7.2 DNA Replication Process

DNA replication is a crucial process in molecular genetics, ensuring genetic information is accurately copied and passed on. This complex mechanism involves various enzymes working together to unwind, copy, and proofread DNA strands.

The replication process follows a semiconservative model, where each original DNA strand serves as a template for a new complementary strand. This occurs at the replication fork, with leading and lagging strands synthesized differently to maintain accuracy and efficiency.

DNA Replication Enzymes

Essential Enzymes for DNA Replication

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• DNA helicase unwinds and separates the double-stranded DNA molecule into two single strands by breaking the hydrogen bonds between complementary base pairs
• DNA polymerase synthesizes new DNA strands by adding nucleotides to the growing chain in the 5' to 3' direction, using the existing strands as templates
• Primase synthesizes short RNA primers (8-12 nucleotides long) that provide a starting point for DNA synthesis by DNA polymerase
• DNA ligase joins the Okazaki fragments on the lagging strand to create a continuous strand of DNA by forming a phosphodiester bond between the 3' end of one fragment and the 5' end of the adjacent fragment

Proofreading and Error Correction

• DNA polymerase has a proofreading function that checks for errors during DNA replication and removes incorrectly paired nucleotides
• The proofreading activity of DNA polymerase ensures high fidelity of DNA replication, with an error rate of approximately one mistake per billion base pairs
• Mismatch repair enzymes, such as MutS and MutL in E. coli, recognize and correct errors that escape the proofreading function of DNA polymerase

Replication Process

Semiconservative Replication and Replication Fork

• Semiconservative replication is a model of DNA replication in which the double-stranded DNA molecule separates, and each original strand serves as a template for the synthesis of a new complementary strand
• The result of semiconservative replication is two identical DNA molecules, each containing one original strand and one newly synthesized strand
• The replication fork is the Y-shaped structure formed when the double-stranded DNA molecule is unwound and separated by DNA helicase, exposing the single-stranded templates for DNA synthesis
• DNA replication occurs in both directions from the origin of replication, a specific sequence where the replication process begins

Leading and Lagging Strands

• The leading strand is the strand of DNA that is synthesized continuously in the 5' to 3' direction by DNA polymerase, following the movement of the replication fork
• The lagging strand is the strand of DNA that is synthesized discontinuously in short fragments (Okazaki fragments) in the 5' to 3' direction, opposite to the movement of the replication fork
• Okazaki fragments are short segments of DNA (1,000-2,000 nucleotides in eukaryotes and 100-200 nucleotides in prokaryotes) synthesized on the lagging strand by DNA polymerase
• The synthesis of Okazaki fragments on the lagging strand requires the repeated action of primase to create RNA primers for each fragment, which are later removed and replaced with DNA by DNA polymerase