Synthetic DNA versus PCR-amplified inserts

A synthetic fragment and a PCR product can share a gene name and still be different molecules. PCR copies a template and can write polymerase errors and primer-derived bases into the insert. Synthesis starts from a file, so it can change codons and leave the old plasmid's mistakes behind, and the tube that arrives still has to match that file. You are choosing which source to use for a short designed insert, then which checks both sources still owe you. This is a research explainer. It does not mean a gene is being made on your behalf.

How a polymerase copies DNA is in how polymerase chain reaction works. How the insert becomes a colony is in plasmid cloning from insert to colony.

What each source is doing chemically

PCR uses two primers and a polymerase to copy the stretch between those primers. The new strands become templates. A classical proofreading polymerase removes many mismatched bases and still has a non-zero error rate. Follow that enzyme's sheet for extension time and fidelity claims. A non-proofreading enzyme is a poor choice when the product will be cloned as a reference. Primer tails are copied into the product on purpose: restriction sites, overlaps, or a start codon you added. Those extra bases are real sequence. They are not a smudge you can ignore at the junction.

The template is whatever you added. If the parent plasmid already has a mutation, PCR will often hand it to you faithfully. That is not an enzyme failure. It is copying. A band of the right size shows length. It does not show the bases. Endpoint PCR does not report how many copies you started with, and it does not certify the insert.

Chemical synthesis assembles a sequence that was written in a file. Oligonucleotides are made and joined until the designed string exists as DNA. Because the process does not read your old plasmid, it does not inherit that plasmid's polymerase scars: the stray substitutions from an earlier amplification, or the primer tails you wish were not there. You can change codons for the expression host, remove a restriction site that would have complicated the clone, or build a short designed fragment that no tube in the freezer contains. The physical DNA that is delivered is still a sample. It can disagree with the file. Treat the paperwork as the claim and the sequence read in your hands as the check.

What has to be true before you choose

You need the exact bases, not a gene symbol. Pull a reference from NCBI GenBank or from the map of the plasmid you hold, and write the differences you want in a second file. You need to know whether a trusted template already exists. You need the ends the cloning method requires: restriction overhangs, blunt ends, or assembly overlaps. Enzymes and primers are classes in the molecular biology catalogue. The file decides which class.

A short fragment is a special case of visibility. On a gel it may be a faint band, and a synthesis tube may contain little mass until you amplify or clone it. Plan the confirmation before you choose the source. A Sanger sequencing enquiry reference is how to frame the read that both routes need. Ask whether a quotation is possible. The read covers the insert and the junctions. One primer window may be enough for a short insert and is not enough for a long one.

Choose from the template you actually have

If the sequence exists only as a design, synthesise it. PCR has nothing correct to copy. Codon changes across a whole open reading frame belong in that design file, not in a stack of mutagenic primers you hope will all land.

If a sequenced plasmid in your freezer already has the coding sequence, PCR with a proofreading polymerase is usually enough. Put the new ends in the primers. Clone, and sequence at least the insert of the colony you will keep. Choose synthesis instead if you need many codon changes, if an internal site must disappear, or if PCR from that parent keeps returning deletions because the template is repetitive or structurally difficult.

If the parent might have drifted, do not amplify it and call the product the original gene. Sequence the parent or replace the insert with a file you can defend.

Both routes then meet the same cloning checks. A synthetic linear fragment still needs a restriction digest or an assembly that matches its ends, and a colony still needs a sequence compared with the file. A PCR insert needs the same. Restriction ligation will show orientation only if the ends were directional or you test it. Write that test down. Survival on antibiotic shows the marker, not the insert.

Where the choice goes wrong

Ordering a synthetic copy of a plasmid you have already sequenced, with no codon change and no site change, spends time to recreate a template PCR could have copied. Do that only if the parent is suspect.

Amplifying a designed sequence you do not hold, by hoping a related gene is "close enough," copies the related gene. Homology is not identity. Blast the parent against the file you think you are cloning before you design primers.

Skipping the read because the synthesis report looked complete leaves your junctions unchecked. Skipping it because the PCR band was bright leaves polymerase error unchecked. Mixed peaks after either route mean a mixed colony. Restreak and read again.

A faint short band cut from a gel can be the wrong faint band. If the synthetic fragment is also short, do not identify it by eye. Use a digest that the design predicts, or go straight to a junction sequence.

Safety and what the sequence is

Changing codons does not change your duty to say what the product is. A toxic or regulated coding sequence is an institutional biosafety question before anyone synthesises or amplifies it. This page is not that approval. Follow the committee that covers your bench. Waste from either route, including PCR products and recombinant plates, is handled as your rules say.

Heat on the way to the bench

A dry synthetic fragment and a liquid PCR tube do not want the same journey. A PCR product left in a hot courier bag can degrade or simply arrive at a volume you no longer believe. A dried fragment still needs to be resuspended as its sheet says, and it still needs the sequence check. Do not leave either one on a warm receiving bench over a weekend and then treat the concentration on the old label as current.

What the enquiry has to contain

For a designed fragment, the custom gene synthesis enquiry reference is a place to frame the sequence, the host, the vector if you want it cloned, and the sites you must keep or avoid. It is an enquiry reference for that specification. Sending the question does not imply that a synthesis run is already booked. For primers and polymerase class, use the catalogue and the quote request. Say whether a trusted plasmid already exists. That one fact is the whole choice. Ask whether a quotation is possible. Attach the file you want compared with the eventual read.