Microbiology Recall: Viral Structure And Replication

Front 1

What are viruses?

Back 1

Obligate intracellular parasites incapable of inependent replication, protein synthesis, or energy production.

Front 2

What are obligate intracellular parasites?

Back 2

Organisms that can only reproduce within cells because they cannot synthesize proteins or generate energy.

Front 3

What is the normal range for the size of a virus?

Back 3

20-300 nanometers

Front 4

How does the size of a virus compare to other organisms?

Back 4

100 times smaller than bacteria
1000 times smaller than eukaryotic cells.

Front 5

What is the significance of the size of viruses?

Back 5

They are too small to be seen with light microscopy, but can be visualized with electron microscopy.

Front 6

What are two basic components common to all viruses?

Back 6

Genome (nucleic acid)
Capsid

Front 7

What additional component is seen with some viruses?

Back 7

Envelope

Front 8

What nucleic acid is found is viruses?

Back 8

Either DNA or RNA, but not both.

Front 9

What type of genome do most DNA viruses contain?

Back 9

Double-stranded

Front 10

What is the exception of DNA viruses having double-stranded genomes?

Back 10

Parvoviridae, which contain a single-stranded DNA

Front 11

What type of genome do most RNA viruses contain?

Back 11

Single-stranded genome

Front 12

What is the exception to RNA viruses having single-stranded genome?

Back 12

Reoviridae, which contains double-stranded RNA.

Front 13

What type of ssRNA genomes exist?

Back 13

1) Positive ssRNA
2) Negative ssRNA

Front 14

What is the significnce of +ssRNA?

Back 14

1) Nucleic acid that can be directly translated by host machinery, thus making it analogous to host mRNA.
2) Positive ssRNA viruses do not require RNA-dependent RNA polymerase in the virion for viral replication.

Front 15

What is the significance of -ssRNA?

Back 15

Nucleic acid that is incapable of being translated directly as messenger RNA and synthesize a positive strand by serving as a template

Front 16

How is a positive strand of RNA made from a negative strand?

Back 16

1) The virus must contain its own RNA-dependent RNA polymerase to synthesize the positive strand.
2) RNA-dependent RNA polymerase is found in the ivrion of negative ssRNA viruses because it is required for viral replication.

Front 17

What is a capsid?

Back 17

A protein coat that surrounds and protects viral nucleic acid.

Front 18

What is a capsomer?

Back 18

The functional subunit of a capsid consisting of one of more proteins that determine the viral geometrical symmetry.

Front 19

What are two forms as geometrical symmery found in viral capsids?

Back 19

Icosahedral
Helical

Front 20

What is icosahedral symmetry?

Back 20

A sphere-shaped structure composed of 20 triangular faces and 12 vertices

Front 21

What is helical symmetry?

Back 21

Rod-shaped structure composed of capomers arranged in a hollow coil.

Front 22

What are two functions fo capsid proteins?

Back 22

1) Protect nucleic acid containing genetic material
2) Mediate viral attachment to host cell receptors

Front 23

What is an unintentional effect of capsid proteins?

Back 23

Contains antigens that may stimulate an immunological response.

Front 24

What is a viral envelope?

Back 24

A lipoprotein membrane surrounding the capsid

Front 25

From where is the viral envelope derived?

Back 25

Host cell membrane

Front 26

What families are nonenveloped?

Back 26

DNA viruses: "PAP"
Papovaviridae (dsDNA)
Adenoviridae (dsDNA)
Parvoviridae (ssDNA)

RNA viruses: "CPR"
Caliciviridae (+ssRNA)
Picornaviridae (+ssRNA)
Reoviridae (dsRNA)

Front 27

What is a name for nonenveloped viruses?

Back 27

A nucleocapsid or a 'naked' virus.

Front 28

What are defective viruses?

Back 28

Viruses with nucleic acid and proteins that are able to invade but incapable of replication.

Front 29

What is an ecample of a defective virus?

Back 29

Hepatitis D virus

Front 30

What are three unconventional infectious agents?

Back 30

Prions
Viriods
Pseudovirions

Front 31

What are prions?

Back 31

Potentially infectious agents that are composed solely of proteins and lack nucleic acid.

Front 32

What diseases are caused by prions?

Back 32

Transmissible spongiform enephalopathies, including Creutzfeldt-Jakob disease and kuru

Front 33

What are viroids?

Back 33

Small infectious agents that consist solely of RNA that do not code for proteins
Viroids only cause disease in plants.

Front 34

What are pseudovirions?

Back 34

Agents with a capsid containing host cell DNA instead of viral DNA incapable of replication.

Front 35

What are the two stages of the viral growth curve?

Back 35

Eclipse period
Exponential growth period

Front 36

What is the eclipse period of the viral growth curve?

