THE RELATION BETWEEN CELL STRUCTURE AND CELL FUNCTION

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THE RELATION BETWEEN CELL STRUCTURE

AND CELL FUNCTION

Charles U. Lowe, M.D.

Department of Pediatrics, University of Buffalo School of Medicine, and the Statler Research Laboratories, Children's Hospital, Buffalo

T HE DAY is past when the cell can be considered a bag containing a nucleus and a host of enzymes swimming freely within the cytoplasm. It is now clear that the cell is a highly organized, carefully integrated and delicately controlled syn thetic and catabolic organ with well de fined particulate structures to which many specific functions may be assigned.

The nucleus contains two specialized

structures, the genetic material or the chromosomes, and the nucleolus. The chromosomes are strands of desoxynibo polynucieotide (DNA) and histone-like pro teins which are invisible as organized struc tures until mitosis is well established (Fig. 1). The DNA consists of double strands of polynucleotide with specific configuration wound in the form of a helix. The chroma tin during mitosis can be visualized as multi ple indented strips, which have sufficient in dividuality as to appear unique. Each chromosome strand is made up of many molecules; a locus on a chromosome with specffic genetic potential has been termed a

cystron, and a structurally related group of

cystrons form a chromomere. The individual

cystrons have not been clearly visualized

but it seems reasonable to suppose that each represents a single molecule. Why particular genetic material is associated with specific chromosomes is not known.

The number of chromosomes appears to be characteristic of the species, and within a given organism all resting nuclei, except those of the germ cells, contain the same number of chromosomes and the same amount of DNA. Certain nuclei, for ex ample a small number of hepatocytes, con

tam multiples of diploid DNA.

The lateral portion of each chromosome has in association with it a nibose poly nucleotide (RNA or PNA), and that region is a site of rapid synthesis of this substance. Some of the RNA leaves the chromosome and collects in the nucleolus, which is also a locus of RNA synthesis. Since the relative specific activity (turnover rate) of nuclear RNA is much higher than that of the RNA found in the cytoplasm, some people have speculated, not without supporting experi mental data, that all RNA in the cytoplasm

FIG. 1. Upper: Diagram of two â€œ¿lampbrusW' chro

mosomes from the giant nucleus of the oÃ¶cyte of the common newt (Triturus viridescens). Total

length approximately 400 @&.Lower: Postulated

structure of a single chromomere. Only the cen

tral section containing the cystrons gives a posi tive stain with Feulgen reagent, suggesting that DNA is found only here. The ioops contain pro

tein which can be removed by treament with tryp

sin. The clusters around the loops are RNA. (From: Gall, J. G.: On the Submicroscopic Struc ture of Chromosomes. Brookhaven Symposia in Biology, No. 8â€”Mutation. Upton, New York, Brookhaven National Laboratory, 1955, by per

mission.)

Presented as part of a Symposium on the Cell at the IX International Congress of Paediatrics,

Montreal, Canada, July 1959.

ADDRESS: 219 Bryant Street, Buffalo 22, New York.

PEDIAmIcs, September 1960

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455 SPECIAL ARTICLES

Nuc@cAcid go on even when the apparently associated RNA is depolymenized.

The mitochondnion is not an empty cigar but rather a tube filled with baffles called cnistae. The wall of the mitochondrion and the cristae are made up of a series of en zymes, the most important of which are the cytochromes or electron-transferring enzymes. It has been suggested by recent work that a map can be made of the wall of the mitochondnion showing the locus of each of these enzymes. Within the mito chondrion, probably in a highly organized pattern, are the enzymes of the Krebs tn carboxylic acid cycle. Though possibly for tuitous, it is certainly an advantage to have this organized system of enzymes in close juxtaposition to the cytochrome system.

The morphology of the mitochondnion, at least in the kidney, can be experimentally affected by the introduction of thyroxin into the suspending medium and under ap propriate conditions will either swell, fill ing with water, or contract while expelling water. The juxtaposition of the micro somes to the mitochondion and their ar rangement as endopiasmic reticulum around the mitochondnion is also appar ently of advantage, for the mitochondrion can by this means supply the energy re quired for protein synthesis.

