repro via fission/budding, sexual repro via conjugation

Question 24

Name two diseases caused by protozoans and give the genus name of the protozoan in each case

Answer 24

Giardia lamblia (has cyst stage)

Trichomonas vaginalis (has no cyst stage)

Question 25

List the distinguishing characteristics of parasitic helminths

Answer 25

Phylum: platyhelminthes (flatworms) => trematodes (flukes) & cestodes (tapeworms) and phylum: nematoda (roundworms)

Reduced digestive sys.
Reduced nervous sys.
Reduced locomotion
Complex repro: can be monoecious (hermaphroditic) M & F in 1 animal, or dioecious (seperate M & F)

Question 26

What is a definitive host?

Answer 26

host where adult stage of parasite emerges & is able to produce offspring that are passed to an intermediate host

Question 27

What is an intermediate host?

Answer 27

A host that the parasite cannot produce offspring in

Question 28

Describe a parasitic infection which has as a definitive and intermediate host

Answer 28

Dog tapeworm Echinococcus granulosus causes hydatid disease.

SEE DIAGRAM.