Origins of Life: The Chicken or the Egg?
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The basic unit of life is the cell (i.e. the prokaryote). What is a cell? A modern cell is a lipid-protein membrane enclosing a volume of chemicals at different concentrations. A cell is very complex. It has a selectively permeable membrane to allow certain molecules in or out. A cell has various metabolic pathways that help maintain cell integrity, and therefore prolong the existence of it's complexity. A simple lipid bilayer is impermeable to polar molecules. On the other hand, a protein membrane is quite "leaky", allowing all kinds of molecules to pass through. A cell must retain certain molecules while removing others. For example, if a cell just had a simple lipid membrane enclosing different chemicals, simple chemical reactions would go to "completion" or to equilibrium. More chemicals would need to be brought in and waste products would need to be removed. It's the classic "the chicken or the egg" paradox. Which came first, the membrane or the cytoplasm? Did a membrane develop first to allow various chemicals in and out? Or were the chemicals concentrated enough to allow metabolism before the membrane developed? Similarly, how did the a genetic system develop? Did the proteins come first or the genetic code? Encapsulation (or compartmentalization) is the isolation of some part of the environment. There are two views dealing with encapsulation and the origins of life. One is the liposome first model, and the other is the protocytosol first model. A typical cell membrane is made up of phospholipids. Phospholipids are a complex molecule, and phosphorus is a limiting element. A cell needs to have some mechanism to synthesize phospolipids to maintain membrane integrity. And, there is the problem of permeability. If an encapsulation event occurred first, the protocell is faced with immmediate starvation, since there would be a limited influx of new materials. Blobel, in 1980, suggested that the first vessicles were empty aggregations of lipids. These aggregations would grow in complexity (e.g. would gain proteins and other components on its surface), and at some point there was an internalization event. In effect, a lipid bilayer forms around a cargo of organics. But, there is still the problem of permeability. Proteins are needed in the membrane to transport various ions and molecules into and out of the cell. One species of bacteria, Halobacterium, indicates that the first membrane might have been made of proteins rather than lipids. Its membrane is about 75% protein (each protein has 7 transmembrane segments). All of the lipid component of the membrane was removed experimentally, and the cell could still function. Pohorille and Wilson in 1995, suggested that the permeability problem may not be so important. They constructed a synthetic membrane using GMO (glycerol-1-monooleate). Now, modern cell membranes are made up of phospholipids and phosphorus is a limiting element on Earth. Just because phospholipid membranes predominate today, doesn't mean that primitive membranes were phospholipid-based. GMO is a small lipid, as it has only 18 carbons in the tail, and it does not have a phosphate group. Ions were able to cross the synthetic membrane passively. The problem is that we don't know if GMO was present on early Earth, however, Pohorille and Wilson showed that it is possible to have a simple lipid membrane. Yanagawa simulated different environmental conditions (e.g. a tide pool, a deep sea vent, and a sea medium) to observe how protein envelopes might form. Under different conditions, different aggregations (spheres) of proteins formed. Yanagawa then described five requirement sfor a viable membrane. It should be made of simple molecules capable of self-assembly (i.e. simple reactions). The boundary is a barrier to the external environment and should also retain water. It should have selective permeabilty (ability to block certain molecules). It should have fluidity (i.e. capable of fussion and fission). And finally, it should have catalytic functions. Protein membranes are "leaky" and they have a small internal compartment, low fluidity (but high stability), and have catalytic functions. Lipid membranes are impermeable (since phospholipids pack closely) and they have a large internal compartment, high fluidity (low stability), and no catalytic functions. What is the ideal membrane? It is one that has the best features from both proteins and lipids. Modern membranes are in fact, made up of both proteins and lipids. In 1979, Day described a membrane model for the generation of a lipid-protein membrane. It suggests that a protein membrane acted as a platform in which passing lipids could be incorporated. Fluidity eventually increased, while permeability decreased. A membrane with high permeability would soon be selected against. Therefore, over time, a membrane comparable to the modern version would develop. It may be possible that a lipsome developed first, but again there are the permeability problems and a requirement for phospholipid synthesis. Also, the development of genetic system and a metabolism might be difficult to reconcile. The other view is the protocytosol first model proposed by de Duve. This model holds that there was no need for an early encapsulation event. Instead, a prebiotic metabolism developed first, and only later was there encapsulation. With this model a genetic system could evolve and all the necessary components for metabolism would be in place for a late encapsulation event. Why would there be encapsulation at all? Compartmentalization would be an advantage when either an energy system develops that requires electron tranpost across a membrane (e.g. ATP formation), or when there is some selective advantage with a membrane once it sustains self-growth. Why are cells so small? A large volume to surface area ratio would allow for more faster reactions, thus evolution of biochemical pathways would be rapid in a smaller compartment. All modern cells have a genetic system. The genetic system