Early Replication in Lambda Phage, LetterTheta, Cyclic Copies, Cell Membrane, Genome Length, Mature Phage

Early Replication in Lambda Phage

	Early Replication in Lambda Phage - During early replication the cyclic DNA molecule associates with the host cell membrane and replicates symmetrically, generating cyclic copies. Replication has a unique origin site (orf) located within gene 0, at 81.7% of the lambda genome length' (measured from the left terminus of the mature phage DNA).
The initiation of replication requires the functions of viral genes 0 and P and host functions. The products of these genes may be involved in nicking the closed circle at the origin. Replication is divergent and bidirectional (it may be occasionally unidirectional), proceeding in opposite directions from the point of origin. In P2, another temperate phage, replication is always unidirectional. A replicating intermediate of lambda DNA engaged in early replication resemble the Greek 'letter theta' (9) and is called the 0, structure. This structure has two branch points which represent the replication forks.
The replication fork is the point at which the non-replicated parental duplex joins the two daughter chromosomes. The distance between the two branch points is always the same on each branch. This suggests that each branch represents a newly replicated daughter, strand. If the two branches are called a and the third segment, then a-l-b will always equal unit length, i.e the characteristic length of the lambda DNA molecule (17 µm).
The branches a and a' represent' the replicated segments, while b is the segment which has yet to undergo replication. Theta structures, have also been reported in E. coli replication (Cairns, 1963). This suggests that the lambda phage and E coli have similar replicating mechanisms. Replication terminates where the two forks meet.

Index >> Bacteriophages - Part Two >> Early Replication in Lambda Phage

Early Replication in Lambda Phage