Circular dsDNA

The energy source for the process is provided by cleavage of ATP. DNA gyrase untwists relaxed circular dsDNA one turn at a time and reseals the cut ends. A reverse gyrase from certain bacteria twists the relaxed DNA more tightly. In both cases the change causes the DNA to form superhelical turns.67193-197 These may be either solenoidal or plectonemically interwound (as a twisted thread Fig. 5-18). 

DNA polymerase activity and 3 —> 5 direction exonuclease activity. Because there are 2 strands to be replicated the 2 strands of the dsDNA have to unwind and in a circular dsDNA bidirectional replication results in a replication bubble bounded by two Y-shaped replication forks that move around the circle. The continuous or leading strand is made unbroken around its ssDNA template. However the other (antiparallel) strand (the lagging strand), proceeding from the same starting point as the dsDNA opens up, is made with the same 5 3 ... 

Note that the insert and plasmid need to be specially modified to link together. "Sticky ends" must first be attached to the insert and the plasmid. The sticky ends attached to the linear insert consist of stretches of deoxycy tidy lie acid (CCCCCCC), The sticky ends attached to the linear plasmid consist of stretches of deoxyguanidylie acid (GGGGGGGG) The sticky ends consisting of "CCCCCCC" can hybridize with those consisting of "GGGGGGGG." The result is a closed, circular dsDNA. The sticky ends are attached to the insert and plasmid by terminal transferase. Terminal transferase catalyzes the attachment of nucleohdes to one end of a linear DNA molecule. 

Note that plasmids exist as circular dsDNA when they occur in bacteria, that is, when they are replicated and transcribed in viw. When plasmids arc extracted from bacteria they also occur as drcular dsDNA, but they can easily be converted to linear dsDNA in a test tube for the purpose of receiving inserts. Special enzymes called restriction enzymes recognize specific and rarely occurring sequences of base pairs and catalyze the hydrolysis of the dsDNA at a specific point within or near the sequence. In this way, a plasmid consisting of several thousand base pairs can be cut at one site only and converted to a single piece of linear dsDNA,... 

Among the retroviruses " are types B and C oncoviruses which induce malignant tumors in mammals and birds and the human immunodeficiency virus (HIV), the apparent causative agent of AIDS. Their RNA functions in a surprising way. Each virion contains a reverse transcriptase, an enzyme that transcribes copies of circular dsDNA copies from the one or two mRNA-like molecules that make up the virus genome. Following action of the reverse transcriptase, one of the transcribed DNA circles becomes covalently spliced into the host s own cellular DNA. There it remains permanently as a provirus. RNA molecules transcribed from the provirus serve as mRNA for virus-encoded proteins and also as the genomes for new virus particles. 

Human papillomaviruses (HPV) comprise a genus in the species Papovaviridae, which consist of small, naked icosahedral viruses containing circular dsDNA. HPV types 16 and 18 infect squamous epithelium of the genitalia and may infect as many as 45% of the female population in developed countries (1). 

One of the most intriguing, but well-established, properties of poly(ADP-ribose) synthetase is its requirement for DNA [20-24]. As reported by Benjamin and Gill [23] and Yoshihara s group ([21] for review see [24]), the enzyme shows preference for DNA ends . Thus, closed circular dsDNA (ColEi or pBR322) shows no activating potential on the enzyme. However, this ability increases proportionally with the number of nicks introduced by DNAase I treatment [21]. 

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