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SCAN0016 - Yr“ Model of replication elongation[madman um...

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Unformatted text preview: Yr“ Model of replication elongation [madman um um alumina snarl 5. quRE 1242:5clremtic model othe relationship between 5:07! replicarion proteins are growing fork. (1] A singie DnaB helicase moves along the laggingstrand nemplate toward its 3' endrlhereby MW ——"\ (2) One can pulymrase (core I) quickly adds nucleotides to the 3’ end ofthe leading smn as I Sing tmdéd template is uncovered by (he helicase action afDnaaThEs leadingstrand pnlymerasamgelherwnh its firsuhunn clamp, remains bound to the DNA, synthesizing the leading strand continuously. (3) A second core {solymerase [core 2) synthesizes the lagging strand discominmuslyas an Okazakl fragment {see Figure l2v9b).Tfie two can: polymerase molecules. are linked via a dimericr protein £4) As each segment ofrhe singlestrandecl templabe fur the tagging strand is uncovered, It becomes coated with 553 protein and terms a lung Once synthesis ofan Okazaki fragment is completed. the lagging—strand polymerase dissociates from the DNA burthe core mums hound m the r-subunir dimerx‘l'he re- leased core polymerase subaequemiy rebinds with the assistance of another [:1 clamp in me raglan ofme primer for the next Dkazaki fragment. See the text for additional details LAdapred from A. Kornberg, 1988,}.3101Ifi2m. 263:1;5. Kim at {$1996. Call 84543] The primase—to-polymerase switch during lagging strand synthesis. (A) DnaB helicase encircles the lagging strand and primase has synthesized a primer The holoenzyme consists of a dimer of tau that binds two polymerase cores, one gamma complex clamp loader, and two beta clamps, Tau and primase interact with DnaB. Primasc must contact SSB to remain on the RNA primer. 03) The chi subunit of gamma complex interacts with SSB, severing the primase-SSB contact and resulting in primase displacement. (C) Primase is then free to synthesize another RNA primer upon contact with DnaB. Diagram (B) also shows that the lagging strand polymerase releases the beta clamp and DNA upon finishing an Okazaki fragment. Diagram (C) shows that afier gamma complex assembles the new beta clamp on the upstream primer, the lagging polymerase recruits the new beta clamp (shaded dark) assembled on the upstream RNA primer for the next Okazaki fragment. ...
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