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GENE221 > Lecture 16 > Flashcards

Lecture 16 Flashcards

1
Q

What is a virus?

A

An infectious agent that must grow/reproduce inside a host cell

  • > considered obligate intracellular parasites
  • > composed of nucleic acid genetic material and protein coat
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2
Q

What is a bacteriophage?

A

A virus that infects bacterial cells

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3
Q

Describe the shape of Phage Lambda

A
  • A double stranded DNA genome

- Long flexible tail and icosahedral head

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4
Q

What is a temperate phage?

A

It can choose between lytic or lysogenic pathway

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5
Q

What does the lifecycle decision of the temperate phage depend on?

A

The state of the host bacterium

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6
Q

Define lytic

A

The lifecycle resulting in bacterial cell lysis upon release of progeny phage

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7
Q

Define lysogenic

A

The lifecycle resulting in stable carriage of the phage (prophage) within the host cell (lysogen)

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8
Q

What is a virulent phage?

A

A phage that’s only able to undergo replication via the lytic cycle

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9
Q

What is a lysogen?

A

A host cell harbouring a prophage during lysogeny

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10
Q

What is a prophage?

A

Latent form of a temperate phage that remains within the lysogen (i.e is integrated into the host chromosome)

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11
Q

Describe the genome in the phage head

A

Linear with staggered ends which give a ssDNA region of complementarity allowing DNA to stick together

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12
Q

Following insertion what happens to the lambda genome?

A

The genome circularises using cohesive end sites (cos sites)

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13
Q

Give the overview for the Lytic cycle

A

(Phage assembly and release)

  1. Head and tail proteins synthesised
  2. DNA packed into heads
  3. Tails added
  4. Host is lysed releasing the new phage
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14
Q

Give the overview for the Lysogenic cycle

A

(phage integration and maintenance)

  1. Lambda integrates
  2. Prophage is stably maintained
  3. Prophage passed to daughter cells
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15
Q

Which of the lifecycles is considered default?

A

The lytic pathway

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16
Q

How does integration occur (lysogeny) ?

A
  • Occurs between two sites
    1. attP (phage)
    2. attB (bacteria)
  • Site specific recombination requiring the enzyme integrase, (rather than using general homology it uses these sites)
17
Q

Is the expression pathway shared up until the decision point?

A

Yes, the same patterns for very early and early gene expression are shared and following the decision -> gene expression patterns differ

18
Q

Describe the Very Early Gene expression events

A

Following injection, the host RNA polymerase transcribes from 2 promotors

  1. PL = produces N
  2. PR = produces Cro
19
Q

What is Cro?

A
  • Cro encodes the Cro protein
  • Expressed early in infection and is a major player in establishing lytic growth
  • Is a dominant protein
20
Q

What does N encode?

A
  • An anti-terminator protein that enable transcription past 2 terminators resulting in expression
  • > enables RNA pol to ignore transcription termination signals
  • Binds to nut site (N utilisation sites)
21
Q

What is the effect of Cro?

A

Is a DNA-binding protein that REPRESSES transcription

->promotes the lytic pathway by switching transcription off at certain times

22
Q

What is the effect of CII?

A

Is a DNA-binding protein that ACTIVATES transcription

->promotes the lysogenic cycle

23
Q

What is the effect of CI?

A

A DNA binding protein that can ACTIVATE OR REPRESS transcription

  • > CI activates it’s own expression
  • > represses genes required for lytic cycle
  • > maintains lysogeny
24
Q

How is the decision of lytic/lysogenic pathway made?

A

Through relative concentrations of CII and Cro

25
Q

In the ‘decision’, what is the deciding protein?

A

CII

26
Q

What would happen for the lytic pathway to be chosen?

A

A healthy cell produces lots of proteases which degrade CII, once this occurs the LYTIC pathway takes place

27
Q

Describe late lytic events

A
  • Cro = represses expression of CI, all other genes, then itself and the replication genes
  • Q = an anti-terminator that enables expression of late lytic genes (head, tail, lysis genes)
28
Q

What would happen for the lysogenic pathway to be chosen?

A

Starved cells produce less proteases therefore CII remains intact and stable

29
Q

Describe late lysogenic events

A

CII is an activator protein that turns on:

  1. Intergrase resulting in lambda integration into host
  2. CI repressor which activates its own expression and binds to OL and OR to repress all other phage genes
    - >maintains prophage state
30
Q

Describe maintenance of lysogeny

A
  • Maintained as a prophage by CI
  • CI is a repressor of all phage genes but is an activator of itself so keeps the phage genome ‘silent’ in bacterial chromosome until induction
  • > keeps fitness cots low for E.Coli cell
  • CII activates integrase and CI, which cause integration of phage and maintenance of lysogeny (respectively)
31
Q

What is PRE?

A
  • Promotor for repressor establishment

- Establishes expression of CI and ensures the phage is silent

GENE221 (29 decks)

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