Organic Chemistry · Biomolecules: Nucleic Acids
DNA Synthesis
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In 30 seconds
DNA synthesis — as opposed to DNA replication — means building DNA molecules chemically, in a machine, with no template and no enzyme. The method used in every DNA synthesizer is the solid‑phase phosphoramidite Protected, activated nucleotide building block for synthesis Full entry → method, developed by Marvin Caruthers and co‑workers in the early 1980s. A short DNA chain (an oligonucleotide Short synthetic DNA (typically 15–100 nucleotides) Full entry →) is assembled one nucleotide at a time on a bead of controlled‑pore glass, with the chain growing from its 3′ end toward its 5′ end — the opposite direction from every polymerase. Each cycle adds one nucleotide through four steps: deprotection of the 5′‑OH, coupling Forming the new phosphite linkage between 5′‑OH and phosphoramidite Full entry → of a phosphoramidite building block, capping Acetylating unreacted 5′‑OH groups Full entry → of unreacted chains, and oxidation Converting P(III) phosphite to P(V) phosphate to convert the new linkage into a stable phosphate.
Because the process is stepwise and chemical, it can be automated and scaled: a modern synthesizer can make a 20‑mer primer in under an hour, and longer assemblies (gene synthesis) are built by stitching many oligonucleotides together. This topic covers the protecting‑group chemistry, the cycle of reactions, and the yield arithmetic that every primer order is really about.
Why this matters
Synthetic DNA is the workhorse reagent of modern biology and medicine. The primers used in PCR and DNA sequencing are made by chemical synthesis — every PCR you run depends on it. Researchers synthesize entire genes from scratch to study protein function, to engineer bacteria that make insulin or biofuels, and to build the DNA templates for mRNA vaccines. Chemically synthesized oligonucleotides are also drugs: antisense oligonucleotides (such as nusinersen for spinal muscular atrophy) and siRNA therapeutics bind mRNA and block harmful proteins, and synthetic guide RNAs enable CRISPR gene editing. DNA is even being explored as an ultra‑dense data‑storage medium — a technology built entirely on writing arbitrary sequences, which is exactly what chemical synthesis does.
The college version
Core Concepts
Why chemical synthesis builds 3′→5′
Enzymes extend DNA at the 3′ end (5′→3′ synthesis) because they need a free 3′‑OH to attack the incoming triphosphate. Chemical synthesis has no enzyme to enforce that direction, so it uses a different trick: the chain is anchored at its 3′ end to a solid support, and each new nucleotide is added at the 5′ end. The growing chain's 5′‑OH attacks the incoming building block's phosphorus. The 3′→5′ direction also makes purification simple — at the end, the product is cleaved from the bead and failures are washed away.
The phosphoramidite building block
The key reagent is a nucleoside phosphoramidite: a nucleoside whose 5′‑OH carries a dimethoxytrityl (DMTr) protecting group, whose exocyclic amine (on A, G, or C) carries a base‑labile protecting group (benzoyl, isobutyryl, or acetyl), and whose 3′‑OH is attached to a P(III) phosphoramidite unit bearing a 2‑cyanoethyl group and two diisopropylamine groups. In words: a protected nucleotide activated for coupling. The DMTr group is the traffic light — acid removes it (revealing the orange DMTr cation, used to monitor coupling), but base does not touch it, while the other protecting groups are removed by base at the end.
The four‑step coupling cycle
Each nucleotide is added in four reactions:
- detritylation Removing the DMTr group with acid Full entry →: dilute acid removes the DMTr from the 5′‑OH of the growing chain (the orange color of the freed DMTr cation lets the instrument measure how much chain is available).
- Coupling: the phosphoramidite (activated by tetrazole) reacts with the free 5′‑OH, forming a phosphite triester — a P(III) connection.
- Capping: acetic anhydride acetylates any 5′‑OH that failed to couple, permanently blocking those failure sequences.
- Oxidation: iodine in water oxidizes the P(III) phosphite triester to a P(V) phosphate triester — the stable backbone linkage.
