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Microbiology Exam 2 > Properties of Viruses > Flashcards

Properties of Viruses Flashcards

1
Q

T/F: Viruses infect bacteria.

A

-True; viruses that do so are called bacteriophages. They help move genetic info around between bacteria increasing spread of drug resistance and affecting pathogenesis
-As an aside, viruses can also infect animals,
plants, and bacteria.

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2
Q

T/F: Bacteria are the most abundant biological entity on earth

A

-false; viruses are

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3
Q

T/F: Viruses are generally 100-1000 times smaller than cells.

A

-True

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4
Q

List 3 properties of bacteria concerning their genome, protein synthesis, and energy generation.

A
  • contain DNA or RNA genome, NEVER BOTH
  • have NO protein synthesis machinery (no ribosomes)
  • have NO energy generating machinery
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5
Q

In all viruses, nucleic acids are enclosed in a ________.

A

-protective protein shell coat called a capsid

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6
Q

2 phases of viral replication

A
  • outside the host: inert collect of macromolecules called a virion
  • inside host: utilize host machinery to copy self and produce more virus
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7
Q

Viruses are ALL obligate intracellular pathogens. How is this term more strict than its use with bacteria?

A

-viruses need to access the cell’s protein synthesis machinery IN THE CYTOPLASM, and as such, they cannot replicate within intracellular vacuoles

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8
Q

Viruses do not _______ or undergo _______, but are assembled from pre-form components.

A
  • grow

- division

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9
Q

2 general categories of proteins that viruses can make

A

-structural and nonstructural proteins

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10
Q

All virions have a protein shell (capsid) surrounding the nucleic acid genome. The full assembly is called the _________.

A

-nucleocapsid

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11
Q

Enveloped viruses have what additional characteristic?

A
  • lipid membrane surrounding the nucleocapsid

- not all viruses have this; those without an lipid envelope are called naked or non-enveloped

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12
Q

What is located on viral surfaces that allow it to attach to host cells?

A

-projections from the surface of the virions called spikes or envelope proteins

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13
Q

What are 2 practical implications of why some viruses have RNA instead of DNA?

A
  1. RNA polymerases usually dont have proof-reading function; thus, RNA viruses tend to mutate more than DNA viruses
  2. RNA viruses tend to replicate quickly, particularly +RNA viruses since their genome is in effect already mRNA
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14
Q

Do RNA or DNA carrying viruses cause chronic infections?

A

-DNA causes chronic infections (think Herpes)

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15
Q

Do RNA or DNA viruses tend to have larger genomes?

A

-DNA viruses

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16
Q

How are viral genomes mainly different from human genomes?

A

-unlike the human genome, there is no wasted space in viral genomes–they are almost completely packed with genes

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17
Q

What are the 4 mechanisms by which viruses conserve “genetic space”?

A
  • make a polyprotein that is cleaved by viral or cellular protease
  • RNA splicing
  • Overlapping reading frames (and splicing): single promoter with overlapping reading frames
  • Ribosomal frame-shifting: when scanning along mRNA, ribosome will slip back one base and continue which gives a frameshift and gives diff. protein production
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18
Q

In addition to being divided by their nucleid acids, viruses can also be divided based on the type of capsid they have; what are the 2 main categories of capsids?

A
  • icosahedral capsids

- helical capsids

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19
Q

5 functions of a capsid

A
  1. packaging and condensation of genome
  2. protection of nucleic acid
  3. transport nucleic acid from cell to cell
  4. provides specificity for attachment
  5. metastable: capsids undergo changes that result in delivery of genetic material into a cell
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20
Q

T/F: All icosahedral capsids are membrane enveloped.

A

-False; all helical capsids are and some icosahedral are

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21
Q

Describe the process of viruses acquiring an envelope

A
  • occurs through a process called budding where the viruses steal membrane from the host cell
  • viral capsid assembles in cytoplasm, and makes integral membrane protein (spike protein) that is transported to the host cell surface where the glycoproteins form patches
  • final step involves membrane fission event when the capsid binds with the intracellular domain of the spike glycoproteins
22
Q

Enveloped viruses have one or more __________. Why are these heavily glycosylated?

