Questions for a Sanger versus NGS decision

The useful question is not which method is newer. It is whether you have one amplicon to confirm, a mixture to resolve, or a haplotype you cannot see in a single Sanger trace. Sanger sequencing reads a population of copies of one template as a chromatogram. Short-read sequencing, the usual meaning of NGS in this decision, reads many individual molecules after a library is made. Both can be the right research tool. They fail in different ways. The short-read path from library to reads is next-generation sequencing from library to reads. How a laboratory commissions either method is commissioning a sequencing or proteomics study. Money is outside this page. The decision below does not use it.

One template, or many questions at once

Ask what the tube contains. A plasmid prep that should be one clone, a PCR product from a diploid clone, or a single colony you want to check across a junction: these are Sanger-shaped questions. You design a primer, you run a reaction, you look at peaks. Mixed peaks mean the template was not pure, or the primer sat down twice. Clean peaks across the junction mean this prep matches the sequence you can read. Cloning practice that ends in that check is the sort of workflow Addgene's protocol collection has long circulated. Use it as a map of the habit, and follow the polymerase and the sequencing service instructions you actually have.

Ask the opposite question too. Do you have dozens of samples, or dozens of targets, or a whole genome? A stack of Sanger reactions can still be the right choice for a handful of loci. It becomes a poor description of the work when the scientific object is a pool. Then the conversation is a library, indexes, and a short-read run, which can be an amplicon panel or a genome. Geometry for those reads is choosing read length and paired ends. Primer design for a locus you might still amplify starts at a tool such as NCBI Primer-BLAST.

Mixtures and the minority base

Sanger's trace is a superposition. If two bases are both abundant at a position, you see two peaks. If one base is only a small minority of molecules, it often fails to rise above the trace's noise, and the read looks homozygous. Laboratories treat that visibility as a property of a strong minor fraction, and they do not treat Sanger as a method for rare alleles. The exact appearance depends on the trace quality. The decision does not require a universal percentage. It requires honesty about your sample. A biopsy-like mixture, a plasmid prep with a subclonal deletion, or a CRISPR pool that is still bulk DNA: if the allele you care about may be rare, a clean Sanger trace does not exclude it.

Short-read sequencing counts molecules, after PCR and with all the duplicate and error caveats in from reads to a result a biologist can question. A minor allele can be visible as a fraction of reads when depth is high enough and the error rate is lower than the fraction. It can also be an artefact. The decision is whether you need that count. If you need it, Sanger is the wrong instrument. If you need to know whether a clone you picked is pure, Sanger's mixed peaks are a direct answer and a pile of reads is an indirect one.

The method class for those short reads is outlined by Illumina. Other short-read classes exist. The mixture logic is about counting molecules versus reading a blended trace, and it survives the brand.

Haplotypes and the plasmid you meant to build

Two heterozygous positions in one Sanger trace do not tell you which alleles share a chromosome. The trace is a blend. Phasing them by Sanger means separating the alleles first, often by cloning the amplicon and reading several clones. Short reads phase the two positions only when a single read or a single fragment pair covers both. If they sit further apart than your insert, the short-read library does not phase them either. Long reads are a third geometry, for events and haplotypes that must be contained, discussed in long-read sequencing for structural variants. Ask whether you need the haplotype. If you do, ask whether one molecule in your chosen method can carry both variants. If it cannot, plan the clone or change the method.

Plasmid confirmation is the question Sanger still answers in a single picture. You want the insert, the junctions, and enough of the backbone to know the clone is the construct you designed. A short-read assembly of a plasmid can also work, and it is a project. For one construct, the trace is usually the record a collaborator can read without a pipeline. For a library of constructs, the indexes and the pool become rational. Write down which of those you have before you pick a service page.

Turnaround is a question you ask, not a number you inherit

People fold "which is faster" into the technical decision. Speed is a planning question for the laboratory that will do the work, and it includes how long your samples spend in a courier. This page has no interval to offer and will not invent one. Ask, record the answer you are given, and keep it out of the scientific justification. A method that cannot see your minor allele is not made appropriate by a short calendar. A method that can see it is not made inappropriate by a longer one. Put the scientific questions in the statement of work, and let the schedule be a separate line that the operator of the instrument is allowed to answer.

Branch when the answer is uncomfortable

If the sample is one locus and one clone, stop designing a library. Write the primer and the expected trace. If the sample is a mixture and the allele might be rare, stop expecting Sanger to acquit it. Design a counting assay and a control for error. If you need phase and the variants are far apart, stop hoping a default paired-end length will save you. Measure the distance against the insert you could actually make. If you need a whole genome, the Sanger questions are for follow-up loci, not for the genome. The genome conversation is a different enquiry.

A wrong method still returns a clean file. Clean Sanger of a mixture looks homozygous. Clean short reads of a single plasmid arrive as a pile of perfect matches that nobody wanted to analyse. The file quality is not the decision quality.

Research limits of either answer

Neither method, chosen well, is a diagnosis. A Sanger trace of a human locus in a research project is a research trace. A short-read VCF of the same locus is a research file. Reporting either to a patient is a regulated activity this article does not authorise. Biosafety of the organism is an institutional decision made before the template is posted. A plasmid from a cloning project and a pathogen amplicon do not share a shipping rule just because both can be sequenced.

Courier time belongs in the planning question

When you ask a laboratory how long a method takes, include the time a tube spends between cities. A hot week can degrade a PCR product that a Sanger reaction needed to be clean, and it can nick a library. Ask how they want the template delivered, and do not treat their planning answer as a promise this page can repeat. If the scientific question is a plasmid check, a delay does not convert it into a genome project. If the scientific question is a rare allele, a short planning interval does not convert it into a Sanger reaction. Keep the two conversations in separate sentences so the second cannot edit the first.

How to describe the decision in an enquiry

Write the contents of the tube, the number of samples, whether a minor allele must be visible, whether two positions must be phased, and whether you need a chromatogram or a count. Ask the planning question about schedule separately. State that you are not asking this page's readers to infer an interval. Consumables for either method are classes in the genomics and sequencing catalogue. The setting is the genomics research pathway.

Discuss one-template traces through the Sanger DNA sequencing enquiry reference. Discuss an amplicon pool through the amplicon sequencing enquiry reference. Discuss a genome through the whole-genome sequencing enquiry reference. Send the questions with the quote request. Each of those pages is an enquiry reference so the method can be discussed. The decision remains the one you can defend from the tube in front of you.