Smaller than the microsome is another particle which can be sedimented only by very high centrifugal fields. Whether or not these small particles are simply small mi crosomes or have specific and separate functions is not clear. They also have a high content of RNA; their enzymatic makeup, however, is unknown.

By high speed uitracentrifugation of

tissue homogenates, one can obtain a parti

cle-free fluid which is faintly pink in color and, for want of a better term, may be called cell sap. This fluid contains proteins, and hence enzymes, as well as RNA. The enzymes of the Embden-Meyerhof scheme, the hexose monophosphate shunt, and spe cific amino acid â€œ¿activatingâ€•enzymes are found here. The RNA in this fraction, in contra-distinction to the other RNA of the cell, is soluble and has a low molecular

78% Nuclear Nucleic Acid RNA 2% DNA 88% 22%

Fic. 2. Diagram of monkey hepatocyte with data

on approximate distribution of RNA and DNA.

originates in the nucleus and passes from the nucleus to the cytoplasm. This hy pothesis is attractive because it is the genetic material which must indicate the type of RNA which is to be synthesized and only within the nucleus could the pre cursor substances of RNA be in contact with the cystron.

Within the cytoplasm, RNA is the only polynucleotide that has been identified. It

is found characteristically in four particular

loci (Fig. 2). The most important concen tration of RNA is in the microsomes. These are tiny particles arranged, according to electron microscopic visualization, on either side of canals formed by a phospholipid protein membrane, and collectively they form the endoplasmic reticulum (Figs. 3 and 4). These canals also envelop the mitochondnion. The canals appear to be in continuity not only with the cell membrane but with the nuclear membrane as well, and have the ability to open, close and re organize, and in this fashion engulf par ticulate material, such as carbohydrate on lipid. One of the principal functions of the microsomes is a unique role in protein synthesis, for it may well be that the RNA in the microsome is the â€œ¿templateâ€•RNA re quired for protein synthesis.

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456 THE CELL

1'.,

Fic. 3. Portion of a notochord cell in Ambly stoma larva. n = nucleus; ne =

nuclear membrane; er = efl(lOphlSrfliC reticulum; m = membrane of er which is

continuous with @iuclear membrane. (@26,666.) (From: Siekcvitz, P.: On the meaning of intracellular structure for metabolic regulation, in Ciba Foundation Symposium, Regulation of Cell Metabolism. Boston, Little, Brown, 1959, by per

mission.)

weight. It serves a very important function,

for without the participation of the soluble

RNA, the amino acid activating enzymes

and the microsomes, there is no protein synthesis. The amino acid activating en

zynies in the cell sap have an affinity for

specific amino acids. It has been postulated,

but not w@@''n, that there is one enzyme for

each amino acid. These enzymes (Fig. 5)

combine with amino acids in a reaction in volving adenosine triphosphate (ATP),

liberating pyrophosphate and yielding an

activated amino acid. The activated amino

acid is then transferred to the soluble RNA

as a step in synthesis of the peptide bond.

The site of attachment on soluble RNA is adenine, which in turn is attached to the

residual polvnucleotide by two cvtosine

molecules. The ii ucleotide sequence beyond

the second cytosine determines which amino acid will be received. More will he said about this later.

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SPECIAL ARTICLES 457

are also filled with multiple indentations which intermittently open and close to per mit continuity between the pericellular ma terial and the endoplasmic reticulum. Hence, material may move through the cell wall without necessarily being controlled by concentration gradients. The nuclear membrane is also indented and in this in stance, it is possible, though not proven, that the unorganized chromatin, which exists in the resting nucleus, surrounds these indentations and in some fashion provides a message for the RNA of the cytoplasm, indicating the type of RNA to be synthe sized (Figs. 6 and 7).

