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An inhibitor of replicative DNA polymerases used to impose replication stress. Increased fragility after treatment supports replication-sensitive vulnerability without identifying every molecular intermediate.

A deoxynucleoside triphosphate is a nucleotide containing deoxyribose and three phosphate groups. DNA polymerases use these substrates to extend DNA; the four standard substrate types carry adenine, cytosine, guanine or thymine.

A replicating DNA polymerase is an enzyme that extends a paired strand end with nucleotides selected against a DNA template. New DNA grows $5^{\prime }\to 3^{\prime }$ while the template is read $3^{\prime }\to 5^{\prime }$. Base pairing specifies the partner; the DNA polymerase catalyzes its incorporation. See Alberts et al.: DNA polymerase.

DNA replication copies a double-stranded DNA molecule into two daughter double-stranded DNA molecules. In semiconservative replication, each daughter retains one parental strand and receives one newly synthesized complementary strand. DNA polymerases build the new strands from incoming nucleotides; they do not make RNA or proteins in this copying step. See Alberts et al., Molecular Biology of the Cell, 4th edition (2002): DNA replication mechanisms.

For a five-base pair example, separate $5^{\prime }\text{-}\mathrm{ATGCC}\text{-}{3}^{\prime }$ from its partner $3^{\prime }\text{-}\mathrm{TACGG}\text{-}{5}^{\prime }$. Base pairing specifies a new TACGG partner for the old ATGCC strand and a new ATGCC partner for the old TACGG strand. With error-free copying, both daughter DNA molecules have the original nucleotide sequence, but different physical strand histories. One initial duplex contains ten nucleotides; one complete round produces two duplexes containing twenty nucleotides, ten retained and ten newly incorporated.

This is useful for inheritance: both daughter biological cells can receive a copy of the DNA record. The elegant feature is that base pairing lets each strand specify its missing partner. Direction matters too: a replicating DNA polymerase reads its template $3^{\prime }\to 5^{\prime }$ and extends the new strand at its $3^{\prime }$ end, so synthesis proceeds $5^{\prime }\to 3^{\prime }$. These are opposite directions on antiparallel nucleic acid strands, not two alternative directions of synthesis. The enzyme needs a pre-existing paired strand end to extend; the diagram omits the machinery that starts and coordinates copying. See Alberts et al.: template-directed synthesis and the requirement for a primer.

The strand accounting predicts observable differences between copying mechanisms. In 1958, Meselson and Stahl grew Escherichia coli with the heavier nitrogen isotope ${}^{15}\mathrm{N}$, then supplied ${}^{14}\mathrm{N}$ and measured extracted DNA by density-gradient centrifugation. After one generation their samples contained intermediate-density DNA; after two, intermediate-density and light DNA occurred in equal amounts. This supported semiconservative replication in the tested Escherichia coli culture. It was an isotope assay of sampled DNA, not simultaneous testing of every kind of DNA or direct colored-strand photography. See The replication of DNA in Escherichia coli by Matthew Meselson and Franklin W. Stahl.

PCR converts a selected DNA sequence into a detectable and manipulable quantity; different downstream measurements give it different uses. A pair of PCR primers defines the interval amplified by DNA polymerase. Repeated denaturation, annealing and extension produce approximately $N_n=N_0(1+E{)}^n$ copies while PCR efficiency $E$ remains constant. At ideal efficiency, twenty cycles give about a millionfold amplification; depletion and accumulating products eventually cause a PCR plateau. Specificity derives from both primer-binding sites and their orientation, and should be checked by product size, melting behaviour or sequencing.

One application is genotyping. A disease-associated substitution can be detected by amplifying the surrounding region and reading the sequence, probing with an allele-specific oligonucleotide, or digesting a restriction site changed by the variant. An early demonstration amplified the human beta-globin region for restriction-based diagnosis of sickle cell disease. Amplification increased sensitivity; the restriction pattern supplied the allele information.^[ref] A negative disease-specific signal is interpretable only if a control locus amplifies, because poor extraction or an inhibitor can mimic absence of an allele. In forensic DNA profiling, several short tandem repeat loci are amplified and fragment lengths compared. A multilocus match supports identity statistically; it does not eliminate contamination, mixtures, allelic dropout or the need for appropriate population-frequency estimates.

