Virus Structures and Classification

Question 1

What are features of viruses that distinguish them from bacteria/toxins?

Answer 1

1. Size - Tobacco Mosaic Virus was able to pass through a filter while bacteria could not
2. Growth - Showed that titer of TMV increased after infecting a plant proving it was not a toxin

Question 2

What are a viruses requirements for living?

Answer 2

Obligate intracellular parasites (Are not autopoietic)

Question 3

What are the two theories of virus origin?

Answer 3

Cellular Origin - Once cellular components but over time separated

Question 4

What is a nucleocapsid?

Answer 4

RNA or DNA CORE that is protected by a protein coat (CAPSID)

Question 5

What is the structure of nucleocapsids?

Answer 5

Repeating protein subunits called capsomeres

Question 6

What is an envelope?

Answer 6

Virus-modified cellular membranes acquired upon exit from host that wraps around the nucleocapsid.

Question 7

What is the range of virus size?

Answer 7

18-300nm

Question 8

What is an advantage of being a large virus?

Answer 8

1. Can carry more of their own genes/proteins to complete replication cycle.
2. Can carry genes which will alter the host immune response

Question 9

What is a disadvantage of being a large virus?

Answer 9

Longer replication time (more immune system opportunities)

Question 10

Why are viruses obligate intracellular parasites?

Answer 10

They NEVER convert RNA into protein

Question 11

What is a replication strategy used by ssDNA?

Answer 11

Hairpin. Repeating ends with ssDNA strand in the middle to which host machinery can bind and copy.

Question 12

What is a replication strategy used by dsDNA?

Answer 12

Rolling circle. Similar to Bacteria

Question 13

What is (+)ssRNA?

Answer 13

RNA that is ready to be processed by a ribosome to make a protein.

Question 14

What is (-)ssRNA?

Answer 14

RNA that must be copied before it can be processed by a ribosome to make a protein.

Question 15

What are the advantages of having a large genome?

Answer 15

Can carry more genes (modulate host immune responses)

Question 16

What do most DNA viruses need that RNA viruses do not?

Answer 16

Because DNA is usually replicated in the nucleus, most DNA viruses need access to the nucleus.

Question 17

What do RNA viruses need that DNA virus do not?

Answer 17

A gene that encodes for an RNA dependent RNA polymerase OR carry a protein in that is capable of RNA polymerase with them into the cell.

Question 18

What is tropism?

Answer 18

The range of hosts a virus is capable of infecting

Question 19

What are different pathways that a virus can enter a cell?

Answer 19

1. Receptor mediated endocytosis
2. Direct penetration (attach and secrete their genome inside)
3. Non-enveloped viruses are not well understood

Question 20

After entering the cytoplasm, what must dsDNA viruses do?

Answer 20

(Nucleocapsid) Attach to a nuclear pore and get their genome into the nucleus

Question 21

Does dsRNA viruses ever release its genome into the cytoplasm?

Answer 21

NO. dsRNA doesn’t occur in host cells and would trigger host immunity

Question 22

Where does dsRNA viruses replicate their genome?

Answer 22

Inside themselves and then exported to cytoplasm

Question 23

What are the different mechanisms of virus replication?

Answer 23

1. Adenovirus - empty protein coat imports genome
2. Reovirus - RNA packaged during capsid assembly
3. Retrovirus - preassembly on a membrane

Question 24

How are viruses released?

Answer 24

1. Lysis - Viral molecules that rupture cellular membrane (bacteriophage)
2. Weak lysis - Depends on membrane breakdown after cell death
3. Budding - Enveloped viruses use cell membrane as outer coating of virus particle (enveloped only)

Question 25

What disadvantage is there for a virus to replicate in the nucleus?

Answer 25

The nucleus must be in a replication stage for there to be replication of the virus genome.

Question 26

What advantage/disadvantage is there with a lytic style of infection?

Answer 26

Advantage: The progeny are able to infect other cells. Disadvantage: The progeny cells are exposed to the immune system.

Question 27

What is the time for one step growth?

Answer 27

Time from the start of the infection to the beginning of the plateau (cell death)

Question 28

How do + sense RNA viruses do replication?

Answer 28

+ sense RNA can be used for translation, makes RNA for template for new + sense

Question 29

How do - sense RNA viruses do replication?

Answer 29

- sense RNA make full-length +ssRNA intermediate for translation and replication

Question 30

Describe the 4 steps in the genome replication process used by dsRNA viruses

Answer 30

1. Virus particle includes viral polymerase 2. dsRNA induces innate immune response to genome inside the particle 3. mRNA is synthesized in the particle and exported to the cytoplasm 4. mRNA serves as the + strand in virus genome, - strand is synthesized during assembly

Question 31

What 3 things must occur for genome replication of DNA viruses (include one virus that is an exception)?

Answer 31

1. DNA virus must gain access to the nucleus (pox viruses are an exception because they already have RNA polymerase in virion) 2. DNA virus must prepare the cell for DNA replication (growth plase, dNTP production, replication machinery) 3. DNA virus must ensure that ends are copied

Question 32

Describe the genome, type of particle, and location of replication for the smallpox/vaccinia virus

Answer 32

1. Genome: dsDNA 2. Particle: Envelope- Brick-shaped 3. Site of Replication: Cytoplasm

Question 33

Describe the genome, type of particle, and location of replication for the Yellow fever/West Nile virus

Answer 33

1. Genome: +ssRNA 2. Particle: Enveloped - sphericle 3. Site of Replication: Cytoplasm

Question 34

Describe the genome, type of particle, and location of replication for the Influenza virus

Answer 34

1. Genome: -ssRNA 2. Particle: 8 segments, enveloped 3. Site of Replication: nucleus

Question 35

Describe the genome, type of particle, and location of replication for the poliovirus/rhinovirus

Answer 35

1. Genome: +ssRNA 2. Particle: Non-enveloped and spherical 3. Site of Replication: Cytoplasm

Question 36

Describe the genome, type of particle, and location of replication for HIV (retroviridae virus)

Answer 36

1. Genome: +ssRNA (2 COPIES!) 2. Particle: Enveloped and Spherical 3. Site of Replication: Nucleus (Reverse Transcriptase)