The cycle repeats for the next nucleotide. After the final nucleotide, concentrated ammonia cleaves the product from the solid support and removes the cyanoethyl and exocyclic‑amine protecting groups, yielding the single‑stranded oligonucleotide.
Yield arithmetic: why every step must be near‑perfect
Because failures are capped and left behind, the fraction of full‑length product after n couplings is the product of the per‑step coupling efficiencies:
overall yield = En-1
where E is the fraction of chains that couple at each step and n is the length. At 99% efficiency, a 20‑mer gives (0.99)19 ≈ 0.83, or 83% full‑length product; at 98%, a 60‑mer gives only (0.98)59 ≈ 0.30. The rest is a mixture of (n−1), (n−2), and shorter failure sequences — which is why long synthetic DNA is purified by gel electrophoresis or HPLC, and why very long genes are assembled from shorter, clean pieces.
How It Works / Step-by-Step Process
- Anchor the first (3′‑most) nucleoside to controlled‑pore glass beads and load the beads into the synthesizer column.
- Detritylate: wash with acid to expose the 5′‑OH; the instrument monitors the orange DMTr color to check the amount of available chain.
- Couple: deliver the next phosphoramidite plus tetrazole activator; the 5′‑OH attacks the activated phosphorus, forming a phosphite triester.
- Cap: treat with acetic anhydride so any chain that missed the coupling is acetylated and permanently inactivated.
- Oxidize: treat with iodine/water to convert the phosphite triester to the stable phosphate triester.
- Repeat steps 2–5 for each nucleotide until the full sequence is assembled (adding 5′→3′ along the chain, which is anchored at 3′).
- Treat with ammonia: cleave the product from the beads and remove the cyanoethyl and amine protecting groups; purify by gel electrophoresis or HPLC.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| DNA synthesis (chemical) | DNA replication (enzymatic) | Chemical synthesis has no template and builds 3′→5′; replication copies a template 5′→3′ using polymerases |
| 3′→5′ chemical growth | 5′→3′ enzymatic growth | The synthesizer adds to the 5′ end of the anchored chain; enzymes add to the 3′ end |
| Phosphoramidite | dNTP | Phosphoramidite is a protected P(III) building block for chemical coupling; dNTP is the natural substrate of polymerases |
| Detritylation removes the 5′‑OH | Detritylation removes the base | Acid removes only the DMTr on the 5′‑OH; bases are protected with base‑labile groups removed at the end |
| Capping extends failed chains | Capping blocks failed chains | Capping acetylates unreacted 5′‑OH so failures cannot grow and contaminate the product |
| Yield = En | Yield = En-1 | A chain of n nucleotides needs n-1 coupling steps after the first anchored nucleotide |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Building DNA in a machine is like building a beaded necklace one bead at a time while the necklace is tied to a table, adding each new bead at the top. After each bead you check that it attached, glue down any bead that didn't attach so it can't cause trouble later, and harden the string so it doesn't fall apart. At the end you untie the necklace and wash it off. The machine repeats "add, check, glue, harden" for every bead in the exact order you want.
Worked example
Example 1: Coupling efficiency and full‑length yield
A synthesizer advertises 99.5% stepwise coupling. The fraction of full‑length product for a 30‑mer is
yield = En-1 = (0.995)29
Computing: ln(0.995) = -0.00501, so ln(yield) = 29 × (-0.00501) = -0.145, and
yield = e-0.145 ≈ 0.865
So about 86% of the material is the desired 30‑mer — quite good. Contrast a 60‑mer made at 98% per step:
yield = (0.98)59 ≈ 0.30
Only about 30% full length, with 70% as failure sequences of every shorter length. This is why long oligonucleotides are much harder to make than short primers, and why stepwise efficiency is the most important quality metric of a synthesizer.
Example 2: Reagent amounts at synthesis scale
A 1.0 μmol‑scale synthesis of a 20‑mer uses 10 equivalents of phosphoramidite per coupling (a large excess drives coupling to completion). The total phosphoramidite needed is
20 couplings × 10 μmol/coupling = 200 μmol
A typical deoxyribonucleoside phosphoramidite has a molar mass near 700 g/mol, so the total mass is
200 × 10-6 mol × 700 gmol = 0.14 g = 140 mg
If the four phosphoramidites are used in roughly equal amounts (about 50 μmol, or ~35 mg, each), a few hundred milligrams of reagents produce a complete primer — a vivid illustration of how little material a synthesis consumes.
