All living things either have DNA, or RNA. So what was the origin of DNA? Did some lightning strike a patch of primordial goo, and nucleic acids, and phosphates just happen to link up? And if that did happen was life created then, or were there just little bits of DNA floating around? Or was life created first with that lightning strike, and DNA created later as a evelutionary advantage? Or was there a lightning strike at all? And where did the nucleic acids come from in the first place? I’ve heard that the first forms of life were RNA based due to the fact that proteins can be made directly from RNA , something which is very useful to life. I know there are no firm answers, but I’d love to hear any theories on the subject. I’m trying to get a scientific answer, no relegious theories please.
As always, could we get a working definition of life/living/alive? I think we can all agree on a def’n of DNA, so I won’t ask.
Or is the general consensus that I should shut up and watch from the sidelines?
The first semblances of life were amino acids (not really life, I know, but it’s where it really began). These acids began joining together to make very, very simple life forms (and, yes, I am generalizing). Eventually, as life got more complex, and eventually multi-celled, it became necessary for each cell to know what the entire group is doing.
I know. Cecil I ain’t (I’m his evil twin, who knows nothing).
Experiments have shown that organic monomers can be created abioticly in the laboratory. The classic experiment combines water vapor, methane, hydrogen gas, and ammonia in a flask. This mixture is hit with a spark of electricity. The result is several sugars, lipids, all twenty amino acids (although my text doesn’t state it, to create methionine some source of sulfer is required), the purine and pyrimidine of DNA and RNA, and ATP if a source of phosphorous is provided.
These monomers will self assemble, although the exact mechanism is not known there are some experimental examples. An amino acid solution dripped onto hot sand or rock will form polypeptides (proteins). The polorimization reacions could have had an inorganic catalyst such as clay or pyrite.
The next step is to isolate these polymers. Phospholipids, which form the cell membranes of every organisim on Earth, will spontaneously for cell like bodies called liposomes when placed in an aqueous solution. If these liposomes formed around some polymers, they could some private chemistry that would not be likely if they floated in the water separately.
These polymers may have done all sorts of chemistry, but the kind our proto ancestors were up to would have been the replicating kind. The liposome polymer partnerships that didn’t self replicate would not have persisted long enough to evolve and change and become more advanced. It is likely one molecule did the replicating and the catalyzing of the replication, because it would only require one special molecule rather than two. One the replicate and one to catalyze. RNA is the most likely molecule.
Proteins don’t replicate well, but a single strand of RNA can provide a template for an opposing strand which can then provide a template for the original strand. DNA doesn’t catalyze reactions well because its helical structure cannot fold into structures that could stabilize reactions. RNA does fold quite well, however, because of the presence of a hydrogen bonding hydrogen on the ribose backbone. Proteins and DNA are specialized at what they do, but RNA can do a little of both. So these proto cells likely were filled with RNA molecules doing some chemistry and self replicating. If the chemistry that the molecules did increased the rate of replication, then that linage began to dominate. Eventually the RNA catalyzed the formation protein, which could in turn do more benifcial chemistry for the proto-cell more efficiently than the RNA. The RNA only produced the protein that was helpful to the cell. Then RNA and protein began producing DNA to store the cell’s information. DNA is more stable than RNA so it was better at long term storage of information. RNA remained as a link between protein and DNA in those cells as it does today in all living cells.
I may have oversimified a bit, but this is the gist of the RNA hypothesis of how abiotic polymers formed the first cells.
Ahhh… but this has an in-built assumption that life requires DNA…
Life, as we know it anyway.
Dr. Lao wrote:
I don’t think this is exactly right. All the experiments that I’ve read about do not suggest self organization. So far, in the lab, we’ve been helping the process along with strands of prefabricated RNA.
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**I don’t think this is exactly right. All the experiments that I’ve read about do not suggest self organization. So far, in the lab, we’ve been helping the process along with strands of prefabricated RNA. **
[QUOTE]
You are right, I couldn’t find any examples of RNA or DNA self assembling. It is a spontaneous reaction for a vial of ATP, CTP, GTP, and UTP to form strands on RNA, it just isn’t very likely to happan on its own. You say that you use a strand of prefabricated RNA. I believe this is called a primer. In that case I’ll assume that you are using polymerase enzymes to create the strands. These enzymes, of course, weren’t available when the first RNA molecules formed. Those strands formed without any active catalyzation (they may have used a substrate, such as clay or pyrite, as a passive catalyst). That sort of self assembly may not be observable on the laboratory time scale. This means that it may take hundreds, thousands, or even millions of years for a strand to form.
I’ve heard of a theory called the seed theory where its theorized that some outer space object (space men, or for the more scientific some space debre) brought primers to earth, and dropped them into the primordial ooze. But that still doesn’t solve the problem of enzyme (DNA polemerase) that needed to be there. I believe what doctor is talking about is PCR (which stands for something I don’t remember) which is way to amplify a specific piece of geonomic DNA. But for that to happen you need geonomic DNA, primers (little pieces of DNA), magnessium, water, and enzyme (DNA polimerase). But DNA, enzyme, and primers wern’t there in the promordial ooze. So I’m back to square one.
Let’s define life as a molecular system that can both replicate itself and evolve. Replication implies some sort of catalytic activity, while the need to store the information intact through generations seems to require a highly stable molecule that doesn’t react easily.
Nowadays, DNA is the stable storage molecule, and proteins provide the catalytic reactions.
But RNA can also store information, like DNA (they are very similar molecules), and can ALSO catalyze certain reactions, including reactions on pieces of itself. It has been hypothesized that the first living molecules were RNA replicases that catalyzed their own replication without proteins, yet stored the information in their own structure, like DNA.
