#DNA sequencing

Sequencing

The Sanger sequencing result reads DNA from:

RNA sequencing analysis begins with mapping reads to a reference genome using splice-aware aligners that handle exon-intron boundaries to identify novel transcripts. This computational step reveals transcript structure, exon connectivity, and novel junctions indicating alternative splicing or gene fusion events absent from annotation databases. Reverse transcription creates complementary DNA copies, while library preparation adds adapters for flow cell binding. Sequencing depth determines sensitivity for low-abundance transcripts, but identification of structural novelty depends critically on alignment patterns. Accurate alignment therefore enables discovery of isoform diversity across conditions.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Which sequencing method avoids use of fluorescence?

Among common sequencing platforms, Ion Torrent uniquely performs label-free, optics-free detection by sensing proton release during polymerization. When nucleotide is incorporated, hydrogen ion acidifies microwell, detected by ion-sensitive field-effect transistor, eliminating costly lasers, filters and fluorescently labeled nucleotides entirely. Sanger sequencing traditionally uses fluorescent dyed terminators or radioactivity, Illumina uses four-color reversible terminators requiring optical scanning, while pyrosequencing uses luciferase bioluminescence but still optical detection. Therefore Ion Torrent is correctly cited as method that completely avoids fluorescence dependence, substantially reducing instrument size and reagent cost.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

The sequencing method with base-specific chemical cleavage is:

Maxam-Gilbert sequencing, also called chemical sequencing, identifies sequence not by polymerase extension but by base-specific chemical modification and cleavage of existing DNA. End-labeled DNA is divided into reactions treated with reagents preferential for G, A+G, C+T, C alone. For example, dimethyl sulfate methylates G, formic acid depurinates A and G, hydrazine attacks pyrimidines. Piperidine then cleaves backbone at modified sites, electrophoresis resolves resulting fragments. Pyrosequencing, Illumina SBS and Ion Torrent all synthesize DNA; only Maxam-Gilbert directly cleaves DNA at specific bases chemically.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

In NGS, what is the principle of sequencing-by-synthesis?

Sequencing-by-synthesis is the core principle of Illumina next-generation sequencing. Library fragments immobilized on flow cell are amplified into clusters, then polymerase synthesizes complementary strand stepwise. Each cycle adds fluorescently labeled reversible terminator dNTPs that block further extension until imaging is complete. After color detection identifying base, terminator and fluorophore are chemically cleaved to allow next incorporation. This iterative labeled nucleotide addition contrasts with ligation-based SOLID, hybridization microarrays relying on probe annealing, or pH change detection in Ion Torrent. Hence labeled nucleotide addition defines SBS.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Ion Torrent sequencing detects:

Ion Torrent semiconductor sequencing, introduced by Life Technologies, determines sequence without optics. Each microwell contains template-coated beads and a polymerase. When a deoxynucleotide complementary to template is incorporated, a covalent phosphodiester bond forms and a proton, hydrogen ion, is released as byproduct. This release transiently acidifies microwell solution, typically by 0.02 pH units, detected by an ion-sensitive field-effect transistor beneath well. Signal amplitude reflects homopolymer length. Detection principle is therefore pH change, not light, electrical charge of DNA itself or fluorescence employed in other platforms.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Which sequencing method produces light after nucleotide incorporation?

Pyrosequencing is a non-electrophoretic sequencing-by-synthesis platform that couples nucleotide incorporation to visible light production. Each cycle dispenses a single dNTP; if it is complementary to next template base, DNA polymerase incorporates it and releases inorganic pyrophosphate, PPi, stoichiometrically. ATP sulfurylase converts PPi to ATP, which drives firefly luciferase to oxidize luciferin emitting photons. Light intensity indicates incorporation and number of same bases in homopolymer runs. Sanger uses termination fluorescence, Ion Torrent measures pH change, and Maxam-Gilbert relies on chemical cleavage without light.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Which base is modified by formic acid in Maxam-Gilbert sequencing?

Maxam-Gilbert chemistry employs formic acid as a depurinating agent. Formic acid weakens the N-glycosidic bond of purines by protonation, leading to loss of adenine and guanine bases and formation of apurinic sites. Subsequent treatment with piperidine cleaves the phosphodiester backbone at those abasic positions, generating fragments ending at purines. Hence formic acid produces an A plus G specific ladder, whereas dimethyl sulfate is tuned for G alone. Pyrimidines C and T are not targeted by formic acid; hydrazine targets them instead.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Sanger sequencing usually reads sequences up to:

Traditional Sanger sequencing resolution was limited by denaturing polyacrylamide gel electrophoresis that separates fragments differing by one nucleotide. Under optimal conditions with long gels or capillary electrophoresis, reliable read length reaches approximately 800 to 1000 base pairs before band broadening, signal decay and polymerase dissociation reduce accuracy. Hence routine Sanger reads are considered about 1000 bp, substantially longer than 100 bp but far shorter than 10000 or 100000 bp. Those very long reads belong to third-generation platforms such as PacBio or nanopore, which exceed Sanger capabilities.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Which of the following is NOT required in a Sanger sequencing reaction?

A standard Sanger reaction requires a single-stranded DNA template, a complementary oligonucleotide primer providing a free 3' hydroxyl, DNA polymerase, all four deoxyribonucleotide triphosphates, small amounts of dideoxynucleotide triphosphates, and magnesium-containing buffer. The polymerase extends the primer synthesizing DNA complementary to the template. RNA template is not required because the technique directly sequences DNA; if RNA were the analyte, it must first be converted to cDNA by reverse transcriptase. Therefore DNA polymerase, ddNTPs and primer are indispensable, whereas RNA template is unrelated to core Sanger chemistry.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

What is missing in ddNTPs that causes chain termination?

Dideoxynucleotides are analogs of deoxynucleotides that lack hydroxyl groups at both the 2' and 3' positions of the ribose sugar. Deoxynucleotides lack only the 2' OH, retaining 3' OH essential for chain elongation. DNA polymerase catalyzes nucleophilic attack of the primer 3' OH on the alpha-phosphate of incoming nucleotide. When a ddNTP is incorporated, no 3' OH remains to attack the next nucleotide, so phosphodiester bond formation cannot continue. Consequently, chain growth aborts. Missing base or 5' OH does not explain termination; absence of 3' OH is critical.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Which nucleotide analogs are used in Sanger sequencing?

Sanger sequencing employs dideoxynucleoside triphosphates, ddNTPs, as essential terminators alongside normal dNTPs. The reaction contains all four dNTPs for strand extension plus limiting concentrations of labeled ddATP, ddTTP, ddGTP and ddCTP in four separate or single combined reactions. DNA polymerase synthesizes complementary strand until a ddNTP is randomly incorporated. Since ddNTPs lack a 3' hydroxyl group, further phosphodiester bond formation is impossible and synthesis halts permanently. Ribonucleotides rNTPs are not used, and dATP alone is insufficient for comprehensive sequence determination.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Sequencing

Who developed the chain termination method of sequencing?

Frederick Sanger developed the dideoxy chain termination method in 1977, revolutionizing DNA sequencing and earning a second Nobel Prize. The method uses DNA polymerase to extend an oligonucleotide primer on a single-stranded template, incorporating normal deoxynucleotides along with small amounts of fluorescently labeled dideoxynucleotides. Random incorporation of a ddNTP terminates elongation because it lacks 3' OH, generating a nested set of fragments whose sizes reveal the sequence after electrophoresis. Gilbert and Maxam developed chemical cleavage sequencing, while Holley contributed to tRNA structure.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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