Being conversant with the understanding and study of the genesis and chemistry of cell is very fundamental and basic to all disciplines under biological sciences. This is not only through the perspective of basic science but also to the numerous advantages in medical diagnosis and prognosis, agriculture, biomedical engineering, biotechnology and many more applications. This is a rapidly growing field of research and application, hence the need to understand its experimental basis and structural makeup.
Cell is the basic structural and functional unit of life of all living organisms. Every living organism may have one cell, two or more cells. There are variations in the cell composition of organisms such as functional variation, morphological and orientational variation. These variations are with respect to the type of organism that possesses the cell, and the specialised functions its components are expected to perform.
Moreover, there are two main divisions of cells; prokaryotic cell and eukaryotic cell. The basis of this division is the attribution or possession of enclosed nucleus and lack of nuclear envelop. The prokaryotic cell has no nucleus with the genetic material naked in the cytoplasm. Also, the cell has simpler and less complex genome with no organelles and cytoskeleton present. This morphological and structural simplicity is limited to the functional requirement of the cell.
However, the eukaryotic cell has a more complex structure and larger genome than the prokaryotic cell. There are membrane bound organelles, an enclosed nucleus separating the genetic material from the free spaces of the cytoplasm. A network of cytoplasmic structures known as cytoskeleton which gives firmness and rigidity to the cell.
Despite all these differences and structural disparities, there are basic similarities which suggest that all present day cells may have evolved or originated from a common ancestry. Amongst these similarities are; similar genetic function and molecular mechanism. These similarities allow certain principles generated experimentally to be inferred and generalised to other cells to some extent. Nevertheless, emergence and existence of life started about 3.8 million years ago. Even though there has since not been a direct experimental reproduction of this occurrence in the laboratory, few other experiments laid down the basis of formation of some steps of the process.
At the first stage of the evolutionary process, it was thought that simple micro-organic molecules could form and spontaneously give rise to macromolecules. The earth, at the time, contained conditions which could enhance spontaneous formation of organic molecules if energy source such as electrical discharge or sunlight was made available. Some of these conditions were abundant presence of Carbon dioxide and Nitrogen gas, in addition to other gases such Carbon monoxide, Hydrogen Sulphide, Hydrogen gas which were present in small amount. This process of spontaneous formation of micro-organic molecules in the presence of the above conditions was demonstrated experimentally by Stanley Miller in the 1950s.
The second stage of evolution was the formation of macro molecules. There was a spontaneous polymerization of micro molecules into macro molecules facilitated by primitive conditions. An experimental example was heating of dry mixtures of amino acids which led to their polymerization into polypeptides. Vitally, the discovery was narrowed down to finding out, from which of the macro molecules could life has arisen. This was with respect to the fact that a life producing macro molecule must possess an attribute of self-replication. Only a molecule with such attribute could facilitate emergence of life and consequently, evolution.
Of all the macro molecules (nucleic acid, protein, lipid, carbohydrate), only nuclei acids have self-replicative quality. They can generate and regulate templates for self-synthesis due to specific base pairing pattern that exist between complementary nucleotides. In 1980s, there was a discovery in the laboratories of Tom Cech and Sid Altman that certain chemical reactions could be catalysed by RNA. RNA could catalyse certain chemical reactions and could regulate self-replication. This initially set the basis for the acceptance of nucleic acid as the genetic carrier of all life on earth. Consequently, the quest to understand and discover the evolutionary source of life began to show off success.
More also, sequential and ordered interaction between RNA and other macro molecules has led to the evolution of present day genetic system known as DNA. This system has taken over the maintenance, and regulation of life of all present day organisms by genetic coding principles. So to say, an organism is able to bring forth its kind because the genetic composition is easily and correctly replicated by the system that governs it. Hence, it is successful passed onto the newly formed organism to enable life to go on.
Eventually, the first cell is presumed to have arisen by enclosure of self-replicating RNA in a membrane which constituted an amphipathic molecule known as phospholipid. Phospholipid molecule contains two sections, a water soluble portion known as hydrophilic head and a hydrophobic tail portion which is water insoluble. This amphipathic molecule constitutes the plasma membrane of all present day cells including both prokaryotic and eukaryotic cells.
More interestingly, if the molecule finds its way in water, there is sudden aggregation into a bilayer, with the hydrophilic phosphate group on the outside and the hydrophobic tail on the inside. Such arrangement gives rise to a firm and stable structure separating two different aqueous media i.e. the interior and the exterior environments of the cell. This enclosure of RNA and other complementary molecules by the phospholipid bilayer might have kept them as a unit and eventually necessitated self-reproductive ability and present day evolution.
To sum up, present day cells initially may have existed in very simple and less complex primitive forms. However, the rapidly changing environmental conditions and the need for adjustment has led to development and evolution into more complex and specialised forms as mostly seen in eukaryotic cells. This was made possible by the genetic coding system associated with the life producing macro molecule (nucleic acid). Eventually, other organelles later developed to enhance specialisation.