DNA Replication and Genome Maintenance
Replication as a Coordinated Process
Semiconservative Replication
DNA replication is semiconservative: each daughter molecule contains one parental strand and one newly synthesized strand. Replication begins at origins, proceeds bidirectionally, and requires helicases, primases, DNA polymerases, sliding clamps, and ligases.
Key Mechanism
DNA polymerases extend a new strand only in the direction, so the lagging strand is synthesized discontinuously as Okazaki fragments.
Fidelity and Proofreading
High fidelity arises from base-pair selectivity, proofreading exonuclease activity, and mismatch repair. The combined error rate is much lower than the raw misincorporation rate of polymerase alone.
Core Replication Components
| Component | Function |
|---|---|
| Helicase | Unwinds the duplex |
| Primase | Synthesizes RNA primers |
| DNA polymerase | Extends the new DNA strand |
| Ligase | Seals nicks between fragments |
| Topoisomerase | Relieves torsional stress |
Why must lagging-strand synthesis occur in fragments?
Antiparallel strands force one strand to be made discontinuously.
Correct answer: Because DNA polymerase can only synthesize in the direction
What is the purpose of DNA polymerase proofreading?
Proofreading reduces mutation frequency by correcting mistakes during synthesis.
Correct answer: To remove incorrectly incorporated nucleotides and improve replication fidelity.
Replication Workflow
-
1
Step 1: Origin recognition proteins mark the start site.
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2
Step 2: Helicase unwinds DNA and single-strand binding proteins stabilize exposed strands.
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3
Step 3: Primase lays down RNA primers.
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4
Step 4: DNA polymerase synthesizes leading and lagging strands.
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5
Step 5: Primers are removed, gaps filled, and ligase seals the backbone.
Which enzyme relieves supercoiling ahead of the replication fork?
Topoisomerases cut and reseal DNA to prevent excessive torsional strain.
Correct answer: Topoisomerase
Name one major source of replication errors that cells must correct.
Mismatch repair and proofreading help correct these errors before they become permanent mutations.
Correct answer: Mismatched base incorporation by DNA polymerase.