Nucleobases Found In Meteorites And Their Implications For Astrobiology

 

Nucleobases And The Origin Of Life

Meteorites are fragments of space rock that found their way to Earth. Around 70% of them originated from three young asteroid families (Karin, Koronis, and Massalia) formed by collisions in the main asteroid belt 5.8, 7.5, and about 40 million years ago. The rest of discovered meteorites have been found to come from the asteroid Vesta, Mars, the moon, and possibly even small pieces of Mercury and Venus. 

Although meteorites are already very interesting on their own, the biological material found on them only enhance the intrigue that surrounds them. Nucleobases are nitrogen-containing biological compounds that form the fundamental building blocks of the complex macromolecules known as nucleic acids, including DNA and RNA. 

 

Nucleobases are nitrogen-containing, carbon-based compounds that form the fundamental units of the genetic code. The five primary nucleobases are adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U). These bases are essential components of DNA and RNA, with A, C, G, and T found in DNA, while A, C, G, and U are present in RNA. Other than genetic information storage, nucleobases play roles in protein synthesis, cellular metabolism (various metabolic processes, including energy transfer (ATP) and cell signaling (cAMP and cGMP), and enzyme cofactors (enzymatic reactions, such as coenzyme A, FAD, and NAD+). 

While finding nucleobases doesn't quite translate to finding extraterrestrial life, they are biological material that can give us great insight into the formation of more complex biological structures and a firmer understanding of extraterrestrial life. 

 

To form nucleic acids, these nucleobases must combine with specific sugar molecules (either ribose or deoxyribose) and phosphate groups through two types of chemical bonds: N-glycosidic linkages and phosphodiester bonds. N-glycosidic linkages connect the nucleobases to the sugar molecules. These bonds form between the nitrogen atom of the nucleobase and the anomeric carbon of the sugar, creating a nucleoside. 

 

Phosphodiester bonds, on the other hand, link the sugar-phosphate components, forming the backbone of the nucleic acid chain. This combination process requires suitable environmental conditions, such as appropriate temperature and acidity, as well as energy sources. In living organisms, special proteins called enzymes, like DNA polymerase, help facilitate this process. 

 

An important feature of nucleobases is their ability to pair up in specific ways - adenine (A) always pairs with thymine (T) in DNA (or uracil (U) in RNA), and cytosine (C) always pairs with guanine (G). This base pairing is crucial for the structure and function of DNA and RNA. 

 

In modern living cells, specialized enzymes help build these nucleic acid chains. However, when life was first emerging on Earth, other chemical pathways might have existed to join these building blocks together, possibly through alternative mechanisms for forming N-glycosidic linkages and phosphodiester bonds. Some theories suggest that nucleobases and sugars could have emerged from common precursors, potentially simplifying the process of nucleoside formation.

Carbonaceous Meteorites That Have Been Discovered 

A carbonaceous meteorite is any meteorite rich in carbon compounds while a carbonaceous chondrite is a more specific term for a class of stony meteorites that contain carbon compounds and have a particular internal structure characterized by the presence of chondrules (small, rounded grains composed of silicate minerals). 

Several carbonaceous meteorites have been found to contain nucleobases. The most notable of these are the Murchison, Murray, and Tagish Lake meteorites.

These meteorites have provided valuable insights into the potential extraterrestrial origins of life's precursors. In a groundbreaking study published in 2022, researchers used advanced analytical techniques to detect a diverse suite of nucleobases in these meteorites. The study identified both purine and pyrimidine nucleobases, including the canonical base pairs adenine-uracil, guanine-cytosine, and adenine-thymine, as well as some non-canonical ones like isoguanine-isocytosine and xanthine-2,4-diaminopyrimidine. This discovery is significant because it marks the first time that all five nucleobases used in life today - adenine, guanine, cytosine, uracil, and thymine - have been detected in meteorite samples.

 

The Murchison meteorite (a CM2 type carbonaceous chondrite) in particular, has been a rich source of organic compounds. Prior to the 2022 study, it had already yielded seven purine bases and one pyrimidine base. The new research expanded this list, identifying various pyrimidine nucleobases such as cytosine, uracil, and thymine, along with their structural isomers like isocytosine, imidazole-4-carboxylic acid, and 6-methyluracil.

 

These findings suggest that a diverse range of nucleobases could have been available on the early Earth, delivered by meteorites. The similarity between the molecular distribution of pyrimidines in these meteorites and those found in photon-processed interstellar ice analogues indicates that some of these compounds may have been generated by photochemical reactions in the interstellar medium before being incorporated into asteroids during solar system formation.

The spontaneous formation of nucleobases on asteroids and potentially other solid bodies in space is a positive sign that at least these building blocks are not so rare as an early step toward more complex extraterrestrial organisms.

References:

 

Identifying the wide diversity of extraterrestrial purine and pyrimidine nucleobases in carbonaceous meteorites