The Origin and Evolution of Cell
Cells are
divided into two main classes, based on whether they have a nucleus or not. In
prokaryotic cells, nuclear envelope is absent whereas in eukaryotes it is
present which separates the genetic material from the remaining cytoplasm.
Prokaryotic cells are generally smaller and simpler than eukaryotic cells; in
addition to the absence of a nucleus, they contain extrachromosomal DNA called
plasmid and their genomes are less complex whereas they do not contain
cytoplasmic organelles or a cytoskeleton. In spite of having all these
differences, the working mechanisms of both prokaryotes and eukaryotes are
similar, indicating that all present-day cells are descended from a single
primordial ancestor.
The First Cell
According to
various research, we can say that the first cell came into existence around 3.8
billion years ago, approximately 750 million years after Earth was formed. How
life originated and how the first cell came into being are a matter of
guesswork, as the similar condition cannot be reproduced in the laboratory.
Although, several types of experiments give us important evidence about the
evolutionary process.
It was
suggested in the 1920s for the first time, that simple organic molecules could
form macromolecules by polymerization if provided the condition which is
similar to primitive Earth's atmospheric conditions. At the time life first
arose, the atmosphere of Earth is thought to be oxygen-free, instead of O2,
consisting principally of C02 and N2 in addition to smaller amounts of other
gases too such as H2, H2S, and CO. Such an atmosphere provides reducing
conditions in which organic molecules, when provided with a source of energy
such as sunlight or electrical discharge from lightning, can form
spontaneously.
Stanley Miller’s experiment (1950s)
In 1950's it
was demonstrated experimentally that organic molecules can be formed when
provided with necessary conditions. Stanley Miller when showed that the
discharge of electric sparks into a mixture of H2, CH4, and NH3, in the
presence of water, leads to the formation of a variety of organic molecules,
including several amino acids. As it is merely speculation about the
atmospheric condition present on the primitive earth, Miller's experiments
could not exactly establish the conditions of primitive Earth, but they very
clearly showed the possibility of the continuous synthesis of organic molecules
when provided with the basic materials from which the first living organisms
are formed.
Moving further
towards evolution, the formation of macromolecules was the next step. The
monomeric organic molecules which were the building blocks of macromolecules
have been displayed to polymerize spontaneously under favorable conditions. For
example, When a Dry mixture of amino acids is Heated, it results in a polymerization
process which forms polypeptides. A very important characteristic of
macromolecules is their ability to replicate, which leads to the evolution of
life on Earth. Therefore only those macromolecules which had the ability to
replicate further participated in the process of evolution of life on earth.
It is presumed
that the first cell would have been formed when the
self-replicating RNA was bounded in a membrane made up of phospholipids.
Phospholipids are the components which lead to the formation of membrane, it is
present in the plasma membrane of both prokaryotes and eukaryotes. In the
modern period, it is present in all biological membranes. The important feature
of the phospholipids is that it is Amphipathic in nature, meaning that one portion
of the molecule is water soluble whereas the other portion is insoluble water.
Phospholipids have long, water-insoluble tail group (hydrophobic) hydrocarbon
chains joined to water-soluble (hydrophilic) head groups that contain phosphate
groups. When placed in water, phospholipids arrange themselves into a bilayer
with their hydrophilic group (phosphate-containing) present on the outside in
contact with water whereas the hydrophobic tail (hydrocarbon chain) is present
on the interior side in contact with each other. Such a phospholipid bilayer
forms a stable barrier between two cells or the barrier between cell organelle
and cytoplasm.
The enclosure of self-replicating RNA and it's related molecules in a phospholipid membrane would thus have maintained them as a single unit, which made them capable of self-reproduction which further lead to the process of evolution. It can be said that the synthesis of protein from RNA would have evolved till now, in this case, the first cell would have consisted of self-replicating RNA and their encoded protein.
