2. Lambda phage, also known as
bacteriophage .
λ phage, is a type of virus that
infects and replicates within
bacterial cells, particularly
Escherichia coli (E. coli) bacteria.
3. The head is icosahedral shape.
The phage λ contains double
stranded DNA of about 17 µm
in length packed in protein
head of capsid. The head is 55
nm in diameter consisting of
300-600 capsomers (subunits)
of 37,500 Daltons. The
capsomers are arranged in
clusters of 5 and 6 subunits
i.e. pentamers and hexamers.
4. Lambda phage has a
unique lifecycle that
can follow two main
pathways: the lytic
cycle and the
lysogenic cycle.
The choice between
these two pathways
depends on the
environmental
conditions and the
state of the bacterial
host cell.
5.
6. Attachment: Lambda phage first attaches to
specific receptors on the surface of an E. coli
cell.
Penetration: The phage injects its genetic
material (double-stranded DNA) into the
bacterial cell.
Replication and Transcription: Once inside the
host cell, the phage DNA circularizes and
begins to replicate. It also directs the host cell's
machinery to transcribe and translate its genes.
Assembly: New phage particles (virions) are
assembled within the bacterial cell.
7. Attachment and Penetration: Similar to the lytic
cycle, lambda phage attaches to and injects its
DNA into the host bacterial cell.
Integration: Instead of immediately initiating the
lytic cycle, lambda phage integrates its DNA into
the bacterial chromosome. This integrated phage
DNA is called a prophage.
Replication: The integrated prophage replicates
along with the host cell's chromosome during cell
division.
Maintenance: The prophage remains integrated in
the bacterial chromosome without causing cell
lysis. The host cell can continue to grow and divide
normally.
8. Lysis: As the phage particles accumulate, an
enzyme called endolysin is produced, which
breaks down the bacterial cell wall, causing the
cell to lyse (burst). This releases the newly
formed phage particles, which can go on to
infect other bacterial cells.
Induction (Genetic switch): Under certain
conditions, such as exposure to DNA-
damaging agents or stress, the prophage can be
excised from the bacterial chromosome and
enter the lytic cycle.
9. The choice between the
lytic and lysogenic cycles is
controlled by a regulatory
switch in the lambda phage
genome. The decision is
influenced by
environmental factors, such
as the availability of
nutrients and the presence
of DNA-damaging agents.
Lambda phage can switch
between these two cycles to
adapt to its host's
conditions.
