I want someone who believes in evolution to explain to me how a DNA double helix is supposed to have evolved. I have heard some people say that DNA evolved from RNA but that begs the questions, how and why? The only organisms that have RNA and not DNA are retroviruses, which need the DNA of another organism to reproduce, so they couldn't have been living long enough for the necessary mutations to evolve. Also, how does this material evolve to become double-stranded instead of single-stranded? Why the switch to DNA, since it can be argued that RNA would be a better template? It seems to me that DNA screams intelligent design.
Iíll have a stab at this one, although Iím probably not too up to date with current thinking so I might not get it quite right (but Iím sure theyíll be helpful individuals out there who will correct me if I do stray ).
Youíre half right about RNA, itsí a damn useful molecule: it can fold in all sorts of shapes to provide all sorts of functions but one thing that it doesnít do very well is act as a template. The biggest advantage that DNA has is that, because it forms a double helix, it is a more stable form of storing the sequences of bases required for this variety of activities. There is not too much difference at all between the chemical structure of RNA and DNA: a missing oxygen molecule and a slightly different base! So we have a distinct advantage and a relatively small change to get that advantage Ė no need to invoke the magical intelligent designer at all, good old selection comes into play.
Imagine a scenario in which a self-replicating RNA molecule has a sequence which gives it an advantage (enzymatic activity or help with replicating for example) and during the course of events in its environment the ribose on the template strand turns into deoxyribose. Because the storage material is now more stable, it can still produce the ribozymes (or whatever) but lord it over the other RNA molecules, so it will produce more of them Ė another advantage. Soon RNA on itsí own wonít stand a chance and the only thing you have is RNA sequence being carried on DNA: essentially what we still have in living organisms.
The latest Discover Magazine has an article on RNA. They are attempting to create a primitive life form. They suggest they might have it in 3 years. According to these guys there isn't much left to sort out.
If this is to be believed the abiogenesis question isn't as unanswerable as I thought.
The problem with something in Discover Magazine is that is still a designed experiment. Leaving something to sit in a test tube and never doing anything to it will never lead to life. Yes, I am aware of the experiment that is supposed to have replicated 'initial conditions' on Earth that got some amino acids to form, but it only got about 8 out of 22. Not a very good ratio for an experiment that was still designed. I think the key word in one of the posts is 'imagine.' Where does this magical self-replicating RNA molecule come from? And if DNA is more stable, why do we still have RNA, and why can't DNA do everything RNA can if DNA is more evolved? RNA can serve more functions. The simple fact is that DNA is useless without RNA--because DNA is not self replicating because it has no enzyme activity, and RNA cannot replicate itself without DNA either. Both must have come into existence at the same time. Therefore, it must have been created. What is the simplest organism you can think of? Probably some kind of virus, which still has how many genes? The smallest one I have ever heard of still has a few hundred genes. The 'simplest' organism on Earth is still far to complicated to ever have evolved step-wise.
If they do what they are trying it shows that there is not "magic" line between life and none life.
It will supply one pathway for life to arise. I don't know if we will learn enough to know for sure that any possible pathway is the one that actually happened.
As noted, they might already have enough information to show that the path is possible.
They are attempting to use steps that are plausible in the natural environment.
Many theologians have warned the creationists and others how bad the theology of god of the gaps (GOTG) is. This is why. This gap will be closed in a number of years. Many, many such gaps have been. The lesson should be to never, ever argue on a basis of what is not know.
and RNA cannot replicate itself without DNA either
Don't make statements about areas you know little about or be so dogmatic on an area that is simply unknown [i]for now[i]. It seems that this might not be true. I'll quote selected pieces of the article when I have more time.
This message has been edited by NosyNed, 05-13-2004 08:51 PM
Yes, I am aware of the experiment that is supposed to have replicated 'initial conditions' on Earth that got some amino acids to form, but it only got about 8 out of 22.
If you are refering to the Miller experiment, you are half right. The original experiment only turned up about 7 or 8 amino acids. But later experiments by other scientists using the same procedure were able to produce all 20 (not 22) amino acids necessary for life.
This message has been edited by Lam, 05-13-2004 10:36 PM
Where does this magical self-replicating RNA molecule come from? And if DNA is more stable, why do we still have RNA, and why can't DNA do everything RNA can if DNA is more evolved? RNA can serve more functions. The simple fact is that DNA is useless without RNA--because DNA is not self replicating because it has no enzyme activity, and RNA cannot replicate itself without DNA either.
I remember writing about the same thing. You can find it Message 42. I will quote myself on the RNA section only.
quote:The hypothesis suggests that the first replicating organic material were short strands of RNA. Laboratory experiments have shown that nucleotide monomers can naturally assemble into RNA molecules without divine intervention. This process happens without the presence of cells or enzymes. As you can imagine, the result is a pool of RNA strands. Now, what scientists have also observed is that some of these RNA strands actually self replicate without any help from anything whatsoever. Again, no divine intervention necessary.
With that said, who said RNA can't replicate themselves?
This message has been edited by Lam, 05-13-2004 10:45 PM
The answer to this question lies in a scientific accident.
The origin of DNA, which has been scientifically tested, proven, and repeated, is found in the environment of the Archean Eon, 4000 mya-2500 mya. The atmosphere at this time was mainly due to volcanic outgassing, composed mostly of water vapor, carbon dioxide, methane, ammonia, and a variety of other gases. Also, there is evidence of surface water, a higher surface temperature than is found today, and a higher level of ultra-violet radiation. These factors, when combined, result in the spontaneous creation of simple monomers, including amino acids. When dehydrated, such as when the surface water is evaporated from the higher level of heat, polymers can spontaneously form, inluding proteins and DNA. This theory was tested in what is now referred to as the Miller Experiment; gases were pumped into a sealed glass container, along with water. The glass container was then heated and bombarded with UV radiation. Later on, amino acids were found to have developed on the sides of the glass. Then the accident occurred; someone left the heat on the glass too long, and the water evaporated. When examined, however, the amino acids that had collected on the inside of the glass had spontaneously polymerized into primitive DNA.