A, G, U and X

19 02 2012

Ngan said to Lindenmeyr: “Let me show you what we’ve learned so far.”

He moved away from the trough of methanogens and retrieved a tablet from his workbench. The tablet awoke from hibernation, and, in the reflection on Ngan’s faceplate, Lindenmeyr saw a series of brightly coloured images, rotating in imitation of three dimensions.

“If you’ll indulge me,” Ngan began, “I might ask you a question. How do you imagine alien life? How do you imagine something entirely new?”

Lindenmeyr paused for a moment.

“Is this a philosophical question?”

“Oh yes,” Ngan replied. “Imagine, if you will, a new colour. A colour no one has even seen before. Or imagine a taste you’ve never tasted. Anything you imagine in your mind is based on what you know, what you’ve experienced, what is familiar.”

“Of course.”

“We can’t possibly imagine something entirely new and different. And if we saw it, we likely wouldn’t recognise it.”

Lindenmeyr cleared her throat. “Like Stanislaw Lem.”

“Exactly.” Ngan returned to the trough of methanogens with the tablet. “The likelihood that any kind of life that evolved away from Earth would be recognisable to us, much less look like humans with pointy ears, is infinitesimal.”

“And yet we recognise these methanogens.”

“Oh yes. And here we return to that philosophical question.”

Ngan explained that the basic building blocks of life as humans had experienced it were readily and widely available in the Universe. Organic compounds such as hydrocarbons and amino acids were found in comets along with methanol, formaldehyde, ethanol and ethane, even hydrogen cyanide. The emergence of life was not some religious miracle, but rather a simple matter of chemistry – the interaction of methane, water, ammonia, hydrogen and the creation of amino acids, the building blocks of proteins.

Astonishingly, more complex nucleobases could also be formed on meteorites, asteroids and comets. Together with amino acids, these nucleobases could under the right conditions evolve into complex proteins, nucleotides, DNA.

Ngan showed a series of slides to Lindenmeyr who, although not an expert in organic chemistry, recognised the association of molecules of hydrogen, nitrogen, carbon and oxygen into an amino acid.

“So we took it back to the beginning,” Ngan said. “Because the end product is so different, we go back to the building blocks that the methanogens must have started with.”

“Ah. Now I see the point of your question. Here is the experience we use to recognise the alien.”

Ngan nodded.

“To continue with the ‘building blocks’ analogy, we figured that all life starts with the same materials and then goes about building different shapes, forms, assemblies.”

He flipped to another slide that showed a complex structure of tangled lines branching from a single curved strand. Where the tangles were clustered they grew away from the thicker strand, and bunched together like fruit on the limb of a tree.

“What is it?” Lindenmeyr asked.

Ngan chuckled. “Oh, this is the only macromolecule that composes the methanogens.”

Ngan continued and grew more animated as he explained. When tested, the methanogens had not demonstrated chirality because they were composed of neither proteins nor DNA. But here, rotating in false colours, was their analogy for both.

“It is somewhat similar to RNA,” Ngan said. “Essential for all life on Earth. In fact, viruses use RNA for genetic material. But it’s not RNA. We don’t know what to call it. We liken it to RNA only because we need that anchor of familiarity. It is only similar to RNA in so far as both are single-stranded molecules with shorter chains of nucleobases, that in turn produce some quite complex three-dimensional structures.”

Lindenmeyr turned from the spinning macromolecule on the tablet screen to the methanogens arrayed in a line between her and Ngan.

“Wow,” she managed.

“Oh yes,” Ngan replied. “At the moment, we’re half-jokingly calling it FNA.”

“FNA?”

Ngan grinned. “Framnucleic acid.”

And, arrayed around that single-strand backbone, were many of the building blocks seeded throughout the Universe: the primary nucleobases of adenine, guanine, the A, and G from DNA-based life, along with uracil, the U found in RNA; and the modified purine bases xanthine and hypoxanthine. Of these four nucleobases, FNA clustered into groups of two, rather than the groups of three into which DNA clustered.

“Actually,” Ngan said, “the simplicity of FNA is more akin to very, very early precursors to DNA than RNA as we know it. Say, four billion years ago. The precursor used only two nucleobases and a handful of amino acids, and worked well long before life evolved the triplet code it uses now. These doublets seem to work well for such a simple lifeform.”

Conspicuously absent from the image of the FNA macromolecule were thymine and cytosine, two of the nucleobases of DNA. Lindenmeyr asked Ngan why C and T were missing from FNA.

“We have a theory about that,” he explained. “Thymine and cytosine bonds are most susceptible to damage from ultraviolet light; in fact, most skin cancers from exposure to ultraviolet are a result of a thymine dimer, where ultraviolet photons damage the bonds between nucleobases and distort the macromolecule. Because of the direct exposure to ultraviolet light, we think the methanogens have evolved without thymine and cytosine.”

“A clever adaptation to the environment,” Lindenmeyr ventured, “but it cripples their genetic complexity.”

“Oh yes. Unfortunately, the methanogens won’t teach us a new way to deal with ultraviolet light; they’ve simply evolved away that part of them damaged by UV.”

Ngan brushed through the next slides. The absence of proteins dramatically simplified the process of replication, he explained. The FNA in the methanogens did not appear to articulate with a Fram version of ribosomes, and so did not communicate instructions to assemble amino acids into proteins through protein biosynthesis.

But it was so much easier to describe what life on Fram didn’t do, rather than what it did do. This difficulty was related, Ngan said, to his earlier comments about conceptualising ideas through the familiar.

Nonetheless, early work suggested that the FNA contained some amount of genetic information, but rather than communicating that information to assemble cells, the FNA duplicated itself in a manner similar to a virus; it did so, however, without a host cell. This duplication was in part related to the complex structures that the single-strand backbone of FNA allowed. The form of that structure was repeated in each duplication – limiting the opportunity to evolve, but allowing for very durable structures once natural selection identified a viable arrangement of nucleobases.

“These methanogens don’t so much ‘grow’ as they ‘self-copy,’” Ngan concluded. “We still don’t know how FNA forms these fronds, in the absence of both proteins and cells.”

Lindenmeyr turned over the fronds in her hands.

“Is it life?”

Ngan paused. “Yes and no. They are subject to natural selection, as evidenced by the absence of thymine. They possess analogues of genes. But they grow through self-assembly, rather than cell division.”

“Life,” Lindenmeyr said again. “Wow, I don’t even know how to communicate what I’m trying to say. I mean, we’re life, you and I, and we evolved from amino acids and nucleobases, and we go out into the Universe and we find these methanogens, and we stand here in this room and…and life asks if life is life.”

Ngan chuckled.

“Oh yes. I think of it like this: spread around the Universe are kits containing all the parts to make something. But there are no sets of instructions, no recipes, in these kits; not even someone or something to assemble the parts.”

Lindenmeyr nodded. “That’s the magic, I think. As best they can…the kits assemble themselves.”

Ngan spread his hands.

“And here we are.”

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One response

23 02 2012
Peter (@PeteTongLaw)

Thanks for the shout out. I’m still following the saga.

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