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Replication of λ DNA

Lambda (λ) phage DNA replicates through two primary mechanisms: Circle-to-Circle (θ) replication and Rolling-Circle replication, with the latter producing concatemers essential for packaging. The process involves key λ genes O and P, along with host proteins DnaB and DnaG, and is regulated by the Gam protein, which inhibits RecBC nuclease to facilitate concatemer formation. Cos sites play a crucial role in cutting DNA into genome-length pieces for packaging into phage heads.

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Ritesh Chowdhury
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116 views2 pages

Replication of λ DNA

Lambda (λ) phage DNA replicates through two primary mechanisms: Circle-to-Circle (θ) replication and Rolling-Circle replication, with the latter producing concatemers essential for packaging. The process involves key λ genes O and P, along with host proteins DnaB and DnaG, and is regulated by the Gam protein, which inhibits RecBC nuclease to facilitate concatemer formation. Cos sites play a crucial role in cutting DNA into genome-length pieces for packaging into phage heads.

Uploaded by

Ritesh Chowdhury
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Replication of λ DNA

Lambda (λ) phage DNA serves as a model system for understanding DNA replication. The λ
genome is linear within the phage head but circularizes upon entry into a host due to
complementary 12-base single-stranded cohesive (cos) ends, forming covalently closed
circular DNA. DNA ligase joins these ends, enabling full genome replication since there is
always DNA upstream to act as a primer.

I. Circle-to-Circle (θ) Replication

 Mechanism: Similar to θ replication in bacterial chromosomes and plasmids.


 Initiation:
o Begins at the ori site in gene O.
o Requires λ gene products O and P, and host proteins DnaB and DnaG.
o The P protein recruits DnaB helicase by binding to O, functioning
analogously to DnaC (hence, "P for pirate").
 Process:
o Replication proceeds bidirectionally with leading- and lagging-strand
synthesis.
o RNA synthesis, often beginning at the PR promoter, may aid in origin
unwinding and priming.
o Replication forks meet on the opposite side of the circle, and daughter DNA
molecules separate.

II. Rolling-Circle Replication

 Trigger: After several rounds of θ replication, λ switches to rolling-circle replication.


 Mechanism:
o A strand of circular λ DNA is nicked, and the 3′ end acts as a primer.
o DNA synthesis displaces the 5′ end while creating a long single-stranded
DNA.
o The displaced strand is then converted into double-stranded DNA.
 Concatemers:
o Rolling-circle replication continues beyond a single genome length, producing
long DNA strands composed of tandem repeats of λ DNA called
concatemers.
o These are processed into genome-length units for packaging.

Regulation and Transition

 RecBC Nuclease:
o Promotes degradation of linear DNA; normally blocks rolling-circle
replication.
o Gam protein (from λ genome) inhibits RecBC, enabling rolling-circle
replication.
 Importance:
o λ DNA must form concatemers for successful packaging, as each phage head
requires at least two genome lengths linked by cos sites.
o This packaging mechanism necessitated the discovery of cos sites and their
role in DNA processing and recombination.

Genetic Requirements

 λ O and P genes:
o Crucial for initiating replication.
o O binds the origin (ori), while P recruits host replication machinery (e.g.,
DnaB).
 Host Proteins:
o DnaB and DnaG participate in helicase and primase activities, respectively.
 RecBC and Gam:
o RecBC degrades DNA during linear replication; Gam counters this,
facilitating concatemer formation for packaging.

Packaging and Cos Sites

 Cos Sites:
o Required for packaging concatemers into phage heads.
o DNA is cut at cos sites into λ-genome-length pieces.
 Phase 2 of Packaging:
o Begins when a cos site is recognized; packaging continues until the next cos
site is reached.

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