Current theory suggests that the transfer of information concerning the amino acid sequence of proteins goes from DNA to RNA and that RNA forms the template upon which proteins are synthesized. It seems reasonable to state, at the moment, that all protein within the body (there may be some exceptions) is enzymatically active protein. The body does not store protein unassigned to a function. The synthesis of protein which occurs during growth, or for repletion following starvation, must be considered enzyme protein. The high nitro gen balances which can occur in surfeit-fed individuals probably reflect temporary hypertrophy of enzyme systems, the func tion of which will be to catabolize the ex cess protein in the diet.

The genetic material may be considered

Fic.4. Diagram of a cell.Magnificationapproxi

mately that in Figure 2. er = endoplasmic reticu

lum; mito = mitochondrion. (From same source as

Figure 3.)

branes, the cell membrane and the nuclear membrane. Classical histology has indicated that these membranes have many of the properties of a wall and are relatively inert, except insofar as they permit movement of fluid and small molecules through the con ventional processes of osmotic diffusion. It is now clear, however, that the membranes are not simply inert walls; they have spe cific chemical function. Incorporated into their architecture may be enzymes in volved in the transport of material across the membrane. In all probability the walls

aa + E + AT? ..-.---> E-aa + PP Ã· AMP

E...aa + S-RNA.@C--G-A

â€”¿

G

E + S-RNA--C-C-A

I

G aa

Template-RNA + S-.RNA-C-C4 â€”¿.--.-) Template-RNA + S-RNA-C-.C-A4

3 1 I

G aa aa G

Ftc. 5. Theoretical scheme for amino acid activation. aa = amino acid; E = amino acid activating en

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458 THE CELL

as a template. Upon this template, RNA can

he synthesized. The synthesized RNA in

some fashion se1)arateS from the DNA and

is either extruded through the nuclear

membrane or may actually be synthesized

in the cytoplasm having come close to the

DNA in the small invaginations in the

nuclear membrane. This RNA then be

comes a template upon which amino acids may l)e deposited for the synthesis of pro

tein. The deposition of amino acids on

RNA is a function of the microsomes, the

soluble RNA of the cell sap and the amino acid activating enzymes, as already men tioned. A single amino acid, following acti vation, appears to be attached to an

adenine molecule of the soluble RNA and

is so placed on an RNA template that a

peptide bond is formed. Presumably, when

all of the sites on the RNA template are

filled, a specific protein has been synthe

sized and becomes freed from RNA. It

seems reasonable to assume that the RNA

synthesizes protein in relation to enzyme

needs, and it is difficult to visualize how

protein which is not enzymatically active

:4!

)

Fic. 6. Rat liver after centrifugation at 110,000 gray for 90 minutes. (Toluidine blue, 960x.) Di rection of centrifugal force is toward bottom of

illustration. Cell particulates and nuclear chroma

tin have been sedimented. Arrow points to a nu

cleus in which canaliculae are easily visualized. Only those canaliculae persist which would pre sumably not be sheared by centrifugal force. (From:

Lowe, C. U., and Sarkaria, D.: Unpublished data.)

Fic. 7. Same as Figure 5, stained with Feulgen re agent. Canaliculae are Feulgen-positive and hence presumably DNA. (From: Lowe, C. U., and Sar

karia, I).: Unpublished data.)

can normally be synthesized under the con

ditions known to exist in the cell.

It has been known for a long time that

during starvation RNA is lost from cells

known to synthesize protein, and at times

of protein synthesis RNA appears to ac

cumulate in the cell. One can logically

associate RNA accumulation with increased

demand for protein, because protein syn

thesis could not occur without suitable sites

on RNA templates for organization and for

ination of peptide bonds. It has been sug

gested by simple mathematical analysis of

the possible combinations of the substruc

tures of RNA (purine and pyrimidine

bases) that for each amino acid there is a

unique code provided by a simple combina

tion of these bases.

One of the great mysteries at the moment

in cellular metabolism is how this extraor

dinarily complicated organization is regu

lated. There must be signs and signals

within the cell and some which come to the

cell from outside, indicating which enzymes

are to be synthesized, how rapidly they

are to be synthesized, whether energy is re quired for contraction or synthesis, and when the cell has to reproduce itself. This

is the discipline known as cell control; in

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1960;26;454

Pediatrics

Charles U. Lowe