The minimum number of genes

The minimum number of genes needed for life depends on the surrounding environment...

Your genome contains all of your genetic information. If you imagine that your genome is a manual of how to build you, then the language it is written in is DNA; and the DNA is organised into genes, which act like the paragraphs. The genes then tell your body how to make all its proteins, which play many critical roles. 

Your personal genetic manual is pretty long. The human genome project estimated that humans have between 20,000 and 25,000 genes. But what is the minimum number of genes needed for life? 473 is the current answer, according to synthetic biology.

To get this number scientists started with a very simple bacterium which had around 900 genes and gradually deleted genes until they got to 473. They found that if they continued deleting any more genes, the bacterium stopped duplicating and were no longer classed as alive.

But of those 473 genes, which make up the “minimal genome”, scientists still didn’t know what nearly a third of them, 149, did. But now Mark Wass and his team at the University of Kent think they have worked out the specific functions of 66 of the mystery genes.

To achieve this they used a combination of 22 different computational methods. “Each of these methods predicts a different property or feature of proteins,” says Wass. “We take the protein and we compare it to other proteins that we know what they do - like searching a database.”

Of the 66 genes they identified, they found that most were involved in the transport of molecules into and out of the cell: bringing nutrients in and taking waste out. These are particularly important in a nutrient-rich environment.

This led them to the conclusion that the 473 genes in the minimal genome can be divided into two groups. “The first is the set that are essential for life, and probably you would expect to find those in every individual species,” says Wass. The second set is related to the environment where the species grows.

Imagine you are packing for a holiday. You could divide your packing into two distinct groups: firstly, the essentials you need for any holiday: underwear, toothbrush. And secondly, the specifics you need for a certain holiday, like your ski jacket or your bikini. Like your suitcase's contents, the genes a bacteria needs to survive depend on the environment that it is in.

So naturally occuring bacteria might contain a range of genes needed to survive in several different environments. But because the scientists deleted unnecessary genes when the bacterium was in a nutrient-rich environment, the genes needed for the bacterium to survive in a nutrient-scarce environment have been deleted. If someone took you on a surprise holiday you would have to take everything, just in case, but if you knew where you were going, you could pack more lightly.

This research is helping to understand the minimal processes needed for life. Scientists are also looking to repeat the work with a different starting bacterium, so that the minimal genomes can be compared. “We would be interested in looking at the overlap in the genes,” says Wass, “so that we can come up with this real essential set of genes.”

But if you’ve been paying attention: 149 minus 66 still leaves 83 unknown. Wass says these are still a complete mystery, so mother nature isn’t revealing all her secrets just yet.