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Tuesday
Aug162011

More evidence that we’re all a little Extra-terrestrial

My brain has constructed a little movie that plays in my head when I imagine the origin of life. To make a short movie even shorter, it’s basically just an explosion. The Big Bang = Explosion. Life on Earth = Explosion. Invention of the Internet = Explosion.

I will leave the details of the Big Bang to Sarah Scoles, et al. and attempt to understand the beginning of life on Earth. What are the building blocks of life? Amino acids. What encodes amino acids? RNA and DNA. What makes up RNA and DNA? Nucleic acids. Where did nucleic acids come from? AH HA!

A few months ago I approached the topic of RNA being the first building block to life on Earth, suggesting that RNA has the ability to be self-replicating (replicates without other proteins and without a protective cell). However, this hypothesis leaves many open questions, including the question:

Where did building blocks come from to form primordial nucleic acids: RNA and DNA?

Baby steps, what are nucleic acids?

Nucleic acids are the building blocks of life (DNA and RNA). These funky molecules are found in all life forms (excluding perhaps Iron Lisa’s arsenic life forms), with and without nuclei (including cell organelles like mitochondria AND bacteria and viruses). Additionally, these molecules function by encoding, transmitting and expressing genetic information. Together, chains of nucleic acids encode all proteins from bacterial DNA replication machinery to the neurotransmitters dumped into your synapses that make you happy.

How do they come together to form genetic information?

I’ll focus on DNA (although RNA is pretty f’n cool):

DNA consists of nucleobases [adenine (A), cytosine (C), thymine (T) and guanine (G)] attached to a sugar and a phosphate group. The linear structure of DNA is determined by a phosphate backbone connecting sugars that are carrying nucleobases. These sugars and phosphates are connected to each other in an alternating chain and this gives DNA directionality, which is very important for encoding genetic information.

Together two strands of complementary sequences (Watson-Crick base pairing) results in the formation of a uniform double-helix.

Moving on!

Where did these nucleic acids come from?

What do you start looking at first? Nucleobases – A’s, T’s, C’s and G’s.

Where do you start looking first? Outer-space! More specifically carbonaceous meteorites. “Meteorites provide a record of the chemical processes that occurred in the solar system before life began on Earth”,

Dun DUH:

This is not a new idea, carbon-rich meteorites have been implicated in carrying organic compounds required for the beginning of life for decades (amino acids, more specifically). BUT! Nucleobases in meteorites have not been extensively studied, and all meteorite-derived nucleobases could be explained as the result of terrestrial contamination. The nucleobases found on these meteorites were “biologically common and lacked the structural diversity typical of other indigenous meteoritic organics”. Busted. Additionally, it has been more difficult to study nucleobases in meteorites because of the low abundance relative to other organic molecules, meteorite heterogeneity (space is big, I’m assuming the variety of meteorite is even bigger), and experimental artifacts.

However, this week in PNAS, NASA researcher Michael P. Callahan published the article: Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases. Callahan, et al. looked at extracts from 12 meteorites for the presence, diversity, and abundence of nucleobases, and nucleobase analogs. Using high resolution liquid chromatography-mass spectrometry, they found the Murchison (pictured above) and Lonewolf Nunataks 94102 meteorties contained a diverse amount of nucleobases, including 3 unusual and terrestrially rare nucleobase analogs (purine, 2,6-diaminopurine, and 6,8-diaminopurine). They could not detect these nucleobase analogs in any blanks, control samples, a terrestrial soil sample, and an Antarctic ice sample. They believe they have found a nucleobase analog that is not terrestrial, because they found it inhibited DNA replication (not helpful for life).

Additionally, they found that they could recreate the pattern of presence and abundance of these nucleobases and nucleobase analogs, found on the meteorites, by reactions of ammonium cyanide.

Taken together, they hypothesize that ammonium cyanide reactions within asteroids produced these nucleobases and nucleobase analogs, and with meteorites as vehicles, may have seeded Earth for the emergence of life. The presence of extra-terrestrial nucleobase analogs on some of these meteorites has now expanded the inventory of bases that could have been utilized during the origins of life. This discovery opens a few more doors in researching the first steps that led us to walking up-right and making stuffed-crusted pizzas.





ResearchBlogging.org

Callahan MP, Smith KE, Cleaves HJ 2nd, Ruzicka J, Stern JC, Glavin DP, House CH, & Dworkin JP (2011). Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases. Proceedings of the National Academy of Sciences of the United States of America PMID: 21836052

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Reader Comments (2)

I'm glad it always comes back around to stuffed-crust pizza.

So would extraterrestrial life potentially be based on these terrestrially rare nucleobases? Why were some nucleobases 'chosen' for our replication and not others?

August 16, 2011 | Unregistered CommenterSarah Scoles

I think they believe that extraterrestrial life could have sprung from these rare nucleobases, perhaps these rare nucleobase analogs were more stable on our rather intolerable Earth, and with time as the Earth became more situated & stable the nucleobases changed form.

I don't know the exact answer to your second question, but I would guess that A,T,G,C (and sometimes U) were chosen because they were at the right place at the right time, and this would mean the right environment with the right molecules/pH/salt concentration, etc.

And I would guess that like my RNA post a while back mentioned, this probably is one of billions of molecular events that were happening in the primordial soup that "stuck".

Vague enough? Haha, sorry - I'm just speculating at this point.

August 16, 2011 | Unregistered CommenterBrooke N.

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