DNA replication is the fundamental biological process where a cell duplicates its entire genetic material, ensuring that daughter cells receive a complete set of instructions. The mechanism used by all known life forms is called semiconservative replication. In this model, the original double helix separates, and each original strand serves as a template for the synthesis of a new, complementary strand.
The outcome is two new DNA helices, each consisting of one inherited, parental strand and one newly constructed strand. This structural arrangement is foundational to maintaining the accuracy of genetic inheritance, preventing the accumulation of errors that lead to mutations.
The Essential Template Strand
The architecture of semiconservative replication provides the conceptual basis for all subsequent error correction processes. When DNA polymerase synthesizes the new strand, it reads the sequence of the parental template strand. This original strand is considered the correct, verified blueprint for the genetic code.
If a mismatch occurs, the cell is faced with a dilemma of which base to fix. Because the original template strand is physically distinct and chemically older than the newly synthesized strand, the cell’s repair machinery operates on the assumption that the base on the template strand is correct. The correct base pair is preserved on the template strand, while the incorrect base is identified on the newly formed strand.
This critical structural separation ensures that any error made during synthesis can be corrected against the authoritative, undamaged sequence. Without the semiconservative nature, a mismatch would exist between two strands of equal status, making it impossible for the repair machinery to know which base to excise. The existence of the template strand establishes a clear reference point that prevents the repair system from accidentally excising the correct base and turning an error into a permanent mutation.
Proofreading During DNA Synthesis
The first and most immediate defense against errors is an intrinsic property of the enzyme responsible for synthesis, DNA polymerase. This enzyme is an active editor, possessing a built-in error-checking function known as 3′ to 5′ exonuclease activity. This proofreading mechanism operates simultaneously with the synthesis process.
When DNA polymerase incorrectly incorporates a non-complementary nucleotide, the mismatched base pair causes a slight distortion in the double helix structure. This distortion signals the enzyme to pause its forward synthesis and engage its exonuclease domain. The 3′ to 5′ exonuclease activity allows the polymerase to reverse its direction by one nucleotide and cleave the phosphodiester bond that holds the incorrect base to the growing strand.
The enzyme removes the mispaired nucleotide from the 3′ end of the new strand, and then gives the polymerase a second chance to insert the correct, complementary base. This integrated proofreading capability significantly enhances the fidelity of replication, reducing the initial error rate by a factor of about 100- to 1,000-fold.
Post-Replication Mismatch Repair
For the rare errors that slip past the polymerase’s proofreading function, a secondary, highly specific system called mismatch repair (MMR) is deployed after replication is complete. The MMR system scans the newly synthesized DNA for structural deformities caused by remaining base-pair mismatches. This system is entirely dependent on the semiconservative structure to perform its function accurately.
The main challenge for mismatch repair proteins is to reliably distinguish the newly synthesized strand, which contains the error, from the template strand, which contains the correct sequence. In prokaryotes like E. coli, this distinction is made by a process called transient hemimethylation. The parental strand is chemically tagged with methyl groups on certain adenine bases, while the newly synthesized strand remains temporarily unmethylated.
In human cells and other eukaryotes, the newly synthesized strand is identified by the transient presence of nicks, or single-strand breaks, in the sugar-phosphate backbone. Once the mismatch is identified, the MMR system excises a segment of the new, incorrect strand. DNA polymerase fills the gap using the template strand as the guide.