Secrets of the Human Cell As you sit quietly reading these lines, a whirl of activity is taking place in your body. Every second, unseen, unnoticed, millions of new cells are born in the body’s ceaseless programme of self- renewal.
Secrets of the Human Cell
Since they are the “bricks” from which all living matter is made, cells are able to perform baffling chemical transformations, producing infinitely complex vitamins, hormones, proteins. They perform striking feats of “biological engineering” the outstanding example being the formation of the fertilized human ovum. At the instant of fertilization this single cell determines exactly the type of human being to be produced, down to waviness of hair and colour of eyes.
Of the amazing performance of cells, a noted doctor has said: “We are familiar with the great diversification of activity in a large city. It does not seem possible that our own bodies could quietly and steadily be carrying on a more complicated and extraordinary process of manufacturing, storage, repