3. The Evolution of Cells

3.3 Sexual Reproduction

The number of nucleic acid bases actively engaged in protein synthesis is about 106 in bacteria. The probability or error in replication of these bases is about 10-6. An exchange of genetic material seems to be the only way in which living systems at the bacterial level can escape from a divergent evolutive phase. If a bacterial organism is capable of assimilating glucose and another strain galactose, it is in the interest of both strains to share their genes through fusion. The fused organism then carries two sets of genes instead of one and is "diploid". We are diploid organisms ourselves. The only cells of our bodies that carry just one set of genes are our germinal cells, ovum and spermatozoa. These cells, like bacteria, are "haploid". Once a spermatozoid fuses with an ovum, the product is a diploid egg, which will grow into a diploid adult being.

The reaching of such a turning point, even if very unlikely for an individual, becomes a necessity in a large population. This turning point permits further developments of the genetic apparatus that lead to higher organisms by a gradual reduction of error in replication and a concomitant increase in the number of bases involved in the storage of information.

In accordance with the augmentation of the complexity of an organism occurs an augmentation of the amount of genetic material present in a reproductive unit. The amount of genetic material present in viruses is less than the amount present in bacteria, in turn it is less in bacteria than in algae and other higher living forms (fig. 3.17).

Figure 3.17. There is an increase in the amount of genetic material present from viruses, bacteria, and algae to mammals. For each group of organisms, the minimum and maximum genome size recorded is expressed as the number of nucleotide pairs per germ cell (haploid cells). Mammals all have about the same genome size.

The total amount of DNA in a reproductive haploid cell of protozoans contains about 107 pairs of bases. In insects, it increases to about 2 x 108. In mammals it climbs to about 4 x 109. The human genome is about 3.3 billion bases long. However, these sophisticated organisms use only from 1% to 4% of the total amount of DNA available. The number of bases really involved in protein synthesis drops back to 1 to 4 x 107. This reduction is understandable if one is aware that neither mammals nor insects succeeded in reducing the error of replication further than about 3 x 10-11 per cell generation.

Error rates lower than 10-11 were apparently missed by evolution on this earth. However, it is doubted that a lower limit could ever have been reached, as there exists an insurmountable damaging effect of thermal collisions and radiation. We reach here again a divergent phase of evolution, with the development of large functional networks created by higher organisms.

The reduction in errors of replication obtained by eukaryotic cells seems to be due to their use of a proteinic polymer involved in DNA synthesis. This polymer is not an enzyme. In eukaryotic cells, the genetic material is associated with basic proteins, the histones and protamines. Histone closely associates with the double-stranded DNA, by fitting within a groove formed by the twisted DNA molecule (see fig. 4.1). The special place occupied by the histones is emphasized by the fact that cellular messenger RNA’ s for histones are devoid of long stretches of poly-adenylic acid while the messenger RNA’ s for all other proteins of eukaryotic cells have such stretches. These histones, perpetually linked to DNA, are supposed to be in charge of the synthesis of DNA and may be the reason for the stability of the DNA of eukaryotes during cell duplication.

The sexual reproduction perfected by the eukaryotic cell is at the roots of the fantastic development of species. Somewhere between 1.5 to 2 million species are presently described. This is a divergent phase of evolution that allowed the exploration of many evolutive alleys.

The unicellular eukaryotic cells that assembled into superior organizational units by cell aggregation and differentiation, of which sponges reached the highest level, had to evolve means of cell coordination. This was achieved with the utilization of cyclic AMP already in use by bacteria. This cyclic AMP is the fundamental messenger used by pluricellular organisms to prompt specialized cells into action.

Figure 3.18. The increasing abundance of species with the passage of time is indicated in grey. The number of species multiplied explosively after genetic exchange was refined into sexuality by the eukaryotic cell. Another fundamental acquisition of eukaryotic cells, obtained at the stage beyond that of the bacteria are the prostaglandins. These substances modulate and coordinate the lives of a colony on its way to pluricellularity.

In higher organisms, all that remained to achieve was to perfect communication within the organism and acquire a better perception of the world without. With the help of cyclic AMP and prostaglandins, this was achieved by hormones and the nervous system. The nervous system was initially destined to gain knowledge over posture in moving organisms. Suddenly, the nervous system fulfilled a new and unforeseen function. This lead to the convergent evolutive line that culminated with man.

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