DNA Replication Quiz: Two Strands, Two Different Stories

Semiconservative replication means each daughter molecule contains one original parental strand and one newly synthesized strand. The Meselson-Stahl experiment in 1958 confirmed this by growing bacteria in heavy nitrogen-15 medium and then transferring them to light nitrogen-14 medium. After one replication round, all DNA showed intermediate density. After two rounds, both intermediate and light density molecules appeared, exactly matching the prediction of semiconservative replication.

DNA polymerase catalyzes the addition of deoxyribonucleotide triphosphates exclusively to the 3-prime hydroxyl end of the growing strand, extending the chain in the 5-prime to 3-prime direction. This directional constraint arises from the chemistry of the polymerization reaction, in which the 3-prime hydroxyl of the terminal nucleotide attacks the alpha phosphate of the incoming deoxyribonucleotide triphosphate, releasing pyrophosphate and forming a new phosphodiester bond.

DNA replication requires RNA primers from primase to provide the 3-prime OH that DNA polymerase cannot generate de novo. Helicase unwinds the double helix to expose single-stranded templates. Single-strand binding proteins coat and stabilize the exposed templates. DNA polymerase cannot initiate synthesis on a fully base-paired double-stranded template and specifically requires a single-stranded template with an existing 3-prime hydroxyl end to extend.

The directionality constraint of DNA polymerase means both strands must be synthesized 5-prime to 3-prime, but the two template strands run antiparallel. The leading strand template runs 3-prime to 5-prime toward the fork, allowing continuous synthesis in the direction of fork movement. The lagging strand template runs 5-prime to 3-prime away from the fork, forcing synthesis in the opposite direction as short discontinuous Okazaki fragments that are later joined together.

Okazaki fragments are short DNA segments of approximately 100 to 200 nucleotides in eukaryotes synthesized discontinuously on the lagging strand template. Each fragment begins with a short RNA primer laid down by primase. After DNA polymerase extends the fragment, the RNA primer of the adjacent upstream fragment is removed by the 5-prime exonuclease activity of DNA polymerase I in bacteria or RNase H and FEN1 in eukaryotes. DNA polymerase fills the resulting gap, and DNA ligase seals the final nick to produce a continuous strand.

DNA polymerases require a pre-existing strand with a free 3-prime hydroxyl group to begin synthesis because they can only catalyze phosphodiester bond formation by extending an existing chain. They cannot join two free nucleotides without a template-paired nucleotide already in place. Primase, an RNA polymerase, can initiate new strand synthesis de novo without a primer, producing short RNA oligonucleotides of approximately 10 nucleotides that provide the 3-prime hydroxyl end DNA polymerase needs to begin extension.

DNA helicase is a ring-shaped motor protein that translocates along the DNA and catalyzes the unwinding of the double helix by disrupting hydrogen bonds between base pairs ahead of the replication fork. This reaction consumes ATP and progressively exposes single-stranded template regions for both the leading and lagging strand polymerases. Helicase activity creates the positive supercoiling tension ahead of the fork that topoisomerase must relieve to prevent stalling of the replication machinery.

Topoisomerase, DNA ligase, and the sliding clamp all perform essential functions at the replication fork. Topoisomerase cuts and rejoins DNA to relieve helical tension. DNA ligase creates phosphodiester bonds between adjacent Okazaki fragments after gap filling. The sliding clamp dramatically increases polymerase processivity. RNA polymerase does not synthesize the leading strand. Primase synthesizes short RNA primers and DNA polymerase extends these with deoxyribonucleotides to build the new DNA strands.

DNA polymerase possesses an intrinsic 3-prime to 5-prime exonuclease proofreading activity that functions in real time during synthesis. When a mismatched nucleotide is incorporated, the distorted base pair is detected, the misincorporated nucleotide is excised by the exonuclease activity, and the correct nucleotide is then incorporated before synthesis continues. This co-replicational proofreading reduces the error rate from approximately one in one hundred thousand to approximately one in ten million base pairs added.

Each time a linear chromosome replicates, the RNA primer at the very 5-prime end of the lagging strand cannot be replaced with DNA because no upstream strand exists to prime gap filling. This leaves a short unreplicated gap, causing progressive chromosome shortening. Telomerase, a ribonucleoprotein enzyme, uses its internal RNA component as a template to extend the 3-prime single-stranded overhang at chromosome ends, providing additional template for lagging strand synthesis and maintaining chromosome length in dividing cells.

Eukaryotic genomes are far too large to replicate from a single origin within the time available for S phase. Human cells contain approximately 30,000 or more replication origins distributed throughout the genome. Replication initiates simultaneously or in a coordinated temporal program from multiple origins, creating multiple bidirectional replication bubbles that expand and eventually merge. This multi-origin strategy allows the human genome of approximately 6 billion base pairs to be replicated in hours rather than the weeks a single origin would require.

DNA ligase catalyzes the formation of a phosphodiester bond between the 3-prime hydroxyl terminus of one DNA segment and the 5-prime phosphate terminus of an adjacent segment within a nicked double-stranded DNA molecule. This reaction requires energy from ATP hydrolysis in eukaryotes and from NAD hydrolysis in bacteria. DNA ligase joins Okazaki fragments during replication and seals gaps left after nucleotide excision repair, base excision repair, and other DNA repair pathways.

The trombone model of replication fork coordination proposes that the lagging strand template forms a loop allowing both the leading and lagging strand polymerases to move in the same overall direction relative to the replisome. The lagging strand polymerase periodically releases its template upon completing an Okazaki fragment, the loop collapses, and a new loop forms at the next primer to begin the next fragment. This elegant mechanism coordinates both polymerases within the same replisome complex moving processively in one direction.

Multiple replication origins allow eukaryotes to replicate large genomes efficiently. Nuclear compartmentalization means eukaryotic replication is spatially separated from translation. Histone chaperones deposit histones onto newly replicated DNA to restore chromatin structure. Prokaryotes achieve high replication accuracy through proofreading and mismatch repair, but eukaryotes also achieve very high accuracy through equivalent mechanisms. Neither is dramatically more accurate than the other, and the statement that prokaryotes eliminate all errors is incorrect.

Eukaryotic replication initiation begins with binding of the origin recognition complex to licensed origins, followed by recruitment of Cdc6 and Cdt1 which load the MCM helicase complex during G1. In S phase, cyclin-dependent kinases and Dbf4-dependent kinase activate helicases at licensed origins in a regulated temporal program. Early-firing origins are typically located in euchromatin while late-firing origins are in heterochromatin, coordinating replication with the chromatin landscape and ensuring each origin fires only once per cell cycle.