Synthesis and Linking of New DNA Strands

DNA synthesis is bidirectional. Two replication forks advance in opposite directions from an origin of replication. 

The direction of DNA synthesis is from 5 end to the 3 end of the newly formed strand. One strand (the leading strand) is formed continuously, while the other strand (the lagging strand) is formed discontinuously. On the lagging strand, small fragments of DNA (Okazaki fragments) are subsequendy linked. 

Five DNA polymerases have been found in E. coU. Polymersase III is primarily responsible for the synthesis of new strands. The first polymerase enzyme discovered, polymerase I, is involved in synthesis, proofreading, and repair. Polymerase II, IV, and V function as repair enzymes under unique conditions. 

DNA gyrase introduces a swivel point in advance of the movement of the replication fork. A helix-destabilizing protein, a helicase, binds at the replication fork and promotes unwinding. The exposed single-stranded regions of the template DNA are stabilized by a DNA-binding protein. 

The synthesis of new strands is catalyzed by Pol III. The primer is removed by Pol I, which also replaces the primer with deoxynucleotides. DNA ligase seals the remaining nicks. 

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