is very complex, as it requires various proteins and RNA for DNA replication and synthesis of other proteins. One view developed in the 1980s, is the RNA World, by Gilbert (1986), which suggests that RNA was the first genetic system. This view was propounded by thhe discovery self-replicating RNA molecules (transposons), and the discovery of RNA molecules with enzymatic functions (ribozymes). The idea that RNA could have been one of the first genetic systems, was postulated by Francis Crick in 1958. The stages in the evolution of a genetic system would go as follows: (1) RNA molecules catalyse reaction for self-assembly, (2) an early translation mechanism emerged where RNA directs protein synthesis, after which proteins would predominate for enzyme functions, and (3) DNA would evolve to store genetic information, since DNA is more stable than RNA. There are, however, two main problems with the RNA first model: phosphate is limiting on Earth (phosphate is needed for phosphodiester bonds in RNA and DNA), and ribose would also be limiting (ribose is the sugar used in RNA, deoxyribose is the sugar in DNA), so some mechanims would be needed to synthesize ribose. Several classes of molecules have been suggested to be possible RNA precursors. PNA (peptide nucleic acid) is an antisense agent used in clinical studies that can form peptide bonds with between bases. The question is, how would PNA be made in prebiotic times? Another molecule, p-RNA or pyranosyl-RNA, seems to be the more likely candidate as an RNA precursor. p-RNA does have a phosphate group, the of which comes from GAP (glycoaldehyde-phosphate). In formose reactions, GAP plus formaldehyde yields about 33% ribose-2,4-diphosphate. GAP can also self-condense in the absence of formaldehyde, yielding 46% hexose (or a variety of six-carbon sugars). There must be a selective removal of a phosphate from ribose-2,4-diphosphate to yield p-RNA. This is not an energetically favorable reaction, and would require an enzyme or some catalyst to do this (enzymes would not be present in prebiotic times). If p-RNA is made from GAP, GAP needs to made somehow in prebiotic times -- i.e. some source of phosphate would be needed. That source of phosphate could have been from phosphonic acid. Phosphonic acid is made from ethyl-phosphonic acid, which happens to be found in meteroites. So, now we have a potential source of phosphate, and a possible mechanism for the synthesis of p-RNA, which could be a precursor to RNA. Compared to DNA and RNA, p-RNA interstrand interactions is much stronger, the linkage is energetically favorable, and only Watson-Crick base pairs (i.e. Adenine with Thymine or Uracil; Guanine with Cytosine) are allowed, due to molecular geometry. In the 1970s, Cairns-Smith developed a possible mechanism that would have inorganic RNA precursors, using clay. An organism that uses RNA as a genetic system must already be highly evolved (i.e. it has to synthesize ribose, and catalyse unfavorable reactions). Therefore, some primitive system gave rise to the modern genetic system. There are two types of genetic material. Primary genetic material which does not require prior evolution, and secondary genetic material, which does require prior evolution (such as DNA and RNA). Genetic takeovers occurred when RNA replaced the primitive genetic system, and when DNA replaced RNA. Cairns-Smith described two types of genetic crystals. Imagine a crystal shaped like a cube -- i.e. it has six faces designated as A, X or Y. Crystals grow by surface nucleation, but with the A face growth inhibited. Information can be stored in crystals by simple orientation and alignment, yielding some ordered sequence. Type I crystals have four-directional growth. Type II crystals have two-directional growth. Replication occurs by cleavage of the crystals, and then later reconstitution, with the information being maintained. So, the crystals can split and grow, which could have given rise to a primitive mode of peptide synthesis. In order for that to occur, two mechanisms are required: (1) some mechanism that attracts specific amino acids, and (2) some mechanism that links the amino acids together. In modern cells, ribosomes attract specific mechansims for peptide synthesis, so in prebiotic times, some sort of protoribosome would be needed. Clay could have been components of the protoribosome. Cairns-Smith defined clay as "any microcrystalline substance (less than 10 micrometers) that can form from an aqueos solution near the Earth's surface." As demonstrated by Miller's experiments, certain amino acids (glycione and alanine) would have been abundant. However, these amino acids might not have necessarily been used in early proteins. Proproteins could have been as simple as alternating residues of "phobine" and "philine" (i.e. hydrophobic and hydrophilic amino acids), which could have formed a bilayer (as an early membrane). So, you can now see that there are many theoretical possibilities as to how life could have originated on Earth. Panspermia is the theory that spores originated on another planet -- Mars perhaps? or a planet from another galaxy? -- and travelled through space to seed Earth. Life could have developed from protocells that either originated in a "primordial soup" or in the atmosphere; or a protocytosol could have developed first and then a late encapsulation occurred to give rise to life. A genetic system must have developed somehow -- probably not starting with RNA -- and several genetic takeover events have occurred since. Some sort of concentrating mechanism (i.e. a protoribosome, perhaps clay) is required to bring in molecules that are capable of generating self-replication and metabolism. Membranes can form from various materials (most importantly, protein and/or lipids). At the moment, now single theory stands out as the most "correct." We just can't know for certain which series of events actually occurred on Earth, to generate life. However, in another section I will try to develop a scenario that I think seems likely.
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