On the Origin of Cells: From Molecules to the First Cell
Simple Biological Molecules Were Formed Under Prebiotic
Conditions
During the first billion years on earth, there was little free
oxygen and no ozone to absorb UV radiation from the sun. Yet, simple
organic molecules were formed under such harsh conditions. Laboratory
experiments simulating the primitive earth have confirmed that organic
molecules could have been formed. When gases such as CO2,
CH4, NH3, and H2 were heated with
water and energized by electrical discharge or by UV radiation, they
reacted and formed small organic molecules. More importantly, the
organic molecules that were crucial to life (amino acids, nucleotides,
sugars, and fatty acids) were also generated.
Experimental Simulation of Prebiotic Condition:
Water is heated in a closed apparatus containing CH4,
NH3, and H2, and an electric discharge is passed
through the vaporized mixture. Organic compounds accumulated in the
U-tube trap.
Polymers Containing Nucleotides Are Capable of Self-replication
Organic molecules such as amino acids and nucleotides can interact
to form polymers. The polymer of amino acids and nucleotides are
called polypeptides and polynucleotides respectively. These polymers
are capable of directing their own synthesis. For example, a
polynucleotide is able to influence the replication of other
polynucleotides by acting as a template.
The complementary nature of nucleotides is crucial in the origin of
life. Because A preferentially binds to U and G preferentially binds
to C, a polynucleotide is thus able to serve as a template for
synthesizing the complementary strand. However, this complementary
templating mechanism only occurs slowly without the assistance of
certain protein catalysts, or enzymes. Although no such enzyme existed
in the "prebiotic soup," certain minerals and metal ions filled in the
role of enzymes. After a period of time, slow replicating systems of
polynucleotides were established.
Replication of polynucleotide sequence. In step 1,
the original RNA acts as a template for making the complementary
sequence. In step 2, the complementary strand of RNA acts as a
template for reproducing the original RNA sequence. This process is
repeated to generate many copies of the original RNA.
Natural Selection Helps to Shape the Self-Replication Systems
Just as a polynucleotide can serve as the template for synthesis of
the complementary strand, specific portions of a polynucleotide can
also pair with free complementary nucleotides within itself. This
self-complementary binding allow the polynucleotide to form specific
structures. Much like proteins, this structure determines a
polynucleotide's stability and ability to replicate.
While polynucleotides can self-replicate, their replication is not
always error free. Errors during replication could influence the
polynucleotide sequence and subsequently result in a different
structure. Therefore, as polynucleotides undergo numerous cycles
of replication, the ones that have received more beneficial mutations
will survive over the ones that received non-favorable mutations.
Polynucleotides Became The Blueprint for Polypeptides
It has been hypothesized that between 3.5 and 4 billion years ago,
self-replicating systems of polynucleotides began the process of
evolution. Different strands of polynucleotides competed for resources
to ensure their own reproduction. Since then, polynucleotides can no
longer depend on slow replication aided by primitive catalysts. To
ensure the accuracy and efficiency of their own replication,
polynucleotides have adapted polypeptides as catalysts to aid their
reproduction. Therefore polynucleotides and polypeptides became
intertwined in their evolutionary struggle for survival. The details
regarding how polynucleotides became capable of directing synthesis of
specific polypeptides (protein synthesis) is still unknown. However, many research efforts are directed toward understanding that
mysterious leap in the origin of life.
The First Cell
In order to ensure the survival of a specie of polynucleotide,
newly synthesized polynucleotides must be retained in the same
area. Also, the polypeptide that was used to facilitate the synthesis
of polynucleotide must also be kept in the same vicinity. To achieve
this purpose, present-day cells are surrounded by a plasma membrane
that envelops the internal organelles of the cell. Current research has
suggested that the first cell was formed when phospholipid molecules
spontaneously assembled into a membrane structure under the primitive
conditions. The oligonucleotides were then able to evolve and natural
selection was able to select the cells with the most efficient system
of replication machineries.
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