For infectious agents, primers can detect a pathogen-specific DNA sequence. An RNA virus first requires reverse transcription, followed by polymerase chain reaction. In RT-qPCR, fluorescence is monitored during amplification, and a threshold cycle is related to starting RNA using calibrated standards and efficiency measurements. This also measures changes in cellular gene expression: compare a target transcript between conditions after suitable normalization. Include extraction controls, a no-template reaction and, for RNA measurements, a no-reverse-transcriptase control to detect genomic DNA contamination. A positive pathogen signal establishes the presence of target nucleic acid, not necessarily a viable organism or infectious virus. Likewise, RNA abundance is not a direct measurement of protein abundance or transcription rate, since RNA degradation also matters.

PCR is also a preparative method. Amplified coding sequences can be assembled into expression vectors; primer tails can add restriction sites, assembly overlaps, sequencing adaptors or an affinity tag. High-fidelity polymerase reduces unwanted substitutions, but the final clone should still be sequenced. Site-directed mutagenesis introduces a specified substitution using mutagenic primers, whereas error-prone PCR creates a library for directed evolution. Sequencing after selection distinguishes the intended change from other mutations. In chromatin immunoprecipitation, quantitative amplification of recovered loci can test enrichment of a DNA-associated protein, with input DNA and immunoprecipitation controls needed to interpret it.

These examples separate presence, identity, quantity and construction. Endpoint band intensity is a poor substitute for quantitative amplification measured before the PCR plateau; multiplex assays can also suffer unequal amplification. The appropriate answer therefore identifies what is amplified, what is measured, and which alternative explanation each control excludes.

The repetitive tract can cause DNA polymerase slippage during PCR amplification or the sequencing extension itself, generating stutter products. A heterozygous length variant creates overlapping Sanger sequencing traces downstream of the indel because the two templates lose register; this does not by itself indicate random sequencing failure. Tumor heterogeneity or contamination by normal tissue can add further mixed templates. Avoid creating or obscuring the mutation during amplification and avoid interpreting a mixed trace as one clean allele. Use a suitable high-fidelity DNA polymerase, independent amplifications and bidirectional reads, and resolve individual alleles by molecular cloning or an appropriate single-molecule/amplicon sequencing method if necessary. Check for non-specific products and allelic dropout caused by variation in a primer-binding site.

Replication fidelity uses successive discrimination and repair systems, because no single enzyme detects every error. The important distinctions are between an incorrect base pair, a slipped sequence register, the wrong sugar incorporated into DNA, a damaged template and an incompletely or incorrectly restored chromosome. Repair must also identify which information should be retained: recognizing a mismatch without strand discrimination could fix the wrong sequence.

A polymerase can select an incorrect deoxynucleoside triphosphate and insert the wrong base through imperfect discrimination, transient base configurations or an altered active site environment. If the mismatch is extended and survives into another replication round, it can become a stable base substitution. Many replicative polymerases possess a separate 3′→5′ exonuclease proofreading site. A mismatched primer end tends to slow extension and transfer toward this site; removal of the terminal nucleotide permits another attempt at correct synthesis. This is distinct from the polymerase's forward synthetic reaction and from proofreading in biological translation. In mice, selective disruption of DNA polymerase δ proofreading increased mutation and tumour susceptibility, demonstrating an organismal consequence of unrepaired copying errors.^[ref]

Post-replicative DNA mismatch repair corrects many errors that escape the polymerase. In eukaryotes, MutSα, formed by MSH2 and MSH6, recognizes base mismatches and many short insertion–deletion loops; MutSβ, containing MSH2 and MSH3, has an important role in larger loops. Recruitment of MutL complexes, strand incision, excision, resynthesis and ligation replaces a stretch of the newly synthesized strand. Nascent-strand discontinuities and replication-associated factors including PCNA help direct the process. Purified human activities reconstituted nick-directed repair from defined substrates, showing how a mismatch and a strand-break signal can support directional replacement.^[ref] The classic Escherichia coli MutH system instead uses transiently undermethylated daughter DNA at suitable sites; that methyl-directed mechanism is not a universal description of eukaryotic DNA mismatch repair or even of every bacterium.

