DNA replication

The word “mutation” carries some pretty negative connotations. In the world of human biology, mutation suggests malformation, and in our DNA, mutations often mean dangerous changes to our genetic code that could lead to cancer. But without random mutations, our species — and all other life forms on Earth as we know it — wouldn’t exist. As a new video published by researchers in Science shows, not all mutations are so terrible.

The video, published alongside a study by a team of French researchers on Thursday, shows Escherichia coli bacteria duplicating over the course of 200 generations and the mutations they accumulate in that time. In their paper, the scientists describe their new technique for tracking these mutations in real time and show that not all these mutations are necessarily bad for the organism.

“Mutations are less detrimental to the bacteria than expected,” explains the narrator in the video.

DNA mutations can potentially occur every time a cell replicates. When a cell decides it’s time to split into two, one of the first things it does is make a copy of its DNA sequence — written out in strings of the four bases G, A, T, and C — to pass on to the new “daughter” cell.

But the cell’s DNA replication machine, shown in fluorescent green in each of the red bacteria cells in the video, isn’t always perfect. Think of it as a slightly wonky photocopier: Sometimes, the DNA replication machinery makes some typos, spelling out, say, G-A-A-G when it should be G-A-T-G. That little typo counts as a mutation, and it could have disastrous — or totally benign — effects, depending on what gene it occurs in.

Fortunately, in the new study, the researchers show that only about 1 percent of the mutations that occurred in the bacteria over the course of 200 generations were fatal ones. The rest of the mutations — there was about one every 600 hours — didn’t seem to have any detrimental effect. Even more encouraging is the fact that changing the stress level of these bacteria didn’t seem to change the rate at which they gained mutations. Shifts in DNA, the study illustrated, are simply a part of life.

They have yet to track mutations in human cells, so we can’t say for sure whether human DNA mutations happen as quickly and are all as benign, but the new technology the scientists developed will make it possible to do so in the future. Unlike E. coli, which are single-celled organisms, each human has about 37.2 trillion cells to deal with — the vast majority of which replicate repeatedly over the course of our lives. We can only hope our photocopiers are as good — or more efficient — that those of our bacterial brethren.