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Index >> Bacteriophages - Part One >> Single Stranded DNA Synthesis ( RF to SS )
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Single Stranded DNA Synthesis ( RF to SS )

Single Stranded DNA Synthesis ( RF to SS )- Gilbert and Dressler (1968) first suggested that ØX174 ssDNA synthesis is brought about by a rolling circle mechanism. They cited as evidence the fact that a label enters positive strands that are longer than the unit genome.


Dressler (1970) later showed that positive strands up to two unit. genomes in length were formed as intermediates. Only the positive strands took up radioactive labelling during ssDNA synthesis. The negative strands detected by an infectivity assay were closed circles.


The essential features of the rolling circle model are as follows :

(1) Replication starts with a specific single stranded break (nick) in the plus strand of the RP by a viral coded enzyme. The strand thus acquires a free 3' OH end and a 5' phosphate end.

(2) Synthesis starts with the addition of deoxynucleotides to the free 3'OH end.

(3) As synthesis proceeds, the open strand is rolled out as a free tail.

(4) The Dew plus strand is laid down by using the minus strand as a template.

(5) As the strand is rolled out, viral structural proteins bind to the elongating tail. The progeny ssDNA molecules are packaged into, phage particles immediately after synthesis.

(6) Nuclease cuts are made within the hairpin loops on the DNA, releasing the viral plus strand.

(7) Circularization of the DNA molecule takes place by the coming together of the cut ends to form a complete hairpin loop.

(8) The gap is finally closed by a ligase.


The essential difference in the rolling circle mechanism operating in the RF --> RF stage and the RF --> SS stage is that in the former case the 5' ended tails serve as templates for the synthesis of small DNA segments, which are eventually joined by DNA ligase to produce a dsDNA circle. In the RF--> SS stage no complementary strand synthesis takes place on the tail.
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