three meteorites Scientists recently discovered the molecular building blocks of DNA and its cousin RNA. A subset of these building blocks have been discovered in meteorites before, but the rest of the group seemed mysteriously absent from space rocks — until now.
The new discovery supports the idea that a barrage of meteorites may have delivered the molecular ingredients needed to start early life on Earth, about four billion years ago.
However, not everyone is convinced that everything that has been discovered is recent DNA The ingredients are extraterrestrial in origin; Instead, some may have ended up in meteorites after rocks fell to Earth, said Michael Callahan, an analytical chemist, astrobiologist and associate professor at Boise State University, who was not involved in the study. “Additional studies are needed” to rule out this possibility, Callahan told Live Science in an email.
He added that assuming all compounds did indeed originate in space, one subset of their building blocks – a class of compounds known as pyrimidines – appeared in “extremely low concentrations” in meteorites. This discovery indicates that the world’s first genetic molecules appeared not because of the influx of DNA components from space but as a result of geochemical processes that unfolded early on Earth, he said.
said Jim Cleaves, a geochemist and president of the International Society for the Study of the Origin of Life who was not involved in the study. This question is still under investigation.
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The basic building blocks of life in space rocks
DNA components and RNA It has been found in meteorites before, Live Science previously reported. Specifically, these space rocks have been found to contain nucleobases, the nitrogen-containing compounds that serve as the “letters” in the genetic code for DNA and RNA. Nuclear bases come in five basic flavors — adenine (A), thymine (T), guanine (G), cytosine (C) and uracil (U) — but previously, only A, G, and U were identified in meteorites.
Now, in a study published Tuesday (April 26) in the journal Nature CommunicationsScientists have reported finding all five nuclear bases inside carbon– Your meteorites are rich. This included trace amounts of all three pyrimidines: cytosine, uracil, and thymine. “In particular, the discovery of cytosine is surprising,” said Yasuhiro Oba, associate professor at the Institute of Low Temperature Sciences at Hokkaido University in Japan and first author of the study, because cytosine is relatively unstable and likely to react with water. .
Although thymine and cytosine have not been found in meteorites before, laboratory studies have suggested that these nuclear bases may be lurking, undetected, in space rocks that collided with Earth.
For example, in laboratory settings, scientists have recreated the chemical conditions of interstellar space – the space between stars – where massive clouds of gas and dust measure about 10 Kelvins (minus 441.67 degrees Fahrenheit, or minus 263.15 degrees Celsius) and the origin asteroids Meteorites can be found. Through these experiments, the researchers synthesized thymine, cytosine and other primary nucleobases, indicating that all of these compounds could theoretically be detected in meteorites, the study authors note in their report.
So the team went looking for these nuclear bases in three known meteorites. “The Murchison, Murray, and Tagish Lake stones belong to a class of meteorites called carbonaceous chondrites, which are known to contain a lot of organic compounds,” Callahan said.
For example, hydrocarbons and the building blocks of proteins (amino acids) have been identified in the three meteorites, Oba said. in addition to, in previous workOba and colleagues discovered an elusive molecule called hexamethylenetetramine (HMT), which is thought to be an important precursor to organic molecules in space rocks. According to NASA.
In their latest study, the researchers used a technique called high-performance liquid chromatography, which involves using pressurized water to separate meteorite samples into their component parts. In this way, the team extracted the nuclear bases from each sample and then analyzed the bases using mass spectrometry, a technique that revealed the chemical composition of the material in minute detail. This method “enabled us to detect nuclear bases at very low concentrations, as low as parts per trillion,” Oba told Live Science.
Analysis revealed that all meteorites carried adenine and guanine. The Murchison samples also contained uracil, while other meteorites carried at least one uracil isomer, meaning a compound with the same number and types of uracil atoms but in a different spatial arrangement. In addition, the Murchison and Tagish Lake samples carried thymine, and the Murray meteorite contained thymine isomers. All meteorites contained cytosine, along with different isomers of the compound.
To verify that the nuclear bases were extraterrestrial in origin and not the result of terrestrial contamination, the team repeated the experimental procedures without any meteorite material in the test chambers. No nuclear bases were detected during these so-called empty tests.
The team also had access to soil samples from the site where the Murchison meteorite first fell to Earth. Oba said they have detected some nuclear bases in the soil, but that “their distribution and concentrations are clearly different from those of meteorites.” In addition, some specific isomers only appeared in meteorites and not in the soil sample; These “single isomers” are rarely seen on Earth, and so are unlikely to be pollutants from the planet’s surface, Cleaves said.
By comparing the diversity of nuclear bases found in the meteorite with those in the soil, the team concluded that the compounds in the space rock formed in space, Oba said. Because of this, they speculate that nuclear bases “contributed to the emergence of the genetic characteristics of the oldest life on Earth,” the authors wrote.
However, regarding these points, Callahan said, there is still some uncertainty.
The soil sample the researchers analyzed contains higher concentrations of cytosine, uracil and thymine than they found in the Murchison meteorite, Callahan said, “so it’s hard to say how much extraterrestrial versus Earth is in the meteorite.” Furthermore, the team did not identify a specific chemical process that would produce C, U, T and their various isomers; Such an analysis would have supported the idea that all compounds formed in interstellar space.
Another way to determine if nuclear bases actually descend from space is to examine the forms of carbon and nitrogen they contain, Cleaves told Live Science. These elements come in different flavors, called isotopes, which have the same number of protons but different numbers of neutrons. Earth’s material contains different proportions of carbon isotopes and nitrogen isotopes than that found in space, Callahan said, so such analyzes can help distinguish terrestrial nuclear bases from those of extraterrestrial ones. Unfortunately, experiments like this require a fair amount of meteorite material to run, and so can be difficult to perform, Cleaves said.
In any case, even if the elements C, T, and U discovered were extraterrestrial, their negligible presence in meteorites casts doubt on the theory that the first life on Earth was cultured with DNA components from space, Callahan said. “If these results are representative of typical pyrimidine concentrations in meteorites, it is likely that early Earth geochemical synthesis was responsible for the emergence of genetic material, rather than input from extraterrestrial conduction,” he said.
In the future, Uba and his colleagues plan to look for nuclear bases in material collected directly from asteroids, rather than in meteorites on Earth, Uba told Live Science; This can reduce the problem of land-born pollutants. For example, the Japanese spacecraft Hayabusa2 recently brought the asteroid Ryugu to Earth, Live Science previously reportedNASA’s OSIRIS-REx probe is scheduled to touch samples of the near-Earth asteroid Bennu in 2023, According to Space.com.
Originally published on Live Science.
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