Essay on the Regulation and Control at the Cellular Level

To a great extent, regulation and control within a cell are the result of interaction between nucleus and cytoplasm. Strictly speaking, not all these mechanisms are homeostatic within themselves; in fact, as we shall see, the steady state of a cell is maintained by the interaction of many separate controls, and we need not expect every mechanism to correspond to out simplified example of a thermostat.

Thus the complex interactions between nucleus and cytoplasm frequently defy any immediate identification with the steady state; and in the main we shall attempt to describe only important mechanisms in these areas of the cell.

The role of the nucleus because the nucleus usually occupies a central location in the cell, it has long been suspected of governing the remainder of the cell. This hypothesis was strengthened in the latter part of the nineteenth century, when it was discovered that the nucleus exhibits a complicated pattern of events in its division during cell reproduction; the precision with which chromosomes apparently divided seemed to indicate a high degree of organization in the nucleus. Thus it was concluded that the nucleus probably techniques were not available for testing this hypothesis.

However, in 1928 the embryologist Hans Spemann conducted an experiment which indicated something of the role of the nucleus in development. Working with fertilized eggs of a salamander, he used a fine hair to hasten constriction at the first cleavage.

He drew the hair tightly before the nucleus had divided, so that one daughter cell included the nucleus while the other consisted only of cytoplasm. Spemann did not separate the two cells completely, but only sufficiently to prevent the passage of a nucleus from one cell to the other. Under these conditions, the cell with nuclear material continued to divide, but the cell without nuclear material remained undivided.

However, after about sixteen cells had been produced by the original nucleated cell, the nuclei became much smaller, and a single nucleus was allowed to pass into the undivided cell. At this point, Spemann separated the two cell groups by drawing the hair tightly, and the undivided cell began to divide.

Subsequently, a normal salamander embryo developed on each side of the hair constriction. This work indicated strongly that the nucleus exerts a vital control upon cell division and embryonic development.

In one series of experiments, Hammering used two species of Acetabularia, A Mediterranean and A. crenulated, which differ in cap shape. In both species, he found that if a piece of stalk is removed from the region between the cap and its nucleus, it will commence regeneration, but will not develop very far before it dies.

This, of course, is what we would expect on the basis of earlier work involving the role of the nucleus. Hammering undertook some grafting experiments in order to determine the influence of the nucleus upon cap formation, and his results were most significant.

He grafted apiece of A. Mediterranean to a decapitated A. crenulated, and vice versa in each case, the cap started forming in the shape it would normally have many separate controls, and we need not expect every mechanism to correspond to out simplified example of a thermostat.

Thus the complex interactions between nucleus and cytoplasm frequently defy any immediate identification with the steady state; and in the main we shall attempt to describe only important mechanisms in these areas of the cell.

The role of the nucleus because the nucleus usually occupies a central location in the cell, it has long been suspected of governing the remainder of the cell.

This hypothesis was strengthened in the latter part of the nineteenth century, when it was discovered that the nucleus exhibits a complicated pattern of events in its division during cell reproduction; the precision with which chromosomes apparently divided seemed to indicate a high degree of organization in the nucleus. Thus it was concluded that the nucleus probably techniques were not available for testing this hypothesis.

However, in 1928 the embryologist Hans Spemann conducted an experiment which indicated something of the role of the nucleus in development. Working with fertilized eggs of a salamander, he used a fine hair to hasten constriction at the first cleavage.

He drew the hair tightly before the nucleus had divided, so that one daughter cell included the nucleus while the other consisted only of cytoplasm. Spemann did not separate the two cells completely, but only sufficiently to prevent the passage of a nucleus from one cell to the other.

Under these conditions, the cell with nuclear material continued to divide, but the cell without nuclear material remained undivided. However, after about sixteen cells had been produced by the original nucleated cell, the nuclei became much smaller, and a single nucleus was allowed to pass into the undivided cell.

At this point, Spemann separated the two cell groups by drawing the hair tightly, and the undivided cell began to divide. Subsequently, a normal salamander embryo developed on each side of the hair constriction. This work indicated strongly that the nucleus exerts a vital control upon cell division and embryonic development.

The chromatin material of the nucleus is composed largely of DNA and protein. Many lines of evidence implicate DNA as the control material of the nucleus, that is, it serves as a kind of “code” which “instructs” the cytoplasm in a highly specific manner.

According to the Watson – Crick Model of DNA this substance consists of two helical strands of sugar-phosphate molecules between which four kinds of nitrogenous bases are linked together in pairs. Apparently, a DNA molecule is replicated whenever the weak hydrogen bonds between linked base pairs break, the two complementary strands unwind, and a new strand is built up on each original strand from micro molecular components present in the nucleus.

Each new double- stranded molecule then assumes the characteristic helical form. On occasion, however, one of the two DNA strands may “template” a complementary’ strand of RNA, not DNA. Whenever this occurs, the base Uralic is used as a “substitute” for the base thymine. In other words the four nitrogenous bases found in DNA are adenine, cytosine, guanine and thymine, but the four found in RNA are adenine, cytosine, guanine and Uralic.

Whenever RNA is synthesized in this fashion, it does not remain joined to its complementary DNA strand; rather, it moves away, and the DNA strand may serve as a code, or template, for the assembly of another strand of RNA.

By using the first letters of the five different bases involved in DNA replication and RNA synthesis, let us compare these processes in a short segment of a hypothetical DNA molecule. You will note that DNA and RNA are differentiated by colour, and we shall follow this pattern as we present subsequent diagrams.

Note that the synthesized RNA molecule is single-stranded, in contrast to the double nature of the DNA molecule. Furthermore, it appears that RNA molecules are produced in shorter segments than the entire DNA strand from which they are template. However, they are ordinarily much longer that the segment shown in which we have oversimplified for illustrative purposes.

What happens to the RNA molecules thus formed? It appears that they pass out of the nucleus into the cytoplasm, where they eventually carry the code to the ribosomes. For this reason, this particular kind of RNA is called messenger RNA. The RNA which composes ribosomes is called ribosomal RNA and it is apparently formed in a somewhat different fashion from mRNA.

Upon reaching a ribosome a given mRNA molecule attaches in some manner to its surface or to the surface of an aggregate of ribosomes. The mRNA code, consisting of a specific base sequence, is exposed as an outer surface.

Meanwhile, still another kind of RNA is active in the cytoplasm. It is called transfer RNA and its individual molecules are much smaller than either those of mRNA or ribosomal RNA; in each molecule of RNA, a chain of about seventy nucleotides is folded so that it resembles a short DNA molecule. Apparently, the region of the “bend” of the of the open ends is specific as a point of attachment to an amino coded ” almandine site’ on a given mRNA molecule.

It appears that RNA molecules are the same in all kinds of cells; thus, we can visualize twenty different kinds of RNA molecules to correspond with the twenty different amino acids ordinarily involved in protein synthesis. Attachment of an amino acid to a given tarn molecule is accomplished enzymatic ally, and is accompanied by the expenditure of energy on the part of the cell.

Many lines of evidence indicate that three mRNA nucleotides are required to code an amino acid. Thus, if a molecule of mRNA consists of thirty nucleotides, as we have shown in our accompanying drawings, ten molecules of RNA can be accommodated, each of which carries an amino acid to the messenger RNA-ribosomal complex.

Eventually, the specified kinds of RNA, each with an amino acid in tow, attach at specific sites on the mRNA molecule. These amino acids are joined enzymatic ally by peptide linkages, and a protein molecule is thus formed.

The protein molecule breaks away and becomes functional as an enzyme or as a structural protein, and the ten molecules of RNA are released from the mRNA molecule, at which point they rapid turnover rate of mRNA molecules, at least in bacterial cells, where it appears that a given molecule of mRNA may code only a single protein.

After this, it is probable that in some protoplasmic systems mRNA. However, it is probable that in some protoplasmic systems mRNA is longer-lived. Now let us return to a consideration of the role played by DNA in cellular control.

It is obvious that the ultimate template for protein synthesis is the DNA molecule, and it is in this role that DNA exerts its effects upon the remainder of the cell.

As a matter of fact, it appears that the nucleus controls the cytoplasm only through this channel. Protein molecules are so diverse within living systems that their expression within cell (mostly as enzyme molecules) ultimately accounts for most of the activity of living systems at all levels of organization.

Perhaps analogies will serve to clarify the DNA-RNA- Protein synthesis sequence we have outlined above. Let us suppose a certain library possesses a copy of a rare old book, perhaps a Gutenberg Bible. Certainly, the library is not going to lend this book, inasmuch as it might easily be lost or damaged.

However, it might allow the book to be photographed page by page so that facsimile copies could be made. These copies might then be used in private studies for the preparation of scholarly papers, which in turn stimulate learned activity.