replication slippage creates a different substrate. In repetitive sequence, transient misalignment can leave an extra loop in the nascent or template strand. A retained nascent-strand loop can produce an insertion; copying past a looped-out template segment can produce a deletion. Short repeats are therefore vulnerable to length changes, and coding-region insertions or deletions can cause a frameshift if their length is not a multiple of three. DNA mismatch repair recognizes many such loops, although substrate size and structure affect efficiency. Yeast mismatch-repair mutants destabilized simple repetitive tracts, linking repair defects with microsatellite instability.^[ref] This is a DNA-sequence frameshift mechanism, distinct from a ribosome changing reading frame during biological translation.

The layers can be represented by conditional escape probabilities,where the second and third factors refer to escape among errors reaching the corresponding stage. As a purely illustrative calculation, $10^{-5}\times 10^{-2}\times 10^{-2}=10^{-9}$ per copied base. These are not asserted universal measured rates: mismatch identity, sequence, polymerase and repair state all matter. The multiplicative logic explains why disabling a later layer can reveal errors that had previously escaped detection without eliminating DNA synthesis itself.

Polymerases can also incorporate a ribonucleotide instead of a deoxyribonucleotide. base pairing may be correct, so ordinary mismatch recognition is insufficient. RNase H2 can initiate ribonucleotide excision repair by incising near an embedded ribonucleotide, followed by DNA synthesis, removal of the displaced segment and ligation. Reconstitution established this dedicated pathway.^[ref] RNase H1 processing of an RNA–DNA hybrid should not be equated with removal of a single isolated ribonucleotide. Separately, normal RNA primers on Okazaki fragments must be processed and the remaining nicks sealed; persistent primers, flaps or unligated junctions threaten chromosome integrity even without an incorrect base.

A chemically damaged template can miscode or stall a fork. Small base lesions can be addressed by base excision repair, whereas nucleotide excision repair removes a damaged oligonucleotide around many bulky, helix-distorting lesions. These pathways repair chemical damage, not every mismatch made on an undamaged template. Replication across an oxidized base can require coordinated recognition of the lesion and the incorrectly paired partner so that repair does not convert damage into a fixed mutation. translesion synthesis permits passage across some lesions with specialized polymerases; it is damage tolerance, not necessarily lesion removal or error-free copying. template switching can provide another route around a block.

Stalled or collapsed forks can leave gaps, broken DNA and structural rearrangements. homologous recombination can use a sister chromatid to restore information and restart appropriate substrates; inappropriate pairing or template switching can instead change sequence organization. non-homologous end joining can restore continuity after a break but need not preserve every base, particularly when end processing is required. Replication checkpoints help stabilize forks and delay progression, while under-replication or rereplication requires control of origin use as well as lesion processing. Numerical chromosome-segregation errors are a related genome-stability problem but are not corrected by simply excising one mismatched base. Base selectivity and proofreading act during synthesis, mismatch and ribonucleotide repair address distinct residual substrates, and damage/fork-repair pathways restore templates and continuity with different fidelity limits.

aphidicolin increases the abnormal-locus fraction from roughly 40% to about 55–60% in all three lines. Inhibition of replicative DNA polymerases increases replication stress at an intrinsically difficult-to-complete region. Tandem repeats can complicate fork progression or chromosome organization. Slower or incomplete synthesis leaves underreplicated DNA, unresolved replication intermediates or abnormal condensation as biological cells approach mitosis; these can appear as split signals or gaps and can predispose to actual breakage.

