Semiconservative DNA Replication

Why one old strand remains in each daughter duplex

Each DNA strand can specify a complementary sequence. At replication, base-pair contacts separate and polymerases synthesize a new strand opposite each template. Every daughter duplex therefore contains one parental strand and one newly synthesized strand. This is called semiconservative replication. It differs from a conservative model, which keeps the original duplex together, and a dispersive model, which mixes old and new material through segments of both daughter strands.

Meselson and Stahl distinguish the models

Meselson and Stahl grew E. coli for many generations with heavy nitrogen-15 so both strands became dense. They then transferred cells to nitrogen-14 medium and separated extracted DNA in a cesium-chloride density gradient. After one generation all detectable duplex DNA had intermediate density, as expected when each duplex paired one 15N old strand with one 14N new strand. After two generations about half the molecules were intermediate and half light: each hybrid duplex produces one hybrid and one light duplex.

In the ideal model the fraction of hybrid duplexes after n complete generations in light medium is 2^(1−n) for n≥1; the rest are light. Thus after four generations hybrid = 1/8 and light = 7/8. Nitrogen-15 is a stable heavy isotope, not a radioactive tracer. Taylor and colleagues found complementary chromosome-level evidence using labelled thymidine in plant cells.

Replication begins at defined regions

Replication starts at origins where initiation proteins recruit machinery that opens a small DNA region. Helicases separate parental strands, single-strand binding proteins stabilise them, and topoisomerases reduce excess twisting ahead of forks. Short RNA primers made by primase provide a free 3′-OH, which DNA polymerase requires to add deoxyribonucleotides. A replication fork is the Y-shaped moving boundary at which parental strands are exposed and copied. Often two forks move away from an origin in opposite directions.

Why polymerases build only 5′→3′

DNA polymerases select a complementary deoxyribonucleoside triphosphate, add it to the growing strand's 3′-OH and release pyrophosphate; the triphosphate provides the chemical driving force. The new strand therefore grows 5′→3′, while the template is read in the opposite 3′→5′ direction. On the template oriented appropriately toward fork movement, leading-strand synthesis is continuous. On the opposite parental template, lagging-strand synthesis occurs away from the fork as successive Okazaki fragments.

Each lagging fragment starts from a new RNA primer. After elongation, RNA primers are removed, gaps are filled with DNA and DNA ligase seals remaining backbone nicks. The ligase does not create complementary bases or open the helix; its role is to join adjacent DNA pieces into a continuous phosphodiester backbone. DNA polymerase proofreading and other repair systems reduce errors, though occasional mutations remain possible.

Coordinating replication with the cell cycle

In eukaryotes nuclear DNA is replicated during S phase. Replication origins and checkpoint mechanisms help ensure chromosomes are duplicated once before division. If DNA replication happens without appropriate cell division, cellular DNA content can increase and polyploid cells may result. Replication is energetically demanding and requires precise coordination of many enzymes, not a lone polymerase copying an entire chromosome without assistance.

Worked rates and predictions

A bacterial genome of 4.6 × 10⁶ bp copied over 18 minutes corresponds to 4.6×10⁶/1080 ≈ 4,259 bp per second if described as a total chromosome-wide rate. Two simultaneous forks distribute much of this work; the rate of one individual polymerase is not the same as total genome completion divided by time. For the density experiment, a culture beginning with one fully heavy duplex and growing three generations in light medium generates eight duplexes: two remain hybrid and six are light. This illustrates why experimental predictions must identify what is counted.

Taylor's chromosome evidence and isotope logic

A second line of evidence for semiconservative replication came from work on chromosomes of dividing plants. Taylor, Woods and Hughes used radioactive thymidine in dividing root-tip cells of Vicia faba. Autoradiography located the incorporated radioactivity in chromatids through successive division cycles. The resulting patterns were consistent with the distribution of parental and newly synthesised DNA expected during semiconservative copying. This evidence operated at a chromosome level, whereas Meselson and Stahl separated isolated bacterial DNA by buoyant density. The two methods did not measure exactly the same physical property, but their patterns reinforced the same model.

Interpret the heavy-nitrogen results one physical strand at a time. Start with a duplex containing two 15N-labelled parental strands. After one round in 14N medium, each old strand is partnered with a new light strand, producing two hybrid duplexes. A second round gives two hybrid duplexes—one derived from each original heavy strand—and two fully light duplexes. After three rounds, only two of the eight duplexes are hybrid because no further heavy parental strands are created. This is a counting argument about DNA molecules, not the proportion of heavy nitrogen atoms inside every descendant cell.

Following both sides of one replication fork

Because the two parental strands are antiparallel, their templates cannot both be read continuously in the same spatial direction by enzymes that synthesize only 5′→3′. At a fork moving to the right, the top template could run 3′ at left to 5′ at right, permitting a new leading strand to extend toward the advancing fork. The opposing template would run 5′ at left to 3′ at right, forcing each lagging-strand fragment to be synthesised away from the fork. New primers are laid closer to the fork as additional parental DNA is exposed. Once a fragment reaches the previous fragment, primer processing and ligation join them.

DNA polymerase does not normally initiate on a bare DNA template without a primer. Primase supplies a short RNA segment with a usable 3′-OH; the polymerase then extends from it using complementary deoxyribonucleoside triphosphates. Helicase opens the helix, topoisomerases relieve excess twisting and single-strand binding proteins stabilise separated templates. Copying accuracy arises through base selection, polymerase proofreading and subsequent repair. Each enzyme addresses a distinct physical problem, which is why a replication fork cannot be described as a single machine simply moving along DNA.

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