DNA is something like the rare book. It never leaves the nucleus, but instead, copies in the form of RNA are released to the cytoplasm. Finally, proteins are formed, and they become directly involved in cellular activity.

Now we are in a much better position to understand the nuclear transplant work discussed above. In Acetabularia, for example, it seems probable that a grafted stalk piece contains a certain quantity of mRNA and protein formed under the influence of the original nucleus. Thus, development of a cap proceeds in the direction which would have been taken had the stalk remained under the influence of the original nucleus. However, before development proceeds very far, new mRNA replaces the old mRNA, and new proteins take over the developmental process. As a result, the cap which eventually forms is characteristic of the species which receives the graft, not of the species from which the stalk was taken.

Again, let us consider the matter of cell size. This is indicated by the fact that when cells reach a characteristic size, they divide. Furthermore, exceptionally large cells, such as those of certain protozoa, may contain a nucleus in which the DNA content has multiplied in some fashion.

In Acetabularia, which is an unusually large cell, the nucleus is actually a composite of many small nuclei, and it appears that the amount DNA is thus increased to a point at which control is possible. Although other factors may Irmit cell size the nuclear-cytoplasmic ratio is apparently a critical factor in this regard.

At the lower limits, in turn, a cell would have to be large enough to contain a minimum attachment to an amino coded ” alanine site’ on a given mRNA molecule. It appears that RNA molecules are the same in all kinds of cells; thus, we can visualize twenty different kinds of RNA molecules to correspond with the twenty different amino acids ordinarily involved in protein synthesis. Attachment of an amino acid to a given RNA molecule is accomplished enzymatically, and is accompanied by the expenditure of energy on the part of the cell.

Many lines of evidence indicate that three mRNA nucleotides are required to code an amino acid. Thus, if a molecule of mRNA consists of thirty nucleotides, as we have shown in our accompanying drawings, ten molecules of RNA can be accommodated, each of which carries an amino acid to the messenger RNA-ribosomal complex.

Eventually, the specified kinds of RNA, each with an amino acid in tow, attach at specific sites on the mRNA molecule. These amino acids are joined enzymatic ally by peptide linkages, and a protein molecule is thus formed.

The protein molecule breaks away and becomes functional as an enzyme or as a structural protein, and the ten molecules of RNA are released from the mRNA molecule, at which point they rapid turnover rate of mRNA molecules, at least in bacterial cells, where it appears that a given molecule of mRNA may code only a single protein.

After this, it is probable that in some protoplasmic systems mRNA. However, it is probable that in some protoplasmic systems mRNA is longer-lived. Now let us return to a consideration of the role played by DNA in cellular control.

It is obvious that the ultimate template for protein synthesis is the DNA molecule, and it is in this role that DNA exerts its effects upon the remainder of the cell. As a matter of fact, it appears that the nucleus controls the cytoplasm only through this channel. Protein molecules are so diverse within living systems that their expression within cell (mostly as enzyme molecules) ultimately accounts for most of the activity of living systems at all levels of organization.

Perhaps an analogy wills serve to clarify the DNA-RNA- Protein synthesis sequence we have outlined above. Let us suppose a certain library’ possesses a copy of a rare old book, perhaps a Gutenberg Bible. Certainly, the library’ is not going to lend this book, inasmuch as it might easily be lost or damaged. However, it might allow the book to be photographed page by page so that facsimile copies could be made. These copies might then be used in private studies for the preparation of scholarly papers, which in turn stimulate learned activity. DNA is something like the rare book. It never leaves the nucleus, but instead, copies in the form of RNA are released to the cytoplasm. Finally, proteins are formed, and they become directly involved in cellular activity.

Now we are in a much better position to understand the nuclear transplant work discussed above. In Acetabularia, for example, it seems probable that a grafted stalk piece contains a certain quantity of mRNA and protein formed under the influence of the original nucleus. Thus, development of a cap proceeds in the direction which would have been taken had the stalk remained under the influence of the original nucleus. However, before development proceeds very far, new mRNA replaces the old mRNA, and new proteins take over the developmental process. As a result, the cap which eventually forms is characteristic of the species which receives the graft, not of the species from which the stalk was taken.

Again, let us consider the matter of cell size. This is indicated by the fact that when cells reach a characteristic size, they divide.

Furthermore, exceptionally large cells, such as those of certain protozoa, may contain a nucleus in which the DNA content has multiplied in some fashion. In Acetabularia, which is an unusually large cell, the nucleus is actually a composite of many small nuclei, and it appears that the amount DNA is thus increased to a point at which control is possible.

