Science can move at a startling pace. In 2003, biologists from the Human Genome Project announced that they had learned how to read an entire human genome. A few days ago, they revealed that they now want to press ahead to try to write one. In other words, researchers have reached the stage where they want to build the genetic instructions that form the blueprint for living cells. The idea, outlined in Science last week, is exciting, audacious and also controversial.

So why put forward such a plan? Why court controversy with such a seemingly outlandish proposal? In fact, the idea goes back to the results of the original Human Genome Project that are now providing greater understanding of the causes of cancer, heart disease and schizophrenia while also shedding light on human evolution.

This information has certainly catalysed genetics, but we can only learn so much from simply reading our genetic code. For a start, there is the puzzling fact that only about 2% of the sequences that make up our genes actually direct the production of proteins. This leaves nearly 98% of the human genome apparently unaccounted for. To understand more fully how a genome works we therefore need to build one to try to elucidate the role of this 98% “junk DNA”. In the process, we will learn the importance of how genes and genomes are ordered and regulated and how this changes in disease states.

The proposed project, provisionally called Human Genome Project – Write (HGP-write), is led by Professor Jef Boeke, of New York University, and Professor George Church, of Harvard. Both are distinguished scientists, which is just as well, because their proposal faces a number of problems. For a start, there is the cost of making the DNA base pairs that are strung together to form a gene and which they need to create their artificial genomes.

In 2001, DNA cost about $12 to manufacture per base pair. That cost has dropped to three cents. It sounds promising. However, the human genome contains about 3 billion base pairs so a drop of a further thousand-fold in costs will have to be achieved in the next 10 years to make the project achievable. At the same time, an ethical framework for genome-scale engineering, as well as for transformative medical applications, will have to be established.

These are major hurdles, though clearing them will give us unprecedented insights into new mechanisms of human biology and disease that could form the basis for new therapies and disease interventions. But is the project actually practical? This is a very different question. Genomes have already been synthesised. In 2010, Craig Venter and colleagues at the J Craig Venter Institute synthesised a simple bacterial genome, while in Britain, Tom Ellis’s lab at Imperial College London and Patrick Cai’s lab at the University of Edinburgh are collaborating with an international consortium known as Synthetic Yeast 2.0. It is working towards the synthesis of the 12 million base pairs that make up all 16 chromosomes of baker’s yeast and expects to complete the work by 2018.

But human chromosomes are huge in comparison. The largest human chromosome is made up of 249 million base pairs and would need an enormous amount of highly expensive DNA for its creation. At present, estimates suggest it would cost around $100m (£69m) to create a human genome. And that is just the cost of making the DNA. The technology to handle these long and fragile lengths of DNA and place them into mammalian cells where they can be grown and studied does not exist at present. However, these are just the sorts of technologies the project’s researchers believe they could develop.

But is the project ethical? Is this another route to the creation of designer babies? This project has certainly been billed as controversial – but is that fair? Medical researchers already replace sections of human DNA in cell lines, which are grown in laboratories, to investigate how specific human genes work and to establish what happens when they go wrong. There are very tight regulations concerning such research. We should also note that the HGP-write project is explicit about only developing cell lines and not altering ova or embryos.

In short, this is not a sinister route to the creation or cloning of babies by stealth. In fact, its proponents are adamant that they want this project to be carried out with public involvement. Common goals for both the public and for the scientist would be identified from the start as part of an open debate. If we can do that, we can then determine how best to move the science of genetics into the latter half of the 21st century.

Susan Rosser is professor of synthetic biology at the University of Edinburgh