Viruses & Subviral Particles

Question 1

Viral Structure

Answer 1

• genetic material
• capsid (protein coat)
• sometimes an envelope

Question 2

Viral Envelope

Answer 2

• surrounds capsid
• composed of phospholipids and virus specific proteins
• sensitive to heat/detergents

Question 3

What is a virus?

Answer 3

obligate intracellular parasite

Question 4

Do envelopes make viruses more or less resistant?

Answer 4

• envelopes make a virus less resistant to sterilization

- virus without an envelope is more likely to persist on surfaces for an extended period of time

Question 5

Can viruses reproduce independently?

Answer 5

No, so considered obligate intracellular parasites

Question 6

Explain the role of a host cell in viral reproduction

Answer 6

• virus must express and replicate genetic information within a host cell
• virus must use host cell b/c they lack ribosomes to carry out protein synthesis

Question 7

List the basic steps of virus reproduction

Answer 7

(1) virus hijacks cell’s machinery
(2) virus replicates
(3) virus produces viral progeny (virons) which can be released to infect other cells

Question 8

Bacteriophages

Answer 8

• viruses that specifically target bacteria
• do not enter bacteria but inject genetic material into bacteria
• made of nucleic acid genome, capsid, tail sheath, tail fibers

Question 9

Tail Sheath

Answer 9

act like syringe in injecting genetic material into bacterium

Question 10

Tail Fibers

Answer 10

help bacteriophage recognize and connect to cell

Question 11

What is the viral genome made of?

Answer 11

• double stranded DNA
• single stranded DNA
• double stranded RNA
• single stranded RNA

Question 12

Positive Sense Virus

Answer 12

• single-stranded RNA virus
• genome can be directly translated to functional proteins by ribosomes of host cell
• use host enzymes for protein translation but RNA dependent RNA polymerase to replicate genome

Question 13

Negative Sense Virus

Answer 13

• singe-stranded RNA virus
• require synthesis of a RNA strand complementary to the negative sense RNA strand which can be used as a template for protein synthesis
• must carry a RNA replicase in viron to ensure synthesis of complementary strand

Question 14

Retrovirus

Answer 14

• enveloped, single stranded RNA virus
• viron contains 2 identical RNA molecules
• carry an enzyme known as reverse transcriptase
• DNA integrates into host cell genome where it replicates/transcribes its own DNA

Question 15

Reverse Transcriptase

Answer 15

synthesizes DNA from single stranded RNA

Question 16

Steps of Viral Life Cycle (4)

Answer 16

(1) Infection
(2) Translation and Progeny Assembly
(3) Progeny Release
(4) Lytic and Lysogenic Cycles

Question 17

Two Basic Steps of Viral Life Cycle:

Answer 17

(1) attachment (adsorption) – specific binding, not yet infected
(2) injection (penetration) – genomes goes inside host

Question 18

Describe Viral Infection

Answer 18

• virus binds to specific receptor on host cell

- enveloped viruses fuse with plasma membrane of cell and release viron

Question 19

How do Bacteriophages infect cells?

Answer 19

use of their tail fibers to anchor themselves to host cell and then use tail sheath to inject viral genome into host cell

Question 20

Describe Viral Translation and Progeny Assembly

Answer 20

• translocation of genetic material to correct location in cell
• most DNA viruses go to nucleus to be transcribed into mRNA which goes to the cytoplasm and is translated into proteins
• translation of RNA to protein (usually structural capsid proteins) uses ribosomes + tRNA + amino acids + enzymes of host cell

Question 21

Positive-Sense RNA Virus Translation

Answer 21

genetic material from these type of viruses stays in the cytoplasm where it is directly translated to protein by host cell ribosomes

Question 22

Negative-Sense RNA Virus Translation

Answer 22

these type of viruses require synthesis of complementary RNA strand via RNA replicase which then allows for translation of proteins

Question 23

Retrovirus Translation

Answer 23

DNA from these types of viruses travels to nucleus where it is integrated into host genome

