Theoretical origin of life; Lack of code bases (Introduction)

by David Turell , Thursday, November 10, 2016, 20:45 (2935 days ago) @ David Turell

What might arrive from space by comet or meteorite to supply the base code for RNA and then DNA. Not everything:

https://blogs.scientificamerican.com/guest-blog/did-the-seeds-of-life-come-from-space/?...

"To investigate whether our own nucleobases could have been delivered to Earth inside meteorites, Pearce started by examining the organic content found in meteorite falls.
Nucleobases come in five flavours: guanine (G), adenine (A), cytosine (C), thymine (T) and uracil (U). Pearce confirmed three of these were commonly listed in the meteoritic record, but there was no hint of cytosine or thymine. Intriguingly, these missing members are the complementary pairs to the discovered nucleobases, with guanine pairing with cytosine (G-C) and adenine pairing with thymine (A-T) in DNA.

***

"Using comet composition as a starting point for their model, the scientists calculated the expected yields of the five nucleobases that make up our genetic code.
What they discovered was that while four of the nucleobases formed in measurable amounts, the elusive cytosine was missing. In truth, cytosine was created. However, it rapidly decayed within a few years to produce the more commonly found uracil nucleobase and ammonia. This meant the chances of finding cytosine in meteorite samples were practically zero.

"'It's not that we simply haven't found any cytosine in meteorites,” Pearce exclaimed. “It seems that it can't be found!"

"While this closed the case on the missing cytosine, the other absent nucleobase, thymine, seemed to be produced in detectable abundances. So why is it never seen?
It turns out that thymine decomposes in the presence of hydrogen peroxide; the same chemical in bleach and disinfectant. Hydrogen peroxide has been spotted in comets, making it a potential culprit for destroying any thymine that was formed.

"These models explain the presence of only three nucleobases in the meteorite samples, but they leave a clear conundrum: if our genetic code needs five nucleobases, where did the missing two partners come from?

"These models explain the presence of only three nucleobases in the meteorite samples, but they leave a clear conundrum: if our genetic code needs five nucleobases, where did the missing two partners come from?

"It is a question that still lacks a satisfactory answer. It is possible that these life seeds began on Earth, although this presents a number of tricky problems. Our early atmosphere was inhospitable for creating organic molecules, while our oceans run the risk of also producing only three nucleobases. A promising option is that the sun’s ultraviolet rays triggered the formation the organics on dust grains within our Solar System, which were then captured by the Earth’s pull.

“'This is a big question!” Pearce concludes. “And at the moment, we don’t know the answer.'”

Comment: Some truth about the problem for those who think life occurred naturally. It was rocky Earth that had no organic molecules at first and needed specific ones to start life. Still looks miraculous.