Friday, May 27, 2022

The Beginning of Cellular Life on Planet Earth

A Possible Structure for a Primitive Cell- Biology Libre Texts

A necessary first step in examining the evolution of cellular-based life from the pre-biotic world is to discern what constitutes the makeup of cells in the broadest possible terms. In deconstructing cell structure from the viewpoint of prokaryotes, I propose the following necessary prerequisites for a living cell:

· Cell Membrane to delineate the cell and protect it from the local environment

· Cell infrastructure composed of the most basic structural components including actin, myosin, etc.

· Source of readily available energy

· Proteins especially enzymes involved in catabolic and anabolic activities

· Signal transduction pathways that allow for both intracellular and extra-cellular communication.

· Nucleic Acids: DNA and RNA to serve as information stores for the cell.

· Mechanisms for DNA and cellular replication.

Any attempt to propose a mechanism by which primordial cell-like structures evolved into the complex cells that exist today, strongly suggests a gradual stepwise process that took eons to accomplish. It also suggests, in my mind, that the process would involve steps in which cellular organization would grow in complexity from the level of simple molecules (substrates) through proteins and nucleic acids and finally through protein-nucleic acid interaction to the encoding of the genetic material. In this way, selection pressures and processes would enhance each succeeding step.

Taking all this into account, I propose the following model for the evolution of cellular life from primordial beginnings.

Elements of the Hypothesis:

There existed an aqueous environment (possibly shallow ponds or along coastal regions or possibly the sea floor) where there was an abundance of nucleotides, fatty acids, amino acids, peptides and polypeptides. It is possible that some of this organic material may have been seeded by meteorites.
In these organic-enriched regions, conditions were appropriate for the spontaneous formation of cell-like structures.
 
These cell-like structures developed semi-permeable membranes formed from the spontaneous assembly of proteins and lipids (probably a more primitive structure than found in present day cells) and highly permeable to dissolved organic matter in the local environment.
 
The local environment was such that amino acids, nucleotides, fatty acids and carbohydrates could readily penetrate the cell membranes of these primordial cells and concentrate there.
 
Ambient conditions including oxygen concentration, temperature, abundance of ammonia and methane made the spontaneous synthesis of proteins and nucleic acids not only possible but highly likely.
Assuming that spontaneous formation of tRNAs were a likely scenario, these tRNAs could bind to their appropriate amino acids. These amino acid carriers collided with each other and resulted in the formation of random polypeptide chains. Subsequently, polypeptides that were capable of binding to carbon sources such as glucose stabilized these small proteins and gave them a competitive advantage over more non-specific proteins. Since the metabolic pathway for glucose metabolism is universal to all life, one must assume that glucose was abundant in pre-biotic times. This same argument can be applied to the presence of ADP/ATP, since this molecule is the essential ingredient for all energy sustaining life activities.
 
Some of these selected proteins also possessed catalytic capabilities and were able to breakdown carbon rich substrates and ultimately capture energy in ATP molecules. This energy may have been used in accelerating the synthesis of more complex molecules and intra-cellular structures, the precursors of cellular organelles.  
 
There is mounting evidence that strongly suggests that RNA may have played a pivotal role in information storage in the early evolution of cellular life. As I have postulated above, tRNAs may have been abundant. Additionally, evidence for the role of RNA in information storage includes:
 
  • The discovery of RNA that possesses catalytic activity referred to as ribozymes. There is a ribozyme that has been found in the core of ribosomes.   
  • The discovery of small pieces of RNA that can readily bind to a variety of organic molecules and that are found on the ends of mRNA in prokaryotes. These pieces function as switches that can turn translation on or off and are referred to as riboswitches. 
  • Double-stranded RNA that can silence gene transcription in a complex referred to as RISC.
  • What is now referred to as the anti-codon region of tRNA may have been used to make mRNA possibly happening spontaneously utilizing an environment rich in small pieces of RNA or assisted by a ribozyme. These nascent mRNAs served as templates for the further synthesis of specific and biologically valuable proteins. Whether or not such associations are possible today in conditions that simulate the pre-biotic environment would need to be tested. It is possible that “ancient” RNA had a different structure than the current form. This early mechanism was probably inefficient and prone to error.
 
These early cells were infiltrated by a competing entity that gradually assumed a symbiotic relationship and was to become what is now referred to as ribosomes. These structures contributed a much more efficient mechanism for the synthesis of proteins.  In addition, the mitochondria found in eukaryotic cells and the chloroplasts found uniquely in plant cells have their own nucleic acid and most probably were once independent organisms that also assumed a symbiotic relationship with their host cells.
 
Messenger RNAs were no longer able to sustain the growing complexity of cell life as embodied in metabolism and energy transfer mechanisms. A more highly conserved store of information was required. The appearance of an enzyme capable of using mRNA as a template to make highly stable double-stranded DNA encouraged the further development of cellular complexity and evolution. This transition was necessitated by the fact that the extra-cellular environment was no longer as rich in nutrients and building materials as was previously the case. It has become clear that large portions of the genome of humans and other complex organisms are made up of retrotransposons. There are relatively small pieces of DNA that code for reverse transcriptases that allow for copying of these segments and ultimately inserting them in other places in the genome. These were originally discovered by McClintock and referred to as so-called “jumping genes.” Integration of these pieces in the promoter or structural regions of active genes can have profound impacts on gene expression. Certain diseases have been associated with this process. Furthermore, there are retrotransposons that have been conserved among and between organisms. This suggests that the increasing complexity of the genome as seen in evolution may be in large due to retrotransposons. In addition, retrotransposons have many characteristics similar to retroviruses suggesting that retroviruses may have played a significant role in delivering novel genetic material to the genome.
In conclusion, the particular scenario I have outlined represents one possible pathway that may have taken place that was responsible for the evolution of cellular life as we know it from prebiotic conditions that existed on planet Earth billions of years ago.

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