Microbiology Procedure

Index >> Bacteriophages - Part One >> Reciprocating Strand Model

Reciprocating Strand Model

	Reciprocating Strand Model - The essential features of this model are as follows. Synthesis of a new viral strand is initiated at the origin in the region of gene A. Priming is brought about either by an RNA primer or by the to protein which properly places a nucleotide. After the synthesis of a short sequence of DNA, there is accumulation of positive superhelical twists.
The strain imposed by these twists is releived by nicking of the parental duplex. This is thought to be brought by the gene protein, together with other cellular proteins involved in the initiation of DNA synthesis. The 5' end of the newly synthesized viral strand is joined to the parental viral strand.
Unidirectional and continuous synthesis of the viral strand takes place in a clockwise direction around the genome. The synthesis of the complementary st rand is discontinuous, and lags behind the synthesis of the viral strand.
The synthesis of the fragments of the complementary strand is primed by RNA. Repeated exchange reactions between the nascent viral strand and the parental strand (reciprocation) are necessary to permit the unwinding of the parental strands.
	At the completion of viral strand synthesis, the two parental strands are completely unwound. A strand exchange now releases the circular but incomplete duplexes. The viral strand has a single gap of variable size. The other strand has many gaps separating segments of complementary strand DNA.

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