Back 36

The period of time when no intact virus is found inside the cell, form initial entry and disassembly of the virus to the assembly of the first progeny.

Front 37

How long is the eclipse period for most human viruses?

Back 37

1-20 hours

Front 38

What is teh exponential growth period in the viral growth curve?

Back 38

The period of itme when the number of progeny virus produces within the infected cell increases expoentially and then plateaus.

Front 39

How long is the exponential growth period for most human viruses?

Back 39

8-72 hours

Front 40

Wht is the incubation period for a virus?

Back 40

The period of time between initial infection and onset of symptoms.

Front 41

What is the burst size?

Back 41

The number of virions released from an infected cell.

Front 42

What is the typical range for the burst size of a virus?

Back 42

100-10,000 virions per infected cell

Front 43

What are the eight stages of the viral growth cycle?

Back 43

1) Adsorption
2) Penetration
3) Uncoating
4) Early transcription and translation
5) Viral genome replication
6) Late transcriptoin and translation
7) Assembly
8) Release

Front 44

What is adsoprtion and how is it achieved?

Back 44

Attachment of virion to host cell receptors.
1) Viral envelope glycoprotein interaction with host cell receptors
2) Folding of capsid proteins creating a site for host cell receptor interaction.

Front 45

What is penetration?

Back 45

Passage of virion from the cell surface into the cytoplasm.

Front 46

What are the two main mechanisms of penetration?

Back 46

Direct membrane fusion
Receptor-mediated endocytosis

Front 47

What is direct membrane fusion?

Back 47

Fusion of the viral envelope with the host cell membrane enabling the nucleocapsid to enter the cell.

Front 48

What is receptor-mediated endocytosis?

Back 48

Host cell membrane invagination after viral adsorption, creating an intracellular virion-containing endocytic vesicle (endosome).

Front 49

How is the virion released into the cytoplasm from the endosome?

Back 49

Direct membrane fusion between the virion and endosome.
Virion proteins create pores within the endosome or cause the membrane to dissolve, facilitating viral entry into the cytoplasm.

Front 50

What must occur for a virion in an endosome to be pathogenic?

Back 50

Release of virus into the cytoplasm before the endosome fuses with the lysosome.

Front 51

Why must release occur before endosome fuses with the lysosome?

Back 51

Lysosomes contain degradative enzymes.

Front 52

What family of viruses is the one exception to this rule?

Back 52

Reoviridae (dsRNA) can survive and partially uncoat within lysosomes

Front 53

What is uncoating?

Back 53

Disassembly of the virion

Front 54

What is early transcription and translation?

Back 54

Nucleic acid is transcribed into mRNA, which is then translated into proteins used for viral genome replication.

Front 55

What is the first step in viral gene expression?

Back 55

Synthesis of mRNA

Front 56

Where do most DNA viruses replicate?

Back 56

In the nucleus

Front 57

What family of DNA viruses does not replicate in the nucleus?

Back 57

Poxviridae, which replicate in the cytoplasm.

Front 58

Wht is the advantage of viruses replicating in the nucleus?

Back 58

They can use host cell enzymes for viral replication.

Front 59

Where do most RNA viruses replicate?

Back 59

In the cytoplasm

Front 60

What RNA viruses do not replicate in the cytoplasm?

Back 60

Retroviruses adn influenza virus which have replication steps in the nucleus.

Front 61

What are two challenges that RNA viruses must overcome for replication?

Back 61

1) Host cells that lack RNA polymerase used to synthesize complementary RNA strands from the RNA genome.
2) RNA viruses frequently contain only one molecule of RNA, but must express two or more proteins.

Front 62

What are three groups of RNA viruses that have different patterns of replication to overcome the two main challenges to replication?

Back 62

+ssRNA
-ssRNA
dsRNA

Front 63

What are two mechanisms by which +ssRNA replicate?

Back 63

1) Replication with a complementary -ssRNA intermediate.
2) Replication with a DNA intermediate

Front 64

What are two roles of +ssRNA that replicate with complementary -ssRNA intermediate?

Back 64

1) Serves as mRNA for translation by host machinery.
2) Template for synthesis of complementary -ssRNA.

Front 65

What is the product of translation?

Back 65

A single protein

Front 66

How are multiple proteins generated from a single protein from translation?

Back 66

The protein contains protease domains that allow it to be cleaved into multiple proteins, e.g., RNA-dependent RNA polymerase and structural proteins.

Front 67

How does +ssRNA replicate?

Back 67

1) The +ssRNA serves as a template for generating a complementary -ssRNA with RNA-dependent RNA polymerase.

2) The -ssRNA can serve as a template for generating progeny +ssRNA via RNA-dependent RNA polymerase

Front 68

What is the initial step in replication of +ssRNA which replicate wiht a DNA intermediate?