A
  • spike proteins (some viruses have 1 for receptor binding and membrane fusion, other viruses have separate proteins for these functions)
  • since carbs are not immunogenic, this functions as a shield to protect the protein from antibodies
23
Q

3 functions of spike proteins

A
  • attach virus to cell surface
  • mediate fusion with cell to get virus into cell
  • targets for neutralizing antibodies
24
Q

Describe the difference in transmission of enveloped vs. nonenveloped viruses

A
  • while lipid membranes are stable, they are sensitive to osmotic stress, dessication, and extremes of pH and salt; thus enveloped viruses are transmitted by transfer of body fluids
  • nonenveloped are much more stable outside of the body and can be transmitted by a much wider variety of mechanisms(fluids, fomites, fecal oral, etc) (not killed by simple soaps)
25
Q

6 steps in the life cycle of a virus

A
  1. Attachment: virus binds to a specific R (R determines tropism)
  2. Penetration:
  3. Uncoating: genome is inside capsid, so it needs to be uncoated for replication to process
  4. Translation, transcription, replication
  5. Assembly
  6. Release
26
Q

What is a common way to measure the amount of virus in solution?

A
  • plaque forming units (PFU)
  • viral solution is serially diluted and placed on a monolayer of cells; as it infected cells, monolayer will develop holes or plaques
27
Q

When cells are infected with a virus, you can collect the media at different times after infection and measure the amount of PFUs present. At first the amount of virus recovered ___________ and then with time, the amount of virus will _________. The period of time between the start of infection and when the first progeny viruses emerge is called the ___________.

A
  • decrease; the virus you add binds and infects cells
  • increase; new viruses are made and released from infected cells and the PFU goes up again
  • eclipse phase
28
Q

5 things a viral receptor may do

A
  1. binds the virus to the cell
  2. can just act to merely tether the virus
  3. can itself initiate entry
  4. can trigger conformational changes in capsid or glycoprotein needed for entry
  5. may send signals to cell that in some way prepare it for infection (virus induces cell to become more permissive host)
    * *need receptor for virus to infect a cell**
29
Q

Viruses can use all sorts of different cell surface proteins as receptors. Most receptors are proteins, but some viruses bind to _______.

A
  • carbs

- influenza binds to sialic acid which is common and explains the wide tropism of influenza

30
Q

Do viruses usually bind to 1 or more than 1 receptor to attach to a cell surface, initiating the entry process?

A
  • usually 1

- rarely more than 1

31
Q

Define viral tropism and what determines it.

A
  • spectrum of tissues and cell types infected by a given virus
  • some have restricted while some have broad tropisms
  • often explained by the presence of absence of the appropriate viral receptor
32
Q

What are the 3 ways viruses cross a cell’s membrane and which type of viruses use each method?

A
  • membrane fusion*: ALL ENVELOPED viruses have one or more membrane proteins (spike) that catalyze fusion between the viral and cellular membranes
  • pore formation: some nonenveloped viruses form pores in membrane
  • Membrane lysis: some nonenveloped viruses enter cells by endocytosis, then lyse the vacuole to escape to the cytoplasm
33
Q

Because nonenveloped viruses do not have a membrane, what method of entry into a cell can they not do?

A

-membrane fusion because they have no membrane to fuse with!

34
Q

5 stages of enveloped virus entry

A
  1. attachment
  2. receptor engagement
  3. Trigger event (pH dependent or pH independent)
  4. conformational change
  5. membrane fusion
35
Q

What is an added difficulty of enveloped viruses infecting cells?

A
  • its genetic material must traverse both the viral and cellular membranes
  • all enveloped viruses encode and express on their surface a protein or proteins that under the right conditions changes its structures in a way that mediated fusion and affords cytoplasmic access to the viral genome
36
Q

What does pH-dependent trigger event mean for an enveloped virus?