This is an increase of approximately 15–20 percentage points, not proof of a 15–20-fold change. The two independent measurements per condition support a reproducible direction, but the graph does not provide enough information for a precise significance calculation or a quantitative model of fork failure. aphidicolin need not bind NVNA1 or the repeat sequence directly. Because these are uninfected biological cells, the result establishes susceptibility of the host locus to replication stress independently of viral protein expression.

The evidence supports NVNA1 as a sequence-specific organizer of viral chromatin, with possible additional roles in nuclear retention and episome partitioning. Binding to the origin of replication and DNA bridging explain assembly at separated viral repeat arrays. Access to recognition sites in nucleosomes and weakening of histone–DNA association provide a plausible way to make an origin accessible to cellular replication machinery. The density-labelling result is consistent with a latent genome using a host-coordinated copying programme; it does not establish that NVNA1 is itself a DNA polymerase or DNA helicase.

Association with host chromosomal DNA could also retain episomes in the nucleus and help distribute them during biological cell division. Such tethering is a plausible maintenance function, but the experiments here do not directly visualize a physical bridge between an episome and a host chromosome. Binding at a vulnerable host repeat could instead be an incidental consequence of recognition specificity. The enrichment of host chromosome rearrangements supports a damage connection; it does not show that chromosome breakage is required for viral replication or that integration is part of the normal latent programme. chromatin opening and host-locus interference are mechanistic interpretations of the presented binding data, not proof of one obligatory replication pathway.

PCR controls are scientific controls that separate different explanations for a readout. A no-template reaction contains the PCR primers and copying reagents but replaces the added template with water or sample buffer. Amplification there can indicate carried-over DNA or a primer dimer; a clean result does not exclude contamination confined to one sample.^[ref]

Different comparisons answer different questions. A known amplifiable template tests whether the PCR primers, DNA polymerase and thermal cycler can produce the expected amplicon. An extraction blank follows sample preparation without adding the sampled material. A separate amplifiable interval in the sample can test whether recovered DNA supports copying, although success there does not guarantee success at the target. A length standard calibrates gel electrophoresis; it is not a control for the copying reaction. These comparisons are an analytical design extension to the teaching example below, not a claim that its student protocol includes every comparison.

PCR denaturation is the high-temperature stage that separates a DNA duplex into single-stranded templates for the next copying step. It disrupts the pairing and stacking arrangement of nucleobases; it does not require cutting the phosphodiester bonds along either strand. The distinction matters: breaking the backbone would damage the ordered nucleotide sequence that is meant to serve as a template.

For an original eight-base pair duplex, complete PCR denaturation exposes two eight-nucleotide strands. It leaves sixteen nucleotides altogether: separation has made the templates accessible without yet making more DNA. PCR extension, rather than heating alone, adds the new material. Already copied duplexes also need to separate at the beginning of subsequent cycles.

Separation is a condition-dependent process. Template composition and the buffer solution affect the required temperature and duration; heating longer is not automatically better because prolonged heating can reduce DNA polymerase activity. Incomplete separation leaves fewer accessible templates even when the PCR primers themselves are well chosen.^[ref]

PCR extension is template-directed construction of DNA from an annealed PCR primer by a DNA polymerase. Incoming deoxynucleoside triphosphates supply the new nucleotides. Each addition lengthens the growing strand at its $3^{\prime }$ end, so the product grows $5^{\prime }\to 3^{\prime }$ while the template is read $3^{\prime }\to 5^{\prime }$. The direction refers to the nucleic acid backbone, not to left or right on a page.^[ref]

For a complete worked copying step, start with template $3^{\prime }\text{-}\mathrm{TCAGGGAC}\text{-}{5}^{\prime }$ and paired PCR primer $5^{\prime }\text{-}\mathrm{AGTC}\text{-}{3}^{\prime }$. The next four template letters are GGAC, encountered in the template's $3^{\prime }$-to-$5^{\prime }$ direction. The base pairing rule therefore selects CCTG in order, givingThe PCR primer supplies four nucleotides, and PCR extension adds four more; this copying event consumes four deoxynucleoside triphosphates. On the oppositely oriented template $5^{\prime }\text{-}\mathrm{AGTCCCTG}\text{-}{3}^{\prime }$, the other PCR primer is $5^{\prime }\text{-}\mathrm{CAGG}\text{-}{3}^{\prime }$. It pairs at the right and extends leftward. Reading that template from right to left encounters CTGA after the paired region, so addition of GACT gives $5^{\prime }\text{-}\mathrm{CAGGGACT}\text{-}{3}^{\prime }$. Both finished products are the reverse complements of their respective templates.