Although other factors may Inmate cell size the nuclear-cytoplasmic ratio is apparently a critical factor in this regard. At the lower limits, in turn, a cell would have to be large enough to contain a minimum number of the macromolecules which constitute the cellular machinery. Several lines of evidence suggest that a cell could hardly be less than 0.05 m in diameter and contain the minimum number of molecules; the smallest cells known, the pleuropneumonialike organisms, are about 0.1 m in diameter.

The regulation of protein synthesis Perhaps our discussion of nuclear control, presented above, has conveyed the impression that the protoplasmic mechanisms involved are reality simple. This would be an unfortunate impression, because even this one aspect of control is fantastically complex.

As is so frequently true in science, the answer to one question may breed a dozen new questions, and this has occurred with regard to protein synthesis. For instance, assuming that we have answered the question of control in cells how is protein synthesis regulated?

This question is an urgent one, especially in view of the fact that numerous kinds of mature cells of an organism are so variously specialized in their functions. For instance, how is it that a human liver cell differs in its chemical activities from a muscle cell?

How did each cell become what it is? It the DNA content of each cell is identical, why are the same proteins not produced by way of the DNA-RNA- ribosome pathway, or if they are, why do they not govern activity in similar fashion within different ceils? One possible approach to answering these questions would be to investigate cytoplasmic differences.

In 1961, two French biologulation based on their work with microorganism, although it is will microorganism. Although is as still too early to assess the valve with microorganisms. Although it is still too early to assess the valve of this model as a working theory at all levels of organization, it constitutes the most attractive hypothesis yet proposed for the pursuit of further experiments in cellular regulation.

Jacob and Monod found that the concentration of certain proteins fluctuates greatly in some strains of bacteria, depending upon the nutritive utilize a group of carbohydrates called the (3-galactosides by attacking them with an enzyme, (3-galactosidase.

Analysis reveals that when (3-galactosides are assert from the medium no (3-galactosidase is present. Only when the substrate is present does the bacterium produce the appropriate galactosidase is only one; still other enzymes are always produced, whether their substrate is present or not.

Based on this and other evidence, Jacob and Monod proposed a unit of genes called an operand which is postulated to work in conjunction with a regulator gene. Just here, we must point out that we are using the term gene to correspond to a given segment of a DNA molecule which codes a segment of RNA, which in turn codes a protein molecule. At least, this is one kind of gene, and to distinguish gene.

According to the Jacob-monody model, two other kinds of genes exist, at least functionally. These are model; two other kinds of genes exist, at least functionally. These are known as regulator and operator genes, respectively; it appears that these two types of genes do not govern the ultimate formation of metabolic enzymes, but rather, they code for substances which exert their effects within the operand system.

Now let us see how the system works. Let us assume that the operand consists of four genes: The operator gene and three structural genes, A, B and C. Gene A represents the DNA segment responsible for the production of (3- galactosidase, and B and C represent genes which code for associated enzyme molecules with which we will not be concerned at this point.

The operator gene acts as a switch; when it is open, the structural genes are active, and when it is closed, the structural genes are inhibited. The operator remains inoperative as long as a repressor substance, produced by the regulator gene, is present. Whenever (3- galactoside is present in the environment, however, it reacts with the repressing the operator gene.

At this point, the operator switches on, and the structural genes under its influence commence to code their characteristic mRNA. Thus, (3-galactosidase and two associated enzymes are produced, and the (3-galactosides are attacked. With the disappearance of the substrate, the system returns to its former operation, that is, the repressor substance turns off the operator and enzyme production by the structural genes ceases.

Although this model is based on work performed upon micro-organisms-and it has not yet been shown that comparable systems are widespread in the cells of the more complex organisms-it constitutes a very promising hypothetical explanation of cellular regulation.

At least, it is possible to visualize how genes might be turned on and off, and how some cells produce certain substances which other cells do not produce. Because the type of work involved is very tedious, it will probably be some years before the full significance of the operand concept will be known, but it is a start toward solving some extremely difficult problems in regulation and control.

Whether the details of such regulation as we have just described are accurate or not, let us note that the operand system is a homeostatic mechanism.

The addition of p- galactosides to the environment of E. coli brings about the production of p-galactosidase, which bring about the degradation of (3-galactosides, and so on. This is only one of many stead-state controls which apparently exist at the cellular level of organization.