The Alien Genome

Craig Venter and Jonathan Rothberg want to send sequencers to Mars.

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A pity the article didn't

A pity the article didn't mention Gary Ruvkun, who has been working on this concept for quite a while.

Many of the comments indicate areas the writer should have covered those areas better: in particular that this is positing a common origin of life and the fact that if that is true, DNA should provide the best ability to distinguish terrestrial contamination from Martian organisms even in the face of extreme terrestrial contamination.

The suggestion to send

The suggestion to send automated DNA sequencers to Mars seems rather audacious at first glance, but it does have merits. With only minor variations, all living organisms on this planet share the same genetic code. For DNA sequencers to work on genetic fragments from extraterrestrial creatures, they would have to use similar chemistry and DNA bases. As argued below, this may indeed be plausible.

On Earth, 20 amino acids are commonly encoded by 61 nucleotide triplet sequences in DNA. When one examines the redundancy of these amino acids, there is some correlation between the distribution of the different amino acids in proteins and their degree of redundancy. For example, the rarest of these amino acids tryptophan and methionine are both encoded by only one nucleotide triplet codon each, whereas the most common amino acids leucine and serine are both specified with six nucleotide triplets each. It has been speculated that the genetic code is optimized to minimize the disturbance of protein structure with genetic mutations. However, if that was the case, one might also expect that there was a relationship between the degree of the interactivity of these amino acids within pairs and their degree of redundancy in the genetic code. For example, arginine, lysine, aspartic acid and glutamic acid, which are critical in electrostatic interactions inside and between proteins, might be predicted to have higher redundancies with their corresponding nucleotide triplets. Yet despite relatively high frequencies of occurrences of these amino acids in proteins, only arginine is specified with 6 codons and the others are encoded with only two different codons. There is unlikely to be significant changes in the genetic code since life started on the Earth, because the viability and reproducibility of the life is critically dependent on the functioning and interactions between thousands of different proteins at once. Any change in the genetic code may be too disruptive.

Over 500 different amino acids are known, but an astronomical number of structures for amino acids are possible. Yet, we see only these 20 amino acids that are ubiquitous in known life. With few exceptions, all of these amino acids are L- isomers, whereas sugars are D-isomer in the Earth's organisms. Basic metabolism appears to be fairly universal with minor variations that arise in extremophiles. It seems that all life on this planet may have arisen when a stowaway bacteria-like organism on a meteorite or chunk of ice collided with and contaminated a once sterile Earth. If it was a meteorite, there is a reasonable chance that it may have come from Mars, or that similar microbe-bearing ice or meteorites rained on both the Earth and Mars. Consequently, it is not so far fetched that oligonucleotide sequencers designed for terrestrial DNA and RNA sequencing might actually work on Mars if evidence of life still remains there.

It is somewhat ironic and humbling to think that a lowly microbe might have been the mother of all life on Earth when so many people on the planet today still look towards gods or ancient intelligent aliens to explain the living world. The discovery of genetically-related life on another world would represent a leap forward in our understanding of our origins from the stars.

Earlier spacecraft have

Earlier spacecraft have revealed that the UV flux at the Martian surface is very high and the Martian soil and may have perchlorate present. Temperatures shifts on Mars are extreme, from -17C to -107C at the Viking sites. Further, initial GC/Mass Spec results from Viking showed an absence of carbon components in the soil at the detection level of those instruments. The CG/Mass Spec on Curioisity is more sensitive, but if it also comes up with no detectable carbon compounds, (especially given the favorable landing site in Gale crater, which looks as if there was water present at some point), it would be hard to imagine that DNA was preserved over hundreds of millions of years in those conditions that could be in detectable amounts. And, on top of all that there is the above set of circumstances that assumes a transfer of bacteria from Mars to Earth, and hence a similar genetic composition.

It would be hard to argue the case for hauling a DNA sequencer to Mars for life detection purposes. I'm not saying it shouldn't be tried, but that would have to be one "clean" spacecraft to insure a positive resukt is real.

A recent publication showed

A recent publication showed that DNA has a half-life of only 521 years (Article about it on http://www.nature.com/news/dna-has-a-521-year-half-life-1.11555 - original paper (behind paywall): http://rspb.royalsocietypublishing.org/content/early/2012/10/05/rspb.201...). This was estimated based on DNA samples obtained from bones of the Moa bird in NZ which has been extinct for several centuries, and the bones having been estimated to be between 600 to 8000 years old. Of course this estimation of DNA half-life is based on samples found under very specific environmental conditions, so there might be more or less favourable situations which have impact on DNA stability. But based on these findings I would be extremely surprised if traces of DNA could be found in soil/rock samples on Mars.