NASA researchers most recently discovered that while Mark and Scott Kelly are still identical twins, Scott's DNA reacted to space in a unique way.
Astronaut Scott Kelly, who retired from NASA in 2016, is the subject of a study comparing his DNA with that of his identical twin (and fellow astronaut) Mark.
Astronaut Scott Kelly spent an entire year in outer space, spending 340 days in the International Space Station (ISS) in 2017, almost double the amount of time spent by other astronauts, which generally ranges between 5 to 6 months.
His gene expression - the transcribing and translation of genes, not the genes themselves - was what actually changed during his year on the Space Station.
This was revealed in preliminary results from the space agency's twins study, according to CNN.
Some of the genes that seem to have changed permanently involved DNA fix, bone formation and how the cells use oxygen.
Live Science reported that 7 percent of astronaut Scott Kelly's DNA changed when he was in space.
For additional detail on preliminary findings, visit NASA Twins Study Investigators to Release Integrated Paper in 2018.
"Scott's DNA did not fundamentally change", a NASA statement. Gene expression refers to how active a particular piece of DNA is.
Bailey said her research, and nine other studies, aren't finalized yet and should be released from NASA later this year. NASA and the other researchers collaborating on these studies expect to announce more comprehensive results on the twins studies this summer'.
After returning to Earth, Scott started the process of readapting to Earth's gravity.
The Kelly brothers have almost identical genomes, allowing for an unprecedented look at the physical effects of long-term spaceflight.
The altered genes are related to Scott Kelly's immune system, DNA fix and bone formation networks, among other bodily functions.
Although 93% of Kelly's genetic expression returned to normal once he returned to Earth, a subset of several hundred "space genes" remained disrupted.
NASA's update came after some media outlets initially misreported that Scott Kelly's DNA itself had changed. While some private space firms are looking to take people to the red planet very soon, it's clear there's still a great deal we don't understand about the effects of space travel on humans.
They have also verified that Kelly's telomeres - the caps on the ends of chromosomes - grew to be longer than Mark's.
Kelly's one-year mission is a scientific stepping stone to a planned three-year mission to Mars, NASA said. Scott was two inches taller when he landed. And he did not have 7 percent of his genes altered by space travel.
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