Genome synthesis: how to write a genome at scale

Genome synthesis is the ability to write DNA from scratch at the scale of entire chromosomes and genomes. Where gene editing modifies existing sequences one change at a time, genome synthesis builds complete genetic programs to specification. It is the difference between editing a document and writing a new one.

What is genome synthesis?

Genome synthesis (also called genome writing or synthetic genomics) is the construction of complete, functional DNA sequences from chemical building blocks. Rather than cutting, pasting, or modifying existing genes, genome synthesis assembles new DNA sequences base by base, then builds them up into chromosomes that can drive a living cell. The resulting organism contains a genome that was designed and built, not inherited.

Genome synthesis vs genome editing

Gene editing tools like CRISPR-Cas9 make targeted changes to an existing genome: swapping a few bases, inserting a short sequence, or disabling a gene. This is powerful for single-gene modifications but fundamentally limited when the goal requires changes across thousands of sites simultaneously, or when the desired genome has no natural starting point.

Genome synthesis removes that constraint. It does not start from an existing genome. It writes the entire sequence from scratch, incorporating as many designed changes as the project requires. The 2019 creation of Syn61, for example, required 18,214 codon changes distributed across 4 million base pairs. No editing tool could have made those changes practically or simultaneously.

How to build a genome in 4 steps

Step 1, synthesise building blocks: short DNA fragments of up to 10,000 base pairs are generated by chemical or enzymatic synthesis. These are the basic units of construction.

Step 2, assemble larger pieces: building blocks are joined into segments of up to 100,000 base pairs, typically within bacterial artificial chromosomes (BACs) that serve as construction scaffolds.

Step 3, combine into chromosomes: the larger segments are assembled into complete chromosomes. For bacteria, this means sequences of 500,000 to 12 million base pairs. For eukaryotes, chromosomes range from 50 million to 300 million base pairs.

Step 4, validate: the final assembly is verified for correct sequence, structural integrity, and biological function. Sequencing costs have fallen roughly a millionfold in 20 years, making comprehensive validation practical at scale.

Why synthesise genomes?

Three categories of application are driving genome synthesis forward.

In fundamental research, synthesising genomes with targeted modifications allows scientists to study gene regulation, chromosome architecture, and non-coding DNA in ways that single-gene editing cannot address.

In agriculture, genome synthesis enables the design of entire chromosomes optimised for traits like nutritional content, pest resistance, or environmental adaptation, where many valuable traits involve coordinated changes across dozens or hundreds of genetic loci.

In genome recoding, rewriting the genetic code creates organisms with properties not found in nature: resistance to viruses, genetic isolation from wild organisms, and the ability to produce proteins containing non-canonical amino acids.

What genome writing makes possible

Once you can write genomes at scale, the design space for biology changes fundamentally. Organisms can be engineered not just by modifying what exists, but by specifying what should exist. This enables recoded genomes like Syn61, synthetic yeast chromosomes (the Sc2.0 project, which had synthesised over 50% of the yeast genome by 2023), and entirely new biological manufacturing platforms.

How Constructive Bio applies genome synthesis

Constructive Bio's technology enables assembly of large DNA segments of more than 200 kb with verified sequence accuracy, creation of megabase-scale custom DNA in E. coli, and complete genome recoding for expanding the genetic code. This genome-writing capability is the foundation of the company's platform for producing programmable biomolecules using non-canonical amino acids.

Frequently asked questions

What is the difference between genome synthesis and gene synthesis? Gene synthesis builds individual genes (typically under 10,000 base pairs). Genome synthesis builds entire chromosomes and genomes (millions of base pairs), requiring additional assembly steps and whole-organism validation.

How long does it take to synthesise a genome? Timelines vary by genome size and complexity. A bacterial genome of several million base pairs can take months to assemble and validate. The field is accelerating as assembly methods improve.

Is genome synthesis the same as cloning? No. Cloning copies existing DNA. Genome synthesis writes new DNA to a designed specification. The resulting sequence can be entirely novel.