Back 68

Conversion of the +ssRNA to dsDNA..

Front 69

How is +ssRNA converted to dsDNA?

Back 69

By RNA-dependent DNA polymerase, which is found within the virion and must be transferred into the host cell along with the viral genome for replication to occur.

Front 70

What is another name for RNA-dependent DNA polymerase?

Back 70

Reverse transcriptase

Front 71

What step is required before the intermediate (in +ssRNA converting to dsDNA) can generate viral progeny and proteins?

Back 71

The dsDNA must integrate into the cell genome.

Front 72

What enzyme catalyzes integration into the host genome?

Back 72

Integrase

Front 73

What unique challenge do -ssRNA viruses face?

Back 73

The -ssRNA strand cannot be directly translated by host machinery.

Front 74

How do -ssRNA viruses overcome the problem of not being able to be directly translated by host machinery?

Back 74

They contain RNA-dependent RNA polymerase within the virion that must be transferred into the host cell along with the viral genome for replication to occur.

Front 75

What is the first step of infection with -ssRNA viruses?

Back 75

Synthesis of +RNA by the virion-encoded RNA-dependent RNA polymerase using the -ssRNA as a template.

Front 76

What are two roles of intermediate +ssRNAs?

Back 76

1) Serve as MRNA for translation by host machinery
2) Template for synthesizing -ssRNA which is incorporated into progeny virions.

Front 77

What are two categories of -ssRNA viruses?

Back 77

1) Viruses with a single, linear strand of RNA
2) Viruses with a segmented RNA genome

Remember BOAR for the segmented RNa viruses: Bunyaviridae, Orthomyxoviridae, Arenaviridae, adn Reoviridae

Front 78

How are multiple proteins translated from single, linear and segmented categories of -ssRNA viruses?

Back 78

1) Linear RNA genomes may be polycistronic, generating multiple mRNAs specific for single proteins.

2) Segmented RNA genomes produce multiple positive-strand mRNAs that usually encode single proteins.

Front 79

What is the structure for dsRNA viruses?

Back 79

All dsRNA viruses contain a segmented genome.

Front 80

How is mRNA synthesized in dsRNA viruses?

Back 80

Because host cell machinery is unable to generate positive-strand mRNA from dsRNA, the virion mut contain its own RNA-dependent RNA polymerase to generate +ssRNA from the -ssRNA.

Front 81

What is late transcription and translation?

Back 81

Translation of nucleic acid into proteins that are either incorporated into the progeny virus particle or are required for viral assembly.

Front 82

What is assembly?

Back 82

Construction of viral progeny from necessary nucleic acids and proteins.

Front 83

Where does assembly occur?

Back 83

In the cellular compartment where viral nucleic acid replicates.

DNA viruse that replicate int eh nucleus assemble in the nucleus, and therefore require proteins to be transported form the cytoplasm.

RNA viruses that replicate in the cytoplasm assemble in the cytoplasm.

Front 84

What are two methods of viral release from the host cell?

Back 84

1) Lysis of the host cell.
2) Budding form the host cell

Front 85

What types of viruses are released by budding?

Back 85

Enveloped viruses

Front 86

What are the six steps of budding?

Back 86

1) Synthesis of viral glycoproteins
2) Transport of glycoproteins to cellular membrane
3) Binding of viral nucleocapsid to glycoproteins
4) Cellular membrane envelopes viral neucleocapsid
5) Budding of cellular membrane
6) Release of enveloped virus

Front 87

How long does one viral growth cycle typically last?

Back 87

20 hours for human viruses
30 minutes for bacteriophage viruses

Front 88

Wht is teh lytic growth cycle?

Back 88

A growth cycle in which the virus enters the cell, replicates, and released by host cell lysis.

Front 89

What is the lysogenic growth cycle?

Back 89

A growth cycle in which viral DNA becomes integrated into the host's genome, replicates autonomously under certain conditions, and then is released by host cell lysis or is simply passed along when the host cell multiplies.

Front 90

What are viruses called that replicate with lysogenic growth cycle?

Back 90

Temperate phages

Front 91

What are viruses called that infect bacteria and replicate via the lysogenic growth cycle?

Back 91

Bacteriophages

Front 92

What is the integrated viral DNA called?

Back 92

Prophage

Front 93

What are two conditions that result in prophage extry into the lytic cycle?
What is this process called?

Back 93

UV light
Metabolic stress

Induction

Front 94

What is the mechanism behind UV light induction?

Back 94

Induces synthesis of a protease, which inhibits a viral repressor protein, allowing expression and excision of prophage DNA.

Once this DNA ha been expressed, the phage completes it's replicative cycle and synthesizes progeny virions.