It is significant to understand how life starts. The origin of the primary cell is an event of low probability, as a complete series of events would have had to take place- however this length of time is long adequate for an event of low probability to have occurred. Nowadays we don't believe that life arises spontaneously from nonlife and we state that 'a life comes from a life'. Though, the very first living thing had to have come from the non living chemicals.
Origin of life:
The planet earth came into existence 4 and 5 billion years ago. Life evolved on planet earth around 3.5 billion years ago. At that time it was very hot. The existence of life in any form at that high temperature was not possible.
Therefore, life two questions arise relating to:
1) How did life originated on the earth?
2) How did primitive organisms develop into new forms resultant in the evolution of a variety of organism on the earth?
Probable phases in the origin of life:
1) First phase (the primitive earth):
Theorized early primitive atmosphere comprised mostly of water vapor, nitrogen and carbon-dioxide having small amounts of hydrogen and carbon monoxide with little if any, of free oxygen.
2) Second phase (evolution of small organic molecules):
A specific mix of inorganic chemicals could have reacted with one other to produce small organic molecules (or compound) like glucose, amino acids and nucleotides. Most of the chemical reactions occur in water and the first proto cell undoubtedly arose in the ocean. In the year 1953, Stanley Miller and Harold Urey performed an experiment termed as the (that is, Miller-Urey experiment) which supports the hypothesis that small organic molecules were formed at the ocean's surface. In the early earth, volcanoes erupted continually and the first atmospheric gas would have therefore contained methane (CH4), ammonia (NH3) and hydrogen (H2). Such gases could have been washed into the ocean by the first rains, fierce lightning and unabated ultraviolent radiation would have let them to react and produce the first organic molecules.
3) Third phase (macromolecules):
Once formed, the primary small organic molecules gave rise to still bigger molecules. Then these would have polymerized to macromolecules. There are three primary hypotheses regarding this phase in the origin of life.
a) RNA first-hypothesis: This hypothesis recommends that only the macromolecule RNA (ribonucleic acid) was required at this time to progress toward the formation of the first cell or cell(s). Such RNA molecules are termed as ribosomes as we know that ribosomes exist.
b) Protein-first hypothesis: This hypothesis recommends that amino acids collected in the shallow puddles all along the rocky shore and the heat of the sun caused them to form protenoids, small polypeptides that encompass some catalytic properties. If protenoids are returned to water, they form the properties of a cell.
A few of these proteins could have had the enzymatic properties.
The third hypothesis is put forth through Graham Cairns-Smith. He assumes that clay was particularly useful in causing the polymerization of both proteins and nucleic acids at the similar time. Clay fascinates small organic molecules and has iron and zinc, which might have served as inorganic catalysts for the polypeptide formation. Moreover, clay tends to gather energy from radioactive decay and then release it if the temperature or humidity changes, possibly giving a source of energy for polymerization. Cairns-Smith recommends that RNA nucleotides and amino acids became related in such a manner that polypeptides were organized by, and helped synthesize, RNA. Chemical reactions probable generated the macromolecules we relate with living things.
4) Fourth stage (the protocell):
As macromolecules formed, somewhat similar to a modern plasma membrane was required to separate them from the environment. Therefore before the first true cell arose, there would probably have been a protocell, which could carry on metabolism however not reproduce, formed when lipids and microspheres made a lipid - protein membrane. It has been recommended that the protocell likely was a heterotrophy, an organism which takes in preformed food.
The Heterotroph Hypothesis has been recommended that the protocell probable was a heterotroph, an organism which takes in performed food. Throughout the early evolution of life, the ocean contained plentiful nutrition in the form of small organic molecules. This recommends that heterotrophs proceeded autotrophs, organisms which make their own food.
Once the protocell was able of reproduction, it became a true cell and the biological evolution start.
5) Fifth stage (The true cell):
A true cell is a membrane-bounded structure which can carry on protein synthesis to generate the enzymes which allow DNA to replicate. The central theory of genetics illustrates that DNA directs protein synthesis and that information flows from DNA to RNA to protein. It is probable that this sequence developed in phases. Once the protocell acquired genes which could replicate, they became cells capable of reproducing, and biological evolution start.
The hypothesis that the origin of life followed a transition from small organic molecules to the macromolecules to protocell to true cells is presently broadly favored by the scientists.
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