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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
• 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
• 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.
- Fall '14