Growth at the Replication Forks Is Discontinuous

Continuous synthesis on both strands of the replication fork would require synthesis in the 5 — 3 direction on one strand and in the 3 — 5 direction on the other because of the antiparallel nature of the DNA duplex (fig. 26.5a). Continuous synthesis on both strands seems unlikely since the only known enzymes that catalyze DNA synthesis add bases 

Schematic diagrams of two different modes of DNA synthesis at the growth fork(s). In unidirectional replication (a) one growth fork occurs in bidirectional replication (b) two occur. Red indicates regions containing newly synthesized DNA. 

A closer examination of the Okazaki fragments led to detection of short stretches of ribonucleotides at the 5 ends. From this and many other observations on different systems it was determined that a new DNA chain can be initiated only by attaching the first deoxynucleotide through its 5 -phosphate to the 3 -OH of a short RNA polynucleotide. An RNA strand that functions in this capacity is called a primer. Primers are formed at points along the chromosome they base-pair with the single-stranded template DNA in the regions where they are formed. 

Continuous synthesis of leading strand 5 Discontinuous synthesis of lagging strand 

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Growth during Replication Is Bidirectional Growth at the Replication Forks Is Discontinuous Proteins Involved in DNA Replication Characterization of DNA Polymerase I in Vitro Crystallography Combined with Genetics to Produce a Detailed Picture of DNA Poll Function... 

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