The 7% solution
Astronaut Scott Kelly has been extensively studied after a year in space (and so has his identical twin). There’s a new, and pretty dramatic, story about some of the results. For example, IFLScience says NASA Sent One Identical Twin Brother To Space For A Year – And It May Have Permanently Changed 7 Percent Of His DNA. So does Business Insider.
If you know that a chimpanzee’s DNA is only about 1% different from a human’s — or that a mouse’s is about 8% different — that sounds weird. It’s even worse than that: the chance you’d still be alive after that sort of mutation load is pretty small.
So what did happen? Well, the story seems to be an example of accumulated mutations itself. In a recent interview for Marketplace, Scott Kelly said
“I did read in the newspaper the other day… that 7 percent of my DNA had changed permanently,” Kelly said. “And I’m reading that, I’m like, ‘Huh, well that’s weird.’”
We’re seeing reports of someone quoting something from the media, rather than any primary source. If you go to the NASA press release, it says
Although 93% of genes’ expression returned to normal postflight, a subset of several hundred “space genes” were still disrupted after return to Earth.
That seems to be the origin of the ‘7%’ figure.
So what’s the difference? Imagine the genome as a library. A 7% chance in DNA would be like saying 7% of the words in all the books in the library had been altered. A change in expression in 7% of genes would be like 7% of the books having a noticeable increase or decrease in how often they were borrowed.
There were also some small changes in Scott’s DNA. His telomeres, which are the caps on chromosomes that stop them fraying at the ends (like the little plastic bits on shoelaces) were slightly longer — which is probably good. DNA that scientists sequenced from his blood also had “hundreds” of new mutations: more than you’d typically expect, but still only about 0.0000001% of his DNA
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
Except that we should really stop comparing the genome to a library, text, recipe, instruction manual, blueprint, history book, code, language, program etc. It is just a bunch of molecules.
6 years ago
I disagree completely. It’s a very important fact about DNA that to a good approximation it behaves like text. Yes there’s methylation and 3-D structure and stuff, but it’s still a useful analogy, not just a metaphor.
Books are just a bunch of molecules too, but that’s not the right level to appreciate what they do.
6 years ago
Thomas,
thanks for answering.
The crucial difference between a book and a genome is that the text in a book has been written intentionally by a human being with a mind and is likewise read and understood only by anohter human with a mind. The meaning the text conveys is constructed/reconstructed by a mind. The text has no meaning outside that context.
A genome has not been “written” intentionally by someone with a mind, and, more importantly, it is supposed (by defenders of DNA as text) to be “read” by an organisms’ non-genetic developmental resources, which are non-mental entities. However, there is no defensible sense in which DNA in an organism is read in such a way that it can be considered as a text of instructions. We know of nothing outside the realm of (human) minds that can literally give instructions and follow instructions. These notions have been incorporated into molecular biology without proper scrutiny and have been almost universally been rejected by theoretically-minded biologists or philosophers of biology (see e.g. Oyama 1985 and Griffiths 2001).
Oyama, S. The Ontogeny of Information; Cambridge University Press: Cambridge, 1985.
Griffiths, P. E. Genetic Information: a Metaphor in Search of a Theory. Philosophy of Science 2001, 68, 394–412.
6 years ago
I went and had a look to see if Einsteins theories on relativity could explain why Scott’s telomere’s were “younger”. But his trip in space only made him younger by 8.6 milliseconds (compared to people on earth) so probably not.
https://qz.com/370729/astronaut-scott-kelly-will-return-from-a-year-in-space-both-older-and-younger-than-his-twin-brother/
6 years ago
I think there is also a denominator problem in the 7% figure.
In this PR, NASA says “several hundred genes” have ongoing changes in expression. There are ~20 000 protein coding genes in humans, and most genome-wide studies will also measure expression of some non-coding RNAs. So, 7% should be much more than a few hundred genes.
The wording of the PR is a bit tricky, but I think they are actually saying is that of those genes that were deferentially expressed in space 7% appear to have keep their space-like expression on earth.
Then there are the questions (biological and statistical) about how many genes would show this pattern whether or not you sent one twin to space…
6 years ago