In the last essay (Part IX: Organic Molecules), we left off with the question “Did RNA predate DNA. . .?” The problem with an RNA first scenario is that protein enzymes are required for RNA to be copied from DNA. These proteins should not have been present in an RNA world by itself. For a long time it was thought that RNA could not copy itself. However T. C. Cech at the University of Colorado performed an experiment that indicated that RNA has an enzyme like catalytic activity which theoretically could catalyze its own replication. Finding an organic molecule or group of molecules that can self-replicate gets to the heart of the very origin of life from nonlife. (Curtis, 1989) How could any RNA arise from a nonliving environment? Some believe, in all likelihood, that RNA was not the first self-replicating system.
Regardless of what nucleic acid molecule came first, it would have to have come from a simpler inorganic molecule. The simpler inorganic molecule to more complex organic molecule sequence would have needed an energy source. Furthermore, the larger biomolecules once assembled would have to be protected from extreme environmental conditions. All of these present difficult issues.
There is also great debate over where all of these prebiotic and early biotic materials came from. Darwin liked the “warm little pond” scenario (Darwin, 1859). Stanley Miller and Harold Urey in 1953 performed an experiment in which they added gases that at the time were thought to comprise the early atmosphere (carbon dioxide, ammonia, hydrogen, and methane) in some elaborate flasks and tubes and used an electric spark to simulate lightning as an energy source and succeeded in making some amino acids. The implication here is that the “building blocks” of life might have originated in the atmosphere and supported Darwin’s pond hypothesis (Miller, 1993). Later the composition of the early gas atmosphere came into question and thus Miller and Urey’s hypothesis fell into disfavor. Other scientists have offered deep ocean hydrothermal vents that are warm and nutrient rich as the place where life originated. It has also been shown that some meteorites and comets contain amino acids such as glycine, glutamine acid, valine, and proline. In fact, in 1969 a large meteorite fell to earth in Australia which contained lipids and all five nitrogenous bases found in DNA and RNA (Miller, 1993). As you can see, there are many more questions than answers, or shall we say, more unanswered questions than answered ones.
Let’s take a closer look at the “prebiotic soup” or “warm little pond” hypothesis, also known as the Oparin-Haldane hypothesis model.
- Assemble simple molecules such as water (H2O), carbon dioxide (CO2), Hydrogen (H2), and ammonia (NH3) into complex polymers
- Assemble these polymers (i.e. nucleotides and amino acids) that can store information and catalyze reactions
- Add membranes and an energy source to make a living organism
However, the Oparin-Haldane model is not without its own problems. One is the question of whether liquid water was present on the earth at the time of life’s beginning and another one is that each building block along the way needs to be chemically charged (activated) before it can work in a polymer. Something like a cell membrane would be necessary to concentrate this energy but this step in the process precedes the existence of a cell membrane. Another problem associated with the water issue is that although virtually all biological reactions require an aqueous solution, water also hydrolyzes the products almost as soon as they are made. Could polymers have been made that were long enough to be self -replicating?
This is a good place to pause and remind ourselves that all life forms use proteins (from the same 20 amino acids), DNA, and other important biomolecules such as ATP (adenosine triphosphate for energy storage). The incorporation of cellular contents by a cell membrane and internal membranes surrounding the organelles was a significant step. Cells exhibit compartmentalization allowing chemicals to be localized where they are needed. Chemical reactions can occur in small steps, waste products can be concentrated, chemicals can he pumped outside, and the internal parts and chemicals can be protected. However, compartmentalization and small step chemical reactions means that every step in a biochemical pathway such as photosynthesis, cellular respiration, or even something as simple as digestion requires its own enzyme. The entire pathway may require dozens of enzymes and if only one enzyme is missing, the overall reaction stops. Something as simple as lactose intolerance or as complex as Tay-Sachs Syndrome results from such failures,
In the next essay we shall look at some early life forms such as prokaryotes (ie archaea and bacteria) and eukaryotes and ways of classifying living organisms.
References
Curtis, H. N. (1989). Biology. New York: Worth Publishers Inc.
Darwin, C. (1859). “The Origin of Species”.
Miller, K. J. (1993). Biology. Englewood Cliffs, NJ: Prentice Hall.
