bacteriophage life cycle by Prof. Kunal Upadhyay Rajkot India

1. Kunal H. Upadhyay
Lecturer in Microbiology
Gyanyagna College of Science &
Management
Dt. 07/02/2009
Lecture for HNGU, Patan

2.  Infectious agents
 Need electron microscope
 No nucleus, organelles, cytoplasm –
acellular
 Not harmed by antibiotics
 Invade susceptible host cell & take over
 Obligate intracellular parasites
 Uses host cell for replication
 Host specificity

3.  Nucleic acid core
 DNA or RNA – single or double stranded; linear,
circular, segmented
 Capsid
 Protein outer coat
 Covers N.A. & protects; determines shape of
virus;
 Envelope (no envelope – naked)
 Bilayer membrane outside capsid
 Acquired when BUDDING from host(nuclear or
plasma membrane)
 May have SPIKES (glycoproteins) - aid in
attachment
 Keeps virus hidden from host defenses

4.  Largest virus = smallest bacterium
 Most viruses – specific shape
 Helical – ribbon-like protein in spiral
around nuclear core
 Icosahedral – most common
polyhedral shape; 20 triangular faces
 Bacteriophages –
 Viruses that infect bacteria
 More complex shape – heads, tails, tail
fibers – aids in attachment

5.  Host Range – spectrum of hosts a virus
can infect; all living cells
 Viruses are limited to one host type, one
cell type, or tissue type
 EX. Poliovirus – can grow in monkey
kidney cells; infects ONLY HUMANS
 Rabies – attacks CNS of many warm
blooded animals
 Viral specificity - particular kind of cell
virus infects

6.  1. Attachment – determined by receptor
sites on host cell & attachment structures
on virus’ capsid or envelope
 2. Appropriate host enzymes & proteins in
host cell available to virus
 3. Can replicated virus be released from
host cell to spread infection

7.  Type & structure of N.A.
 Method of replication
 Host range
 Chemical & physical characteristics
 Family – highest taxonomic level
 English name , not binomial name
 Virus families based on:
 N.A. type
 Capsid shape
 Envelope
 Size

8. But, most phages have
DNA genomes

9.  One step growth experiment reveals events
during a single infectious cycle
 Adsorption
 Latent period (includes eclipse)
 Rise
 Burst size: the number of particles released per
infected cell

11.  Adsorption
 DNA injection
 Synthesis of early mRNA
 Degradation of host DNA
 Synthesis of phage DNA
 Synthesis of late mRNA
 Assembly
 Host cell lysis and release of ~300 virions
Involves holin and endolysin

12.  3 steps procedure:
 Sheath Contraction
 Unplugging of Core Tube
 Injection of DNA

13.  Sheath is made up of 144 subunits of gp
19.
 They are arranged in 24 layers & each
contains 6 subunits.
 Sheath contraction is ATP dependent
process & sheath will be contracted to
12 layers with 12 subunits in each.
 So length will be half while width will be
doubled.

14.  Gp e is attached on tip of tail fibers
 It has lysozyme activity
 It helps in drilling in membrane

15.  Circular DNA will get converted to linear
during injection
 Diameter of linear DNA is 18nm while
diameter of core tube is 20 nm
 Then DNA will pass through channel
created by gp e & inserted in Host.
 After entering in host it will undergo
circularization

16.  Adsorption
 DNA injection
Energy independent These steps require energy

17. Early Late
Gene Gene
s s
Immediate Delayed
Quasi Late True Late
Early Early

18.  Immediate Early genes : 0 to 3.75mins
 Delayed Early genes : 3.75 to 5mins
 Quasi Late genes : 5 to 11mins
 True Late genes : 11 to release
 Early genes are expressed in
counterclockwise direction
 Late genes are expressed in clockwise
direction

19.  Adsorption
 DNA injection
 Synthesis of early mRNA
Transcribed using host RNA polymerase, new
proteins include modifiers of transcriptional
specificity, especially, a new sigma factor (s), that
recognizes only phage promoters
Other early genes encode proteins required to
take over host cell and synthesize viral nucleic
acids

20.  Adsorption
 DNA injection
 Synthesis of early mRNA
 Degradation of host DNA
Phage DNA is protected from degradation by
incorporation of hydroxymethyl cytosine (HMC),
and glucosylation. Phage DNA is protected from
degradation by viral nucleases and host restriction
enzymes.

21.  169 kb double stranded, linear molecule
(genome 166 kb pairs)
 Encodes 144 genes
 Terminally redundant - 3 kb pairs (2% at
left end repeated at the right end)
 Circularly permuted (circular DNA cut at
different points yields linear DNA, different
permutations)
 Replication forms concatemers
 Tandemly linked genome copies

24. Assembly of T4 Phage

26.  Requires activity of rII & S genes.
 rII gene triggers the lysis of host at 22nd
minute of infection
 S gene is responsible for delay in lysis
until maturation process gets
completed.
 gp e is also involved in lysis as it is
lysozyme in nature

28. 2. Entry-nucleic acid is
inserted into host cell 3. Replication-viral
components are made
4. Assembly-new viruses
are assembled
1. Attachment: virus
connects to host cell
5. Release-host cell membranes are
destroyed by viral enzymes. New viruses are
released and free to destroy other cells.

30.  No genes specific for host cell disruption
 ФX174 has overlapping genes (coding region of
one overlaps another)
Replicative form (RF) is double stranded circle
 fd is filamentous phage that does not kill host
RF is double stranded circle

32.  Small (<4 kb) genome, 3-4 genes
 Positive sense ssRNA genome encodes replicase
to copy RNA (RNA-dependent RNA
polymerase)
 Regulation of gene expression by RNA
secondary structure; alterations permit
ribosome binding

34.  Sigma factor cascade (T4)
 Gene order/slow injection (T7)
 Access to translational start sites (RNA phages)
 Antisense RNA (l, P1)
 Antiterminators (l)
 Frameshift allows expression of 2nd protein
(FX174)

35.  Temperate phages can convert host to lysogen
 Establishment likely with high level of infection
of bacterial culture, starvation
 Stable association of phage DNA with bacterial
cell (prophage)
 Normal growth and division of lysogen
 Environmental cues may induce lytic cycle

36. A) Attachment-virus connects to host
cell
B) Injection-viral nucleic acid is
inserted into host cell and is
incorporated into the host cell’s
DNA as a Prophage. It can remain
dormant for days, months, or even
years.
C) Host cells replicate both the host
cells DNA and the Prophage.
D) The “new” host cells continue to
survive.

37. A) Attachment- B) Injection-viral C) Host cells
virus connects to nucleic acid is inserted replicates both the D) The “new”
host cell into host cell and is host cells DNA and host cells
the Prophage. continue to
incorporated into the
host cell’s DNA as a survive.
Prophage. It can remain
dormant for days,
months, or even years.

39.  Double stranded DNA genome of 49 kb
 Linear DNA has cohesive ends, circularizes
 Early gene expression from leftward and
rightward promoters determines dominance of
cI or cro
 If cI dominates, lysogeny
 If cro wins, lytic cycle

43. cro
cI

44.  Block all  Expression of late
transcription except genes
for cI gene  DNA replication
 Integration of lDNA
into host
chromosome

46.  cI repressor cleaved as a result of DNA damage
 Excision of prophage DNA from chromosome
 Initiation of lytic cycle

48. Radiation or chemicals can cause
the lysogenic cycle to change to
the lytic cycle.