The direction argument is short: the PCR primer and its template are antiparallel nucleic acid strands; extension takes place at the PCR primer's $3^{\prime }$ end; hence forward progress along the new strand is backward progress along the template. Reflecting a drawing reverses its arrows on the page without changing either chemical direction. This is why a leftward arrow can represent $5^{\prime }\to 3^{\prime }$ synthesis.

The time needed depends on how much DNA remains to be copied and the chosen DNA polymerase, while temperature must support that enzyme's activity.^[ref] Separating PCR annealing and PCR extension into different holds is convenient, but some compatible conditions allow a combined hold. The three chemical jobs therefore need not always require three distinct temperatures.^[ref]

A PCR primer is a short DNA strand that supplies a paired $3^{\prime }$ end for a DNA polymerase. Its nucleotide sequence determines where base pairing can initiate copying. Matching a PCR primer at one site is insufficient: both PCR primers must support opposed extension across a copyable interval for sustained selective PCR amplification.

A PCR thermal cycle separates three jobs. During PCR denaturation, heating separates the paired DNA strands without intentionally cutting their phosphodiester bonds. During PCR annealing, cooling permits base pairing between PCR primers and templates. During PCR extension, the chosen interval supports DNA polymerase synthesis from the paired $3^{\prime }$ ends. Each new nucleotide is supplied as a deoxynucleoside triphosphate; its incorporation lengthens the strand. The DNA polymerase is a catalyst, while these nucleotides are consumed substrates.^[ref]

Taq polymerase withstands repeated high-temperature strand separation better than a heat-sensitive DNA polymerase. Its extension activity still depends on temperature, buffer solution and magnesium ions. Heat resistance does not mean constant activity at every stage: the separation temperature serves the DNA, whereas the extension temperature serves the copying enzyme. As one institutional example, Addgene's Basic PCR Program specifies $94^{\circ }\mathrm{C}$ separation, $55^{\circ }\mathrm{C}$ primer binding and $72^{\circ }\mathrm{C}$ extension. These are example settings, not universal properties of all PCR primers or DNA polymerases.^[ref]

For timing arithmetic alone, those stipulated holds give $20+25+45=90\mathrm{s}$ per PCR thermal cycle, and three cycles give $270\mathrm{s}$. Adding an assumed $90\mathrm{s}$ initial separation and $180\mathrm{s}$ final extension gives $540\mathrm{s}=9\min$ of holds. A real thermal cycler also takes time to move between temperatures. The schedule tells us when conditions favour PCR denaturation, PCR annealing or PCR extension; it does not measure the fraction of DNA copied.

An actual institutional protocol makes the distinction concrete. New England Biolabs specifies mixing templates and PCR primers with its OneTaq master mix in a thin-walled tube; a quick spin collects liquid at the bottom. Its $25\mu \mathrm{L}$ preparation uses $12.5\mu \mathrm{L}$ of the twice-concentrated mix and $0.5\mu \mathrm{L}$ of each $10\mu \mathrm{M}$ PCR primer stock, with water completing the volume. Tubes enter a thermal cycler preheated to $94^{\circ }\mathrm{C}$; mineral oil is specified if there is no heated lid. The program gives thirty cycles of $94^{\circ }\mathrm{C}$ separation, $45$–$68^{\circ }\mathrm{C}$ binding and $68^{\circ }\mathrm{C}$ extension, with extension time scaled to product length. The binding setting must be selected for the PCR primers; the range is not a continuously swept setting.^[ref]

The dilution can be checked independently: each PCR primer has final concentration $10\times 0.5/25=0.2\mu \mathrm{M}$, while the mix is diluted twofold. For an endpoint readout, Addgene specifies examining a product aliquot by gel electrophoresis.^[ref] Pair that size comparison with the existing PCR controls: a known template and a no-template tube test different explanations of success or failure. Those comparisons are an analytical addition, not controls documented in the OneTaq schedule. An expected-sized band supports production of a length-compatible amplicon; it does not establish its nucleotide sequence, ideal PCR efficiency or successful separation during every cycle. Neither protocol supplies a measured temperature or yield curve for the schematic above.

