Scientists have created synthetic DNA with four extra “letters”:
A couple billion years ago, four molecules danced into the elegant double-helix structure of DNA, which provides the codes for life on our planet. But were these four players really fundamental to the appearance of life — or could others have also given rise to our genetic code?
A new study, published today (Feb. 20) in the journal Science, supports the latter proposition: Scientists have recently molded a new kind of DNA into its elegant double-helix structure and found it had properties that could support life.
But if natural DNA is a short story, this synthetic DNA is a Tolstoy novel.
The researchers crafted the synthetic DNA using four additional molecules, so that the resulting product had a code made up from eight letters rather than four. With the increase in letters, this DNA had, a much greater capacity to store information. Scientists called the new DNA “hachimoji” — meaning “eight letters” in Japanese — expanding on the previous work from different groups that had created similar DNA using six letters.
The team was able to confirm that the synthetic DNA could be replicated into RNA, and could form the double helix structure. However, they stopped short of confirming that it could replicate itself:
Still, in order for the Hachimoji DNA to support life, there’s a fifth requirement, Benner said. That is, it needs to be self-sustaining or have the ability to survive on its own. However, the researchers stopped short of investigating this step, in order to prevent the molecule from becoming a biohazard that could one day work its way into the genomes of organisms on Earth.
The article takes Hachimoji DNA as evidence that extraterrestrial DNA could be made of different components than the ones found on Earth. However, since they stopped short of replication, I’m not sure we have that evidence yet.
One question that often comes up in biology is, how necessary or arbitrary is a particular solution in evolution? In other words, were there other solutions that life could have taken to solve a particular problem? This is always a difficult question, because we don’t know whether those alternate solutions arose at some point in the past, but were subsequently selected against, or never arose because the right mutation just never happened.
Biological traits arise because of mutations. Mutations can be beneficial, in which case they’ll usually be selected for, or they can be detrimental, leading to them being selected against. Or they can be neutral, in which case whether they propagate may come down to the random fluctuations of genetic drift.
This is complicated by the fact that phenotypic (observed) traits typically arise from the complex interactions of proteins produced by individual genes. So a beneficial trait might be paired with a detrimental one, with whether the combination propagates depending on how the mix of benefit and detriment works out.
An aspect of any trait or mechanism is how much energy it needs. The trait or mechanism might be neutral or perhaps even mildly beneficial, but if it’s costly in terms of energy, it’s likely to end up falling on the detrimental side of the ledger. Although if it’s very beneficial, then even being costly in terms of energy might not matter. (The brain is a prime example of this latter case, an energy hungry organ that nonetheless earns its keep.)
Energy is what I’m not sure about with these additional letters. How much chemical energy do they require to be incorporated into the DNA structure? The answer might not matter for any artificial applications we come up with, such as DNA storage, but if they require more energy to form, that might be why we don’t see them in nature.
Along those lines, I’d be very interested if anyone has seen information on this aspect of the development. Or, as usual, if I’m missing anything here.