Heteroduplex cleavage

Cleavage at a, a and rejoining forms two parallel helices, each containing a heteroduplex region... 

Mashal R, Koontz J, Sklar J, Detection of mutations by cleavage of DNA heteroduplexes with bacteriophage resolvases. Nat Genet 1995 9(2) 177-183. 

The most widely used genetic screening technique, PCR-RFLP detects a mutation at a specific restriction endonuclease cleavage site at the mutation locus [9], The products from other techniques such as ARMS (amplification refractory mutation system), SSCP (single-strand conformational polymorphism), HPA (heteroduplex polymorphism), and CDCE (constant denaturant capillary electrophoresis) and PCR are usually separable using polymer networks. 

Homologous recombination in E. coli is a two-step process that requires the RecA protein. In step 1, a single-stranded DNA invades the target DNA molecule and base pairs with its homologous sequence. In step 2, the heteroduplex formed in step 1 is isomerized to form a Holliday junction which is resolved by subsequent DNA cleavage and ligation reactions. 

As indicated above, R. is a result of crossing over. In the accepted model of the crossed-over intermediate, the nicked strands of two aligned homologous DNA duplexes cross over and pair with their complementary counterparts, followed by sealing of the nicks. Since models of the resulting 4-stranded, crossed-over structure of the heteroduplex (the Holliday structure) show that all the bases are paired without steric strain, the Holliday structure is apparently stable. Two possible mechanisms for resolution of the Holliday structure, i.e. for cleavage of the DNA chains followed by formation of two DNA duplexes without loss of continuity, are shown in the Fig. 

This section is devoted to three representative endoribonucleases (a) bovine pancreatic RNase (RNase A) as the prime model of RNases, (b) RNase H as the model of a special class of RNases that catalyze specific cleavages of the RNA strand in RNA-DNA heteroduplexes, and (c) a guanine-specific nuclease, RNase Tl. 

The reduced rate of cleavage by Hindlll on the DNA RNA heteroduplex compared with that on the DNA homoduplex has been utilized in the Okayama-Berg vector—primer cloning method (21). 

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