Question 24

Describe the different ways Progeny Release occurs

Answer 24

• viral invasion initiates cell death resulting in spilling of viral progeny
• OR host cell may lyse as a result of containing many virons
• OR virus can leave cell by fusing with its plasma membrane via process known as extrusion (keeps host cell alive and allows for virus to stay in host cell in a productive cycle)

Question 25

Lytic Cycle

Answer 25

- bacteriophage makes maximal use of cell's machinery with little regard for survival of host cell - host cell eventually swells with new virons and lyses so other bacteria can be infected

Question 26

Virulent

Answer 26

term used for bacteria in the lytic cycle

Question 27

What happens if virus doesn't lyse bacterium during the lytic cylce?

Answer 27

then the virus may integrate into host genome as a provirus or prophage and begin the lysogenic cycle

Question 28

List the steps of the Lytic Cycle (3)

Answer 28

(1) Transcribe and Translate viral genome -- hydrolase (destroys host cell genome) and capsid proteins allow for synthesis of viral proteins to occur (2) Replication of viral genome -- automatic assembly of genome with capsid proteins (3) Lysis of host and release of new viral particles -- use of lysozyme which puts holes in cell wall so bacteria can no longer resist influx of water by osmosis

Question 29

Lysogenic Cycle

Answer 29

- virus is replicated as the bacterium reproduces because it is part of host genome - environmental factors (radiation, light, chemicals) eventually cause provirus to leave the genome and revert to a lytic cycle

Question 30

Superinfection

Answer 30

- simultaneous infection of a bacteria | - can become less susceptible to this if bacterium is already infected with one strain of phage

Question 31

List the steps of the Lysogenic Cycle (3)

Answer 31

(1) integration of viral genome with host genome (2) normal host activity including reproduction -- use of repressor to turn off synthesis of viral genes and make sure only host transcription/translation occurs (3) excision and lytic cycle -- requires removal of repressor, triggered by stress (pH, salt, heat), viral genome is transcribed/translated to enter lytic cycle

Question 32

What can occur during the excision step of the lysogenic cycle?

Answer 32

- transduction can occur, meaning that there is removal of part of the host gene along with the viral gene - mixed viral and host genome can be transferred to next host and leads to genetic diversity for bacteria

Question 33

What must occur to a retrovirus before it enters into the lysogenic cycle?

Answer 33

- virus must go through a cycle to convert its single stranded RNA into a double stranded DNA version of itself - single stranded RNA of retrovirus can't be inserted into double stranded DNA of host genome

Question 34

What are 2 problems that occur with retroviruses?

Answer 34

(1) once in the host genome, they are permanently there | (2) very rapid mutation of these viruses

Question 35

What is one way that RNA Polymerase differs from DNA Polymerase?

Answer 35

RNA Polymerase has definite stop sites

Question 36

Productive Cycle

Answer 36

- same as lytic cycle but does not have lysis of host cell at end - budding of new virus from host cell which allows virus to acquire a coating of the host cell's lipid bilayer (envelope) - can only occur in cells without a cell wall -- animal viruses!

Question 37

List the advantages of the Productive Cycle

Answer 37

- leads to creation of more virus because cell not lysed in end - enveloping of virus gives virus some immune protection and allows for easier infection of next host

Question 38

Prions

Answer 38

- infectious proteins, nonliving, no DNA or RNA, no membrane or organelles - resistant to things that normally destroy proteins like salinity, heat, pH, etc

Question 39

How do Prions cause disease?

Answer 39

- they trigger misfolding of other proteins - usually involves the conversion of a protein from an alpha helical structure to a beta pleated sheet - leads to reduced solubility of protein and reduced ability of cell to degrade misfolded protein

Question 40

Viroids

Answer 40

- small pathogens consiting of short circular single-stranded positive sense RNA that infects plants - similar to viruses but no capsid - replicate independently - can bind to many RNA sequences and silence genes in plant genome which prevents synthesis of proteins and causes metabolic/structural derangement in plant cell