In others specic termination sequences block further

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Unformatted text preview: scale synthesis DNA ligase links fragments together •Discontinuous synthesis results in the production of Okazaki fragments •These must be connected (“ligated”) at the end of replication ligase 4. Termination • In some DNA molecules, replication is terminated whenever two replication forks meet. • In others, specific termination sequences block further replication. • A termination protein (called Tus in E. coli) binds to these sequences and blocks the movement of helicase. • Tus stalls the replication fork and prevents further DNA replication. Fidelity of DNA replication • Overall, the error rate in replication is less than one mistake per billion nucleotides • Three processes are required to achieve this level of fidelity: 1. Errors in nucleotide selection by DNA polymerase arise only about once per 100,000 nucleotides 3 or 1 Fidelity of DNA replication 2. Most of the errors that do arise are fixed by proofreading • DNA polymerase inserts an incorrect nucleotide into the growing strand • The 3’-OH of the mispaired nucleotide is not correctly positioned in the active site of the DNA polymerase Fidelity of DNA replication • This incorrect position stalls the polymerization reaction • The 3’-to-5’ exonuclease activity of DNA polymerase removes the incorrectly paired nucleotide. • DNA polymerase then inserts the correct nucleotide. • Together, proofreading and nucleotide selection result in an error rate of one in 10 million nucleotides. Fidelity of DNA replication 3. Mismatch repair corrects errors after replication is complete. • Incorrectly paired nucleotides are recognized and excised • The original nucleotide strand is used as a template to replace the incorrect nucleotide. • More on this when we discuss DNA repair For Monday: Read Chapter 12...
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This document was uploaded on 02/25/2014.

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