#Sanger 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 polymerase is generally used in Sanger sequencing?

Classical Sanger sequencing traditionally used the Klenow fragment of Escherichia coli DNA polymerase I, which retains DNA polymerase and 3' to 5' proofreading exonuclease but lacks 5' to 3' exonuclease activity that would degrade primer. This made it ideal for controlled extension and chain termination without unwanted digestion. Later improvements adopted Sequenase, a chemically modified T7 DNA polymerase, and thermostable Taq variants for automated cycle sequencing. DNA ligase seals nicks rather than polymerizing, reverse transcriptase copies RNA. Klenow represents historically taught enzyme for this method.

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 Sanger sequencing, what happens when ddATP is added?

In Sanger sequencing reaction, DNA polymerase extends primer and randomly incorporates either canonical deoxyribonucleotide or dideoxy analog such as ddATP opposite template thymine. Incorporation of normal dATP allows continued elongation, but incorporation of ddATP terminates strand because its ribose lacks 3' hydroxyl required for phosphodiester linkage with next nucleotide. The terminated chain remains stable product whose length marks position of adenine in synthesized strand. Accumulation of such fragments across many templates generates ladder where termination sites indicate base identity, not chain breakage in middle or no effect.

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 the function of polyacrylamide gel in Sanger sequencing?

In manual Sanger sequencing, termination products from synthesis reactions must be ordered by size to deduce sequence. Denaturing polyacrylamide gel electrophoresis provides single-nucleotide resolution due to its highly crosslinked matrix with small pore size, unlike agarose. Fragments migrate inversely to length; shorter fragments run faster and appear at bottom. By running four lanes or four colors in capillary, the ladder reveals base order. Polyacrylamide does not precipitate DNA, detect bases or primarily buffer pH. Its purpose is size separation with near single-base precision essential for accurate sequence reading.

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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