Prebiotic conditions, when simulated, can give rise to RNA oligomers (polymers of the nucleic acids that make up RNA). A randomly constructed RNA replicase, once it occurred, would perpetuate itself and begain to evolve.
Eventually it would have had to associate with proteins to lead down the path of life we see today. That process is actually difficult to rationalize at this point, and is a fruitful one for speculation.
But DNA is essentially a slightly modified version of RNA. It is not hard to see how evolution could have set aside some RNA molecules for catalysis, and other RNA molecules for information storage. In this scenario, the information-storage molecules were then enzymatically modified to make them more stable by removing a hydroxyl group (the missing oxygen in deoxyribonucleic acid), and to mark those molecules as storage rather than active molecules. So if this is correct, DNA arose in an already-living RNA system.
So the answer is, the best guess of current biological thinking is that life preceeded DNA.
Thank you APB9999, that exactly what I was trying to say. I’m glad you came along to give a much more lucid explanation. 
Cartoongod:
I think you are hitting on the biggest problem with the RNA hypothesis. It is fairly easy to imagine how an RNA replicase could work. Scientist believed at one time that RNA was only used in cells as a means of transfering information from DNA to protein. It is now known that RNA is also used in the cell as a catalyst in protein sythesis. The existance of a self replicating RNA molecule shouldn’t be too difficult to grasp. However, the inital creation of the RNA strands is the sticking point. Even under optimal lab conditions in is exceedingly dificult to create RNA molecules without enzymes. Even short RNA strands may be too complicated to make good canididates as the first self replicating molecules.
And you are right, I was talking about PCR. It stands for polymerase chain reaction. However, PCR isn’t a good model for the early self-replicating system, for the reasons you state (no polymerase enzyme, no primers, and no template DNA or RNA strand).
Would this be a good time to mention proteinoid microspheres?
Dr. Lao:
Exactly. I was merely pointing out that your previous post seemed to be saying that you could just throw all of the ingredients into a soup, give a little zap and presto-chango you got yerself some RNA (or DNA). There’s obviously more to it than that, so “self assemble” seems to be a bit generous of a term.
APB:
Hmmm… I’m not sure I completely agree with this conclusion. The best guess is that DNA did not precede a living organism, however this does not eliminate the possibility that DNA and life occured simultaneously - which strikes me as the most likely scenario. Of couse, I guess, once again, it may depend on how you define ‘life’.
tracer:
Have you tried Preparation-H?
So does that mean that the first “life” forms were just strands of RNA floating around which eventually hooked up with other proteins to form more coplicated structures?
How about if we go with a very simple definition of life, anything that can replicate itself. Does that make it easier, or is that too simple?
Essentially, yes. The RNA was probably enclosed in a liposome. A liposome is the structure phosopholipids form in water. They are very similar in size, shape and composition to cell membranes.
Whether you call self-replicating RNA strands living or not is up to you. The point is that they are a good guess about what could have preceded what we know to be living cells.
In reference to Dr. Lao’s first post…
Don’t forget that it is believed that the early-Earth oceans were teeming with all the sugars, amino acids, enzymes, etc. etc., thus rendering random assemblage so commonplace that “self-assemblage” isn’t actually that inaccurate. Of course, it IS still a random assemblage, but the sheer scale would probably have been daunting…
Cartoongod…
Ya, I’ve heard that theory. It’s interesting, and not impossible, though I’ve never really bothered to buy into it, personally. After all, you still have to deal with the problem of life evolving somewhere on its own, so the whole notion of “maybe extra-terrestrials did it” just seems to add an unnecessary complication to the whole thing.
Joeyblades (long time no see, man)…
I agree that “life” and DNA are not both mutually inclusive. In fact, I seem to recall that the earliest forms of life didn’t even have a nucleus, and hence no DNA… but carried a sort of “pattern imprint” that slightly resembled DNA… proto-DNA, if you will. Do you know if this is accurate, or did I just get wrong information, or remember wrongly? (Sorry, no cite… like I said, this just happened to be wafting through my brain at the moment).
SPOOFE:
I’ve been around, I just tend to avoid the Great Debates these days… usually exercises in futility. Occasionally I see a topic that interests me and is in it’s early phases before anyone’s gone off the deep end…
I think you may be referring to early fossilized prokaryotes called stromatolites - they mostly match your description. Check out this link:
http://www.lloydpye.com/essay2.htm
A search on “fossilized prokaryotes” should dig up more links for ya…
I think they technically have DNA, it’s just not organized into chromosome structures.
Ya, thanks (I’ll try to keep my marbles close at hand :D)…
I just remembered where I heard the “sort-of DNA, sort-of not” thing… remember SimEarth? The in-game description of Prokaryotes follows those basic lines.
Yeah, I know… taking my research from computer games… but I did say it just happened to be wafting through my brain at the time.
Don’t forget Cell Theory, which states that all living things are made up of cells. Many don’t consider a virus alive, although it does have DNA and/or RNA. With this definition, nucleic acids had to have formed before they started encasing themselves in cells.
But if I understand evolutionary theory correctly, it’s simply the combination of reproduction with mutation and environmental factors that cause selection. Life is not a prerequisite. E.g., “evolution” begins with the ability to reproduce (along with mutations) and therefore predates anything we’d call life, unless your definition of life is “anything that reproduces”…
ren:
Well, that may be oversimplifying things a bit. The reproductive mechanism must pass on the change and the change must provide a realizable survival advantage to the pre-reproductive parent.
Life requires reproduction, but reproduction does not necessarily imply a living organism.
As long as we’re kicking around the concept, most definitions of life require some sort of metabolism… Many require growth and responsiveness. Personally, I require metabolism, but I’m willing to accept an organism as being alive even if it doesn’t grow or respond to stimulus… actually, I’ve got a relative or two like that…