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Infectious Diseases > Bacteriophages > Flashcards

Bacteriophages Flashcards

1
Q

What are bacteriophages?

A

Typically highly specific viruses (species/strain) that infect bacteria

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2
Q

What makes bacteriophages significant?

A

good models for animal viruses
Evolution via Horizontal gene transfer in bacteria (transduction)
Medical applications

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3
Q

What are the medical applications of bacteriophages?

A

Diagnostics - phage typing
Treatment - phage therapy
Research - phage display

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4
Q

What are the types of phage structures?

A

Head-tail
filamentous / helical
Icosahedral

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5
Q

What are the four main stages of phage replication?

A
  1. Adsorption and Penetration
  2. Transcription and Translation
  3. Replication
  4. Assembly and Release
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6
Q

What is involved in the Adsorption and penetration step of phage replication?

A

Specific receptors on host cell wall
Genome injection
Entry via bacterial feature

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7
Q

What is involved in the Transcription and Translation step of phage replication?

A

Expression of Viral RNA
And production of viral proteins
(by host ribosomes)

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8
Q

What can be involved in the Replication step of phage replication?

A

Terminal redundancy
Rolling circle
Lysogeny

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9
Q

What is involved in the Assembly and Release step of phage replication?

A

Follows a pathway and potentially uses lytic enzymes

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10
Q

What happens in Adsorption?

A

Tail fibres bind specific receptor in host cell wall

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11
Q

How does receptor interactions affect adsorption of bacteriophages?

A

Affects host specificity

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12
Q

What are the ways that bacteria develop resistance to phage?

A

Modify receptors
Hide receptors
Modify O/K antigen

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13
Q

What is an example of a bacteria producing a protein to mask receptors from phage adsorption?

A

Staphylococcus aureus produces protein A

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14
Q

How do phages with a sheath penetrate the bacterial envelope?

A

Phages with a sheath e.g. T4: irreversible binding of the phage to the bacterium results in the contraction of the sheath

the hollow tail fiber is pushed through the bacterial envelope

remainder of phage stays on the outside (some exceptions)

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15
Q

If a phage doesn’t have a sheath what is a mechanism they could use to get through the bacterial cell envelope?

A

Using enzymes that digest various components of the bacterial envelope

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16
Q

For bacteriophages what do class I genes do?

A

Set cell up for replication

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17
Q

For bacteriophages what do class II genes do?

A

Phage genome replication

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18
Q

For bacteriophages what do class III genes do?

A

encode structural components or lytic enzymes (produced very late)

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19
Q

What is the main problem when replicating linear DNA?

A

Replicating the end sequences

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20
Q

What is a strategy to avoid the problem of replicating end sequences regarding linear DNA?

A

Terminal redundancy

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21
Q

What is a strategy to avoid the problem of replicating end sequences regarding circular DNA?

A

No ends in circular DNA!

Rolling circle replication

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22
Q

What is a strategy to avoid the problem of replicating end sequences regarding Passive replication within host genome?

A

Lysogeny

23
Q

What is Rolling circle replication?

A

Endonuclease nicks a strand
3’ end extended by DNA polymerase
Displacing complimentary strand which is then copied in short segments by DNA polymerase
Replication proceeds until one new copy is made

24
Q

What is a concatemer?

A

long continuous DNA molecule that contains multiple copies of the same DNA sequence linked in series

25
Q

What is Terminal Redundancy?

A

Terminal repeated sequences that can anneal to form concatenated forms of the genome
cut at the overlapping sequences every genome length to leave a 5’ overhang suitable for polymerisation to complete the ends

26
Q

How are phage structures assembled?

A

Often via specific pathways involving sub-structures (such as the pro-head)

27
Q

How is a transducing phage produced?

A

A mistake in incoorporation of DNA causes bacterial DNA to be packaged as a transducing phage

28
Q

Is incoorporation of DNA during phage assembly specific?

A

YES

29
Q

Are the assembly and release process for bacteriophages active or passive?

A

Both are active!

30
Q

What is an example of a bystander protein during assembly of bacteriophage?

A

Scaffolding proteins

31
Q

What are Holins?

A

protein that disrupts hosts cytoplasm membrane

32
Q

What do lysozymes do?

A

degrade cell wall

33
Q

What are the steps of the packaging process?

A
  1. Terminase binds viral genome
  2. Terminase-DNA binds procapside portal
  3. DNA translocation
  4. contamer cleavage and packaging completion
34
Q

What is an example of phages that use accessory proteins in the assembly process?

A

Phage lambda

35
Q

What does lysin do?

A

punches holes in bacterial outer cell wall

36
Q

What is a Lytic bacteriophage?

A

capable of replication via lytic cycle only

37
Q

What is a Temperate / lysogenic bacteriophage?

A

able to integrate genome in host genome and replicate passively when host divides, or establish lytic infection depending on environmental conditions

38
Q

What is a prophage?

A

term used to describe lysogenic phage when it has integrated in host genome and is mostly repressed and inactive

39
Q

What is a Lysogen?

A

term used to describe bacterial cell in which a phage has integrated its genome in dormant state (i.e. as prophage)

40
Q

What are examples of lytic phages?

A

T2 and T4

41
Q

What are the steps of the lytic cycle?

A

1) attachment
2) insert of phage DNA and degradation of host DNA
3) synthesis of viral genomes and proteins
4) assembly
5) release

42
Q

What happens during lysogeny?

A

Phage genome integrates at specific site into host chromosome
phage still present as prophage
Under certain conditions, such as UV irradiation, it becomes active and resumes a lytic replicaiton cycle

43
Q

What is a prophage?

A

phage present in the cell as an integrated copy of the genome

44
Q

What is an example of a prophage that goes through the lysogenic cycle?

A

Phage lambda

45
Q

How is circulisation of the phage lambda genome achieved?

A

by complementary base pairing of the ss overhangs

46
Q

For phage lambda what is the site at which it integrates into the host genome?

A

Integrates at specific site (attB) in host genome

att site opposite cos site required for integration

47
Q

What genes serve as the lambda repressor switch?

A 
Lambda repressor protein (cI)
Cro protein (control of repressor operator)
48
Q

What does the lambda repressor protein (cl) do?

A

activates the lysogenic pathway

49
Q

What does the Cro protein (control of repressor operator) do?

A

activates the lytic pathway

50
Q

Why are lysogens immune to “super infection” by more phage?

A

they are full of cI repressor

51
Q

How is the lytic virus replication activated?

A

Certain environmental stimuli (e.g. UV light) indirectly inactivate the cI repressor (via activation of bacterial RecA protein)

52
Q

Why is lysogenic replication sometimes preferred to lytic replication?

A

Poor growth of host = wise to wait for better times (likely to be limited new hosts available for progeny phage)

53
Q

M13 causes chronic infection of host cells how does it achieve this?

A

Rolling circle replication

Does not cause lysis of host cell

Infectious Diseases (4 decks)

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