Modern Concepts About the Structure, Functions and Reproduction of Cells
Section 2.7. In this Section the cell as a generalized notion will be
considered in the light of the chromosome theory and the Watson and Crick
model of DNA, underlying the modern interpretations of its development
and reproduction. Naturally, the results of classical cytology and genetics
will not be ignored, as well as certain deviations from the concepts that
have been imposed for a given period of time as the only correct ones.
If the existing data, opinions and hypotheses about life origins are
fastidiously scrutinized (see Chapter 1) it would not be hard to admit that
the abiotic formation of the monomers by “the lucky combining” of atoms
and molecules in the non-living nature and their conversion under suitable
conditions into biopolymers with the corresponding primitive self-reproductive
mechanisms has given rise to living matter formed initially into
cell structures and only after that into unicellular and multicellular
organisms.
Biopolymers are naturally found compounds of high-molecular weight
consisting of monomers bound by a specific manner into long chains of
strictly defined configuration and biological functions. Since cells represent
in themselves very complex biological systems in their composition, it is
impossible to even make suggestions about their ultimate chemical and
biochemical organization. That is why only the most important biopolymers
will be spelled out — proteins, nucleic acids and carbohydrates, these
being the major cell components.
Proteins
Proteins are high-molecular organic compounds, built from lineary situated
amino acids, with a molecular weight ranging from 1×10⁶
to 1×10¹⁰ daltons.
Out of over the 150 identified amino acids only 20 of them take part in the
composition of protein molecules (Fig. 2–29).
Literature data allow for the possibility of formation of more than 10³²⁰
different proteins with an average length of the chain of about 300 amino acid
residues. Simultaneously it is admitted that such a number of proteins would
have hardly been possible in reality. It is much more likely for their fund to be
much less numerous, the course of evolution eliminating most of them and
only preserving the really needed ones.
Chemical analyses have shown that after water (about 70%) protein
content (more than 50% of the dry matter) comes second as quantity in the
cell composition. Theirs is the biological function to determine the structure,
to keep up growth and take part in the realization of all vital processes
related to their development and division including cell specialization,
differentiation and dedifferentiation which will be further treated in detail.