Example 3: How many primer molecules come out?
A 0.10 μmol synthesis of a 20‑mer primer, after purification, delivers on the order of 0.10 μmol of full‑length product. The number of molecules is
0.10 × 10-6 mol × 6.022 × 1023 mol-1 ≈ 6.0 × 1016 molecules
A typical PCR uses about 1012 primer molecules per reaction, so one small synthesis supplies enough primer for tens of thousands of PCR reactions — one reason synthetic primers are so inexpensive per reaction.
Key takeaways
- Chemical DNA synthesis needs no template and no enzyme; it builds oligonucleotides 3′→5′ on a solid support — the opposite direction of polymerases.
- The building block is a phosphoramidite: 5′‑OH protected with DMTr (acid‑labile), exocyclic amines protected with base‑labile groups, 3′ end activated as a P(III) phosphoramidite.
- Cycle per nucleotide: detritylation → coupling (tetrazole‑activated) → capping (acetylate failures) → oxidation (P(III) → P(V) phosphate).
- Capping is what keeps failure sequences out of the product: blocked chains never grow again.
- Overall yield = En-1: 99% per step gives ~83% for a 20‑mer but only ~37% for a 100‑mer; 98% per step gives ~30% for a 60‑mer.
- Final deprotection (ammonia) removes cyanoethyl and amine protecting groups and cleaves the product from the support.
- Synthetic oligonucleotides are used as PCR/sequencing primers, gene‑synthesis building blocks, antisense/siRNA drugs, and CRISPR guide strands.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
In which direction does chemical DNA synthesis build the chain, and why?
Show answer
3′→5′ (each nucleotide is added at the 5′ end), because the chain is anchored to the solid support through its 3′ end and no enzyme directs the direction.
List the four steps of one coupling cycle, in order.
Show answer
Detritylation (acid removes DMTr) → coupling (tetrazole‑activated phosphoramidite reacts with the free 5′‑OH) → capping (acetylate unreacted 5′‑OH) → oxidation (P(III) phosphite → P(V) phosphate).
What is the purpose of the capping step?
Show answer
To permanently block any chains that failed to couple, so they cannot grow in later cycles and contaminate the final product with (n−1), (n−2), … failures.
A 40‑mer is synthesized at 99% stepwise efficiency. What fraction of full‑length product do you expect?
Show answer
(0.99)39 ≈ 0.68 — about 68% full length (39 couplings for a 40‑mer).
Why can't the same phosphoramidite chemistry be used to make RNA as easily as DNA? (Think about the 2′‑OH.)
Show answer
The ribose 2′‑OH would participate in unwanted side reactions and needs its own orthogonal protecting group, making RNA synthesis substantially more demanding; DNA's 2′‑H needs no protection.
Name three applications of synthetic oligonucleotides.
Show answer
PCR primers, sequencing primers, gene‑synthesis building blocks, antisense/siRNA drugs, CRISPR guide strands (any three).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- oligonucleotide
- Short synthetic DNA (typically 15–100 nucleotides)
- phosphoramidite
- Protected, activated nucleotide building block for synthesis
- solid‑phase synthesis
- Building the chain on a solid bead, washing away reagents
- DMTr (dimethoxytrityl)
- Acid‑labile protecting group on the 5′‑OH
- detritylation
- Removing the DMTr group with acid
- coupling
- Forming the new phosphite linkage between 5′‑OH and phosphoramidite
- capping
- Acetylating unreacted 5′‑OH groups
- oxidation
- Converting P(III) phosphite to P(V) phosphate
- β-Oxidation
- Four-reaction spiral removing two-carbon acetyl CoA units from the β carbon of a fatty acyl-CoA
- Solid-phase synthesis
- Reactions on an insoluble bead; excess reagent and byproducts are removed by washing
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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