A
  • the virus engages the receptor, is taken up by endocytosis into an endosome, which is a highly acidic prelysosome, and this low pH activates the virus to fuse with the lysosome and release its contents into the cytoplasm
  • others do the same thing, but with out pH being a factor (pH-independent)
37
Q

Describe how non-enveloped viruses enter and infect cells.

A
  • usually enter by endocytosis
  • low pH in endosomes induces conformational changes in capsid proteins
  • hydrophobic domains are exposed and insert into endosomal membrane, forming a pore (or lysing endosome)
  • viral DNA or RNA enters cell via pore
38
Q

Describe how large DNA vs. small DNA vs. RNA vs. retroviruses replicate their genomes

A
  1. large DNA: encode many of the enzymes required (DNA pol) and are more “autonomous”
  2. small viruses use host cell DNA polymerase and other enzymes
  3. RNA: encode their own RNA-dependent RNA pol which uses a complementary RNA as template
  4. Retroviruses: copy a ss+ RNA genome into dsDNA which is template for new genome synthesis
39
Q

Where and when does small DNA viruse DNA replication take place?

A

-in the nucleus during S phase because they rely on host cell DNA polymerase

40
Q

T/F: RNA dependent polymerase lacks proofreading, and so RNA viruses mutate quickly and often replicate faster.

A

-true

41
Q

General replication strategy of negative-stranded RNA viruses

A
  • (-)RNA viruses have to carry with them their own polymerase, because this is the only way they can make a + RNA copy of their -RNA genome and a +RNA stand is needed for replication
    1. turn -RNA into + RNA with RNA-dependent RNA Pol
    2. RNA+ (mRNA) is turned into viral proteins and viral RNA polymerase or turned back into -RNA to be packed with RNA pol
42
Q

General replication strategy if positive-stranded RNA viruses

A
  • +RNA can be turned directly into viral proteins, not dependent on transcription into +RNA like -RNA viruses are
43
Q

Compaire +RNA vs. -RNA on presence of viral polymerase in virion, if viral RNA is alone infectious, and the initial event in the cell they infect

A
  • RNA+: no polymerase in virion, infectious alone, translation
  • RNA-: have polymerase, not infectious alone (need polymerase), transcription
44
Q

Why is it beneficial for viruses (RNA viruses) to mutate quickly?

A

-enables them to evolve and adapt to new conditions, including immune escape

45
Q

3 major mechanisms by which viruses mutate

A
  1. simple mutation: during replication, wrong base pair is used–point mutation
  2. recombination: when 2 viruses infect same cell their genomes can recombine
  3. reassortment: restricted to viruses with segmented genomes; if 2 viruses infect the same cell and replicate, their genetic segments mix and result in new generation of new virus; non classical recombination, very efficient
46
Q

Recombination occurs most frequently is what viral types? What about reassortment?

A
  • recombination is common in DNA viruses

- reassortment is for segmented viruses only

47
Q

Fundamental steps of virus assembly

A
  1. formation of individual structural units of the protein shell form one or several viral proteins
  2. assembly of the protein shell by appropriate and sometimes variable interactions among structural units; self assembly
  3. selective packaging of nucleic acid and other viral components
  4. acquire envelope is needed
  5. release for host cell
48
Q

How does the virus selective package its own RNA or DNA and not cellular molecules?

A
  • viral genomes have packaging signals that are recognized by viral proteins
  • packaging signals are usually a particular secondary structure like a stem-loop
  • useful for gene therapy vectors by placing signal upstream of drug of choice
49
Q

2 general mechanisms of virus release for cells

A
  1. enveloped viruses bud from a cellular membrane; this can be the PM or the ER, if the ER they are then secreted
  2. Nonenveloped viruses accumulate in the cell, and are released when the cell dies and is lysed; cell may die from immune system, apoptosis, cytotoxic effects, etc
50
Q

Define burst size

A

-average number of virus particles released from an infected cell

Microbiology Exam 2 (11 decks)

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