The polymerase chain reaction amplifies a selected DNA region using repeated copying steps. Amplified material can be examined by DNA sequencing; appropriate template controls help distinguish a new rearrangement from material already present in the starting preparation.

The essential trick is to choose two PCR primers on opposite DNA strands with their extendible $3^{\prime }$ ends facing into the same interval. A DNA polymerase extends each PCR primer $5^{\prime }\to 3^{\prime }$; the resulting DNA can itself become a template. The amplicon is therefore selected by two opposed PCR primers, rather than by the DNA polymerase recognizing a named gene. This makes PCR amplification useful for inspecting one interval within a much larger genome. Here PCR means polymerase chain reaction; the statistical abbreviation PCR means principal component regression.^[ref]

For a deliberately tiny nucleotide sequence example, display the upper strand as $5^{\prime }\text{-}\mathrm{ACGT}\text{-}\mathrm{GGAATT}\text{-}\mathrm{CAGC}\text{-}{3}^{\prime }$. The left PCR primer is $5^{\prime }\text{-}\mathrm{ACGT}\text{-}{3}^{\prime }$ and binds the lower strand. The right PCR primer, written in its own $5^{\prime }$-to-$3^{\prime }$ direction, is $5^{\prime }\text{-}\mathrm{GCTG}\text{-}{3}^{\prime }$: the reverse complement of CAGC. It binds the upper strand and extends leftward in the drawing. With flanks outside this interval, the bounded amplicon contains $4+6+4=14$ base pairs, including both PCR primer sequences. These four-nucleotide PCR primers illustrate direction and counting only; they are not a proposed experimental design.

A useful way to understand the polymerase chain reaction is to separate recognition from construction. Base pairing positions a PCR primer; a DNA polymerase then joins new nucleotides onto that existing strand. The PCR primer becomes part of the copied DNA, rather than merely marking a place for an independent strand to start. A PCR thermal cycle repeatedly exposes templates, positions starting ends and permits construction. This makes DNA copying possible outside biological cells, with the chosen PCR primers specifying which interval is repeatedly accessible.

The 1993 Nobel chemistry announcement credits Kary Mullis with inventing the polymerase chain reaction method and states that the technique was first presented in 1985. The reusable-product principle explains its power: each successfully copied interval can help specify the next generation. The award attribution is a historical claim; it does not imply that every improvement in DNA polymerases, thermal cyclers or PCR controls came from one person.^[ref]

An RNA polymerase used in transcription is an enzyme that synthesizes RNA from a DNA template. Unlike a replicating DNA polymerase, it can start a new strand without a pre-existing paired strand end; initiation still requires the appropriate DNA signals and associated machinery. See Alberts et al.: RNA polymerase and transcription initiation.

Consider an original teaching template whose next eight positions, encountered after the sequencing primer, areThe sequencing primer binds an upstream region omitted from this display; the template also continues beyond the eight positions shown. DNA polymerase encounters the displayed template from left to right. Applying base pairing at each successive position gives the extensionThese eight letters are added material, not the entire primer-containing DNA molecule.

Here is the full endpoint accounting. A terminal letter denotes an incorporated terminating nucleotide; all preceding letters of that extension were supplied by ordinary deoxynucleoside triphosphates.

The shortest fragments migrate farthest, placing position 1 at the bottom. Reading upward, the lane labels spell A, C, G, T, T, A, G, C: the new strand in its $5^{\prime }$-to-$3^{\prime }$ direction. The adjacent T bands at positions 4 and 5 encode two separate additions. A single unresolved T patch would leave that count uncertain. In the A lane, the position-6 fragment passed position 1 using ordinary dATP and stopped at position 6 using ddATP; it does not skip the intervening nucleotides.

The common sequencing primer is an offset. For primer length $p$, endpoint $n$ has length $p+n$ nucleotides. With an illustrative $p=20$, the drawn fragments would contain 21 through 28 nucleotides, and adjacent endpoints still differ by one. Subtracting the common $p$ preserves their order because $(p+n+1)-(p+n)=1$. This supplies a short justification for Sanger ladder reading: nested fragments share a start, length orders their endpoints, and lane membership supplies each endpoint's letter. A missing early band cannot be relabeled position 1 without independent evidence about the starting point.

To report the original template region, first complement the readout in aligned orientation:If the requested answer is that template written $5^{\prime }$ to $3^{\prime }$, reverse the aligned template to obtain $5^{\prime }\text{-}\mathrm{GCTAACGT}\text{-}{3}^{\prime }$, the reverse complement of the readout. Reversing the readout alone would give CGATTGCA and would be wrong. Complementing without reversing gives the correct template letters in its displayed $3^{\prime }$-to-$5^{\prime }$ direction, but not in the usual reporting convention. Specify the strand and its direction before transforming the answer.

In 1977, Frederick Sanger, S. Nicklen and Alan Coulson tested chain termination on Phi X 174 DNA. They isolated restriction enzyme fragments as primers, heated them with template to $100^{\circ }\mathrm{C}$ for three minutes and annealed at $67^{\circ }\mathrm{C}$ for thirty minutes. Four extension mixtures used the Klenow fragment of DNA polymerase and radioactive dATP. Their Figure 1 used ddGTP, ddATP, ddTTP and araCTP, an alternative terminating substrate; Figure 2 used all four dideoxy types. This distinction matters when relating the original experiments to an all-ddNTP teaching model.^[ref]

Figure 1 used a 12% acrylamide gel at $40\mathrm{mA}$ for fourteen hours, read by autoradiography. Omitting the unlabeled dATP chase caused unwanted A stops across samples: a chemistry scientific control. Ordered bands supported local nucleotide sequences; comparisons could revise provisional assignments. Weak T bands, unresolved long runs and structure-dependent migration limited interpretation. The paper's observations are evidence from particular preparations, not a guarantee that every endpoint is resolved.^[ref]

Sanger sequencing determines a DNA nucleotide sequence by extending a common sequencing primer and occasionally incorporating a dideoxynucleoside triphosphate. A normal deoxynucleoside triphosphate already has hydrogen in place of a $2^{\prime }$ hydroxy group; it retains the $3^{\prime }$ hydroxy group. A dideoxynucleoside triphosphate lacks that $3^{\prime }$ hydroxy group too. It can join the growing strand because the preceding nucleotide supplies the reacting $3^{\prime }$ hydroxy group. Once incorporated, its own $3^{\prime }$ end cannot form the next phosphodiester bond. Thus the blocked step is the next addition, not attachment of the terminator itself.^[ref]

In the historical four-reaction format, each tube receives copies of the same template, the same sequencing primer, a DNA polymerase and all four ordinary deoxynucleoside triphosphates. The tubes differ in their terminating substrate: ddATP, ddCTP, ddGTP or ddTTP. In the A reaction, ordinary dATP can allow copying to continue past one A position, whereas incorporation of ddATP stops another copy there. Competition produces a family of endpoints, rather than every strand stopping at the first A. Adjusting substrate proportions makes later endpoints detectable. A lane identifies the terminating nucleobase in the new strand, not the complementary template letter.^[ref]

The readout requires gel electrophoresis capable of separating single-stranded DNA fragments differing by one nucleotide. Historical denaturing polyacrylamide gel electrophoresis suppressed strand folding and resolved these adjacent lengths; radioactive labels were detected by autoradiography. An ordinary agarose gel electrophoresis comparison with a 100-base pair DNA ladder does not supply that resolution. Four separate reaction lanes carry the identities in this format.^[ref]