Most genetic engineering modifies organisms that already exist. Synthetic biology attempts something different: constructing cells, or components of them, from designed parts.
What has been achieved
Cells with genomes reduced to the minimum set of genes required to live, which turns out to be a surprisingly hard question to answer. Several of the genes that proved essential have unknown function – they are needed and nobody knows what for.
Artificial cell-like structures that carry out specific chemical processes without being alive in any full sense.
And organisms with expanded genetic codes, using base pairs that do not occur naturally, which lets them build proteins from amino acids biology does not use.
Why do it
Understanding, first. Building something is a stringent test of whether you understand it. Every failed attempt identifies something missing from the model.
Manufacturing, second. Engineered organisms already produce pharmaceuticals, materials and fuels more cheaply than chemical synthesis. Designed-from-scratch organisms could be optimised for production in ways evolved ones cannot.
And containment. An organism dependent on a synthetic amino acid supplied artificially cannot survive outside the laboratory, which is a genuine biosafety mechanism rather than a procedural one.
The concerns
Dual use, mainly. Techniques for constructing organisms are techniques for constructing dangerous organisms, and the knowledge does not separate.
Governance rests substantially on screening by synthesis companies and on norms within the research community, which is thinner than the risk warrants.
Where it stands
Real capability, narrow applications, and a governance conversation that is behind the science. That last part is the one worth watching.