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Phage Lambda Flashcards

1
Q

Define lytic cycle

A

Virus infects a host cell
Replicates
Destroys the cell to release newly formed viral particles

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2
Q

Define lysogeny

A

Viral genome integrates into the host cell’s genome
Replicates alongside it without causing immediate lysis

In this state, the viral DNA/RNA is known as a PROPHAGE when integrated into the host genome

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3
Q

What state are viral genes integrated in lysogeny?

A

Genes are transcriptionally silent or express only a few genes = allowing the virus to persist in the host without causing immediate harm

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4
Q

Define prophage

A

A phage genome covalently integrated into a bacterial genome

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5
Q

Define phage immunity

A

The ability of a prophage to prevent another phage of the same type from infecting a cell

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6
Q

What does induction of a phage mean?

A

Under certain condition, the prophage can be induced to exit the lysogenic state and enter lytic cycle

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7
Q

What happens to viral gene expression in induction?

A

Phage DNA is released and enters the lytic cycle

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8
Q

Why may induction of a phage occur?

A

Destruction of a lysogenic repressor
Environmental stress
Changes in host cell

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9
Q

What can a temperate phage do?

A

Can choose between lytic and lysogenic pathway development

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10
Q

What can a virulent phage do?

A

Lytic phage = cannot display lysogenic cycle

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11
Q

What are the periods of lytic development?

A

Early period = before replication
Late period = after replication onset

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12
Q

What are phage genomes and protein particles first assembled into in lytic development?

A

Progeny phages

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13
Q

What are the two early genes?

A

cro = negative regulator
N = antiterminator

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14
Q

What is N gene required for?

A

Expression of delayed early genes

N = synthesizes delayed early genes

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15
Q

What are the 3 delayed early genes?

A

Regulators = cii, ciii, Q

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16
Q

What delayed early genes does lysogeny require?

A

cii and ciii

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17
Q

What genes does the lytic cycle require?

A

cro = early gene
Q = delayed early gene

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18
Q

What genes does the lysogenic cycle not have?

A

Late genes

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19
Q

What order does the lambda repressor bind operators?

A

O1-2-3

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20
Q

What is the outcome of lambda repressor binding operator?

A

Represses expression of lytic genes

Maintains lysogenic cycle

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21
Q

What is the role of cro?

A

Promotes lytic cycle
Prevents lambda repressor from binding

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22
Q

What promoters are the early genes transcribed by?

A

N = promotor L
cro = promoter R

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23
Q

What is the role of the anti-terminator protein? (pN)

A

Allows RNA polymerase to pass the terminators = tL1 tR1

24
Q

What does N mRNA code for?

25
What is the role of Q protein?
Anti-terminator = allows RNA polymerase to transcribe the late genes for the LYTIC cycle
26
What happens to the lambda DNA after infection?
It circularizes = late genes form a single transcription unit
27
What is the late genes' promoter called?
PR'
28
What gene encodes lambda repressorand what are its promoters?
cl gene PRM = promoter right maintenance PRE = promotor right establishment
29
What genes does lambda repressor block from being transcribed?
N and cro Lambda repressor acts at operators = blocking transcription of immediate early genes
30
How is transcription of cl stimulated?
Lambda repressor binding at operator R = stimulates transcription of cl from PRM
31
What is the structure of the lambda repressor?
Monomer has two domains NTD = DNA binding site CTD = dimerization
32
What is required for lambda repressor to bind DNA?
Must be a dimer
33
How is the lytic cycle induced?
Cleavage of the repressor between the two domains = reduces affinity to operators Inducing the lytic cycle Cleavage can occur because of UV or temperature
34
How does lambda repressor bind DNA?
Helix-turn-helix motif = arrangement of two alpha-helices that bind DNA
35
What is the role of the recognition helix?
Lambda repressor recognition helix (helix 3) = recognizes operator sequence in phage genome FIts into major groove
36
Describe the lambda operator sequence
17 bp sequence Axis of symmetry through central base pair
37
Phage immunity occur between phages with the same what?
Immunity region
38
What happens in virulent mutations?
Unable to establish lysogeny because mutation in operator but repressor cannot bind
39
What is serial dilution used for?
To count phage
40
What is a plaque?
Hole in a bacterial lawn
41
What are the different plaque morphologies seen?
Wild-type phages = cloudy plaques Because they contain some cells that have established lysogeny instead of being lysed Virulent mutants = clear plaques Because unable to establish lysogeny so the plaques only have lysed cells
42
What two regions overlap?
Operators overlap with promoters
43
How does the lambda repressors' autoregulatory circuit work?
DNA-binding region of repressor at OR2 contacts RNAp and stabilizes its binding to PRM = promotes cl synthesis Repressor blocks N and cro transcription = blocks entry into lytic cycle
44
Where is the DNA-binding region found in lambda repressor?
Helix 2
45
What does cooperative interaction increase?
Sensitivity of regulation
46
Cooperative ???
47
Why are cII and cIII genes needed to establish lysogeny?
These delayed early genes are need for RNAp to initiate transcription at PRE cIII = protects cII from degradation Transcription from PRE = synthesis of repressor and blocks cro synthesis
48
What is the role of cII protein?
RNAp only binds promoter in presence of cII
49
What is the role of cIII protein?
Protect cII from degredation
50
Why can transcription from PRE block cro synthesis?
...?
51
Where does cII act at?
PRE
52
What are the steps in lysogeny?
Immediate early genes N and cro are transcribed N protein antiterminates allowing delayed early genes to be transcribed = cII and cIII cII acts at PRE = cl is transcribed Establishment of repressor turns off early gene expression Repressor binds OR and OL cl is transcribed from PRM
53
What are the steps in teh lytic cycle?
N and cro are transcribed N antiterminates, allowing delayed early genes to be transcribed = cII and cIII cro binds O3-2-1 affinity Binding OR3 = prevents RNAp binding PRM Cro represses immediate early genes = indirectly blocking repressor establishment pQ activates late expression
54
What is the same between the lytic cycle and lysogeny?
Delayed early stage = both Cro and repressor are being expressed
55
What is deteremines which cycle the phage goes through?
Whether cII causes sufficient synthesis of repressor to overcome action of Cro

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