Evolution of First Cell | Biology

In this article we will discuss about:- 1. Evolution of First Cell 2. Origin of Cell Membrane in First Cell 3. Evolution of Metabolism 4. Prokaryotic Cell.

Evolution of First Cell:

Early living cells were RNA life forms, self- replicating RNA covered by lipoprotein vesicles were the pre-prokaryotes, with time the proteins replaced the catalytic function of RNA, and DNA replaced the coding function of RNA, the progenitors of modern prokaryotes with DNA-RNA-protein functioning types evolved.

Evolution of first primitive cell from RNA world represents a huge gap. Primitive bacterial cell represents an immensely complicated struc­ture with at least 1000 genes in comparison with our ideas about RNA world.

Following problems need to be solved to fill this gap:

1. Dominating role of protein as enzymes over ribozymes.

2. Differentiation of different types of RNA.

3. The shift from RNA to DNA as carrier of genetic information.

4. Origin of genetic code.

5. Formation of chromosome.

6. Increasing genetic information.

7. Phenotypic expression of a genotype.

8. Origin of cell-membrane.

9. Evolution of metabolic process.

Proteins as Enzymes:

Introduction of pro­teins as enzymes resulted in more specific cata­lysis. Enzymatic capability of the RNA strands could be improved if individual amino acids were attached as in tRNA, i.e., amino acids acted as co-enzymes for the ribozymes. The next step is the specialization of RNA so that ‘+’ strand had the role as mRNA and ‘-‘ strand functioned as tRNA and attached to the ‘+’ strand with an anti-codon triplet.

Finally, the amino acids could be coupled together as a polypeptide strand that would further improve catalytic activity. This idea is supported by the fact that tRNAs from different organisms with similar function are more closely related and thus tRNA can be traced back to the origin of the genetic code and to a RNA world.

Differentiation of Different Types of RNA:

Functional specialization of different RNAs was adaptive in increasing the efficiency within proto-cells. Some kinds of RNA (tRNA) specialized in collecting amino acids and others (rRNA) in cou­pling them together are the basis of the code in a third kind of RNA (mRNA).

From RNA to DNA:

Emergence of complex organisms requires the transition from RNA to DNA as genetic material. Double stranded DNA is much more stable than RNA and allows enzy­matic proof-reading and correction in connection with replication and thus reduces the rate of mutation (Fig. 2.10).

The genetic information in RNA organisms corresponds to a maximum of 10 thousand base pairs in comparison to 10 million base pairs in bacterial chromosome.

Replacement of ribose by deoxyribose in the carbohydrate backbone of RNA and replacement of uracil base by thymine resulted in DNA. Deoxyribose is formed in cells through an enzymatically con­trolled reduction of ribose. Enzymatic synthesis of DNA from RNA by reverse transcriptase in RNA virus is well known today.

Origin of Code in First Cell:

Genetic code based on four bases expressed in triplets with redundancy for twenty amino acids is almost universal. Though there is no chemical relationship between the mRNA codon or anticodon of tRNA and the chemical structure of amino acid, but the speci­ficity of given tRNA to a particular amino acid has developed. All these features minimizes the risk of replication errors and rate of point mutations.

Formation of Chromosome:

Free floating RNA molecules once enclosed in a membrane would become adaptive to have genes linked together in a single chromosome. Different kinds of free-floating RNA molecules replicated inside their proto-cells undergo unequal distribution in daughter cells after division of proto-cell with reduced fitness.

This might be overcome by con­necting the RNA molecules into a single strand combined with simultaneous replication which results in equal distribution of genome between daughter cells.

Increasing Genetic Information:

Genome size gets increased with increasing complexity from a couple of genes in virus to 1000 in bacte­ria, 5000 in fruit-fly, and 30 000 in human or higher plants, but not associated with a drastic increase in the number of translatable genes.

The important mechanism of increasing genetic information is gene doubling followed by muta­tion and selection leading to production of new enzyme and biomolecules. Natural horizontal gene transfer as found in bacteria (transforma­tion, conjugation, transduction) could lead to increase in genetic information.

Phenotypic Expression of a Genotype:

Though genes are often correlated to certain phenotypic traits but genes only specify proteins/ enzymes. Variations of a gene (alleles) can have effects on the phenotype through variations in the specified protein. Actually the production of a given phenotype is the result of network of interactions between genes and enzymes and between different enzymes which is far too com­plex to be un-ravelled.

Origin of Cell Membrane in First Cell:

Spontaneous for­mation of molecular double layer on the water surfaces by lipids served as a model for the ori­gin of double layer phospholipid cell membrane. This is due to hydrophobic (mutually attracted) and hydrophilic (attracted to water) end of linear-molecules (Fig. 2.11).

If the lipid films form spheres, the hydrophobic ends are hidden inside the film attaining lowest energy state. Phospho­lipids are easily formed in the presence of lipids, glycerol and phosphate and such spheres can be made experimentally through shaking and sonication.

Constant addition of mass to the contents of the spheres and to the membrane results in budding and division of cells. Residence of most vital functions (energy metabolism, transport channels) of the cell in the cell membrane is based on a variety of embedded protein (Fig. 2.12).

Evolution of Metabolism in First Cell:

The fundamental types of energy metabolism are photo-trophy, respiration, fermentation, metanogenesis; all of which are represented among the bacteria. Dissimilatory energy metabolism (catabolic) refers to the mechanism to generate ATP with high energy rich phosphate bonds (Fig. 2.13).

Assimilatory metabolism (anabolic) refers to metabolic processes that serve to build the com­ponents of the cell from chemical compounds of environment through phototrophic (photosynthe­sis), chemotrophic (chemosynthesis) or hetero­trophic modes. The process of energy meta­bolism are based on coupled redox processes of the type AH2 -1- B BH2 + A.

The important hydrogen carrier found in cell is NADVNADFH or its phosphorylated version NADP/NADPH (Fig. 2.14).

Fermentation represented the most primitive form of energy metabolism whose biochem­istry is simple and does not require an external oxidant (electron acceptor) and independent of O₂. Well known fermentation processes include lactic acid fermentation, ethanol fermentation, butyric acid fermentation. Respiratory carbohy­drate metabolism is initiated by an anaerobic fer­mentation.

First membrane bound electron trans­port mechanism was based on simple functional molecules but without the protein component. The protein component developed later which improved efficiency and specificity.

Such naked molecules like quinine, metal containing por­phyrins, inorganic FeS common in anoxic prebi­otic earth, could have incorporated into primitive cell membrane that can be photo-activated and responsible for a primitive electron transport system or a kind of photochemical energy trans­duction (Fig. 2.15).

Photosensitive porphyrin has become protochlorophyll and cytochrome. Respiring organisms are derived from phototrophic one through secondary loss of chloro­phylls and dependent on external chemical reductants. The photosynthetic purple non- sulphur bacteria has electron transport system almost identical to that of mitochondria (Fig. 2.16).

Mechanism to explain the origin of compli­cated biochemical processes involving many steps and cycles is the fact that these pathways are mostly reversible, catalysing the process in either direction.

Assimilatory reduction of CO₂ with the help of NADFH2 and energy (ATP) may run in a opposite way and become dissimilatory and oxidative pathway, degrading and oxidizing organic matter into CO₂ and release ATP through respiratory glycolytic pathway and citric acid cycle (Fig. 2.17).

CO₂ assimilated through Calvin cycle into organic matter undergoes oxidation through glycolytic pathway which is actually a reverse process of Calvin cycle. The origin of citric acid cycle can be traced by the fact that green sulphur bacteria assimilate CO₂ through a reverse citric acid cycle which is reductive and requires ATP (Fig. 2.18).

Prokaryotic Cell:

From the above discussion it is crystal clear that the chemical evolution on prebiotic earth gave rise to organic molecules which included protein, nucleic acid, etc.; establishment of tem­plate system evolved enzyme systems and a sur­rounding lipid membrane; an energy transfer mechanism involving ATP has evolved.

This may have been the beginning of a stable structural and functional organisation having resemblance of a biological cell. These cells are called prokaryotic because of the absence of membrane bound nucleus and organelles.

Primitive prokaryotic cells were essentially anaerobic cells (anaerobic bacteria) because the early earth was devoid of oxygen. Depletion of organic compounds in the primaeval soup resulted in the appearance of photosynthetic cells (blue green algae) which can fix CO₂ and probably nitrogen also.

Photosynthetic cells were responsible for production of oxygen in atmosphere which resulted in the ori­gin of aerobic cells (aerobic bacteria) with metabolic pathways for aerobic respiration.