Lecture 5 - Phage

Question 1

what is a phage

Answer 1

virus that kills bacteria

Question 2

3 steps to phage attacks

Answer 2

attachment, injection and cell lysis

Question 3

biological significance of phage

Answer 3

drive bacterial evolution by horizontal gene transfer, basis for molecular biology discoveries applications in phage therapy

Question 4

describe the typical structure of a phage

Answer 4

capsid head, neck, tail, tail tip (touches the bacteria)

Question 5

how to classify phage by genome type

Answer 5

dsDNA< ssDNA, dsRNA, ssRNA

Question 6

how to classify phage by morphology

Answer 6

tailed, filamentous, icosahedral, enveloped

Question 7

are phage genome circular or linear

Answer 7

both: linear when outside (packaging) and circular when inside (replication)

Question 8

Describe the genome form transition from linear to circular

Answer 8

injected into host as linear dsDNA, contains cos sites at the ends. cohesive ends areal and ligate to form circular DNA. circularisation stabilised the genome.

Question 9

how are cohesive ends processed during assembly of phage DNA

Answer 9

• overhangs are complementary so they can ligate easily (sticky ends) to form circular DNA
• terminase cleaves COS site to generate double strand break and sticky ends → linear DNA
• ligase can ligate → circular DNA
• terminase is at the neck region (crucial for DNA packaging) and is then transported into the head after packaged. (circular → linear)

Question 10

what is integrated prophage

Answer 10

inserted into bacterial chromosome
- passed onto next generation and coexists with bacteria
- requires DNA recombinase
- bacteria is phage carrier

Question 11

what is the lytic and lysogenic life cycles of phage and how is it represented in plaque formation

Answer 11

lytic - replicates in host and kills lysis, forms plaque
lyosgenic - coexists by inserting itself in chromosome as prophage - no plaques

Question 12

what are the 2 fate mechanism deciding phage lifecycle

Answer 12

arbitrium and CI-CII switch

Question 13

describe the bacterial quorum sensing mechanism

Answer 13

regulation of gene expression in response to fluctuations in cell population density

Question 14

how does quorum sensing allow bacteria to evade phage

Answer 14

senses AHL secreted from other cells and reduce the phage receptor LamBb expression to prevent phage invading

Question 15

examples of bacterial defence systems

Answer 15

RM - restriction modification
crispr
abortive infection
TA systems

Question 16

2 principles of bacterial defence system

Answer 16

attack the phage or kill itself

Question 17

ways of bacterial defence vs phage choices to block bacterial defence

Answer 17

CRISPR - anti crispr
neoprotein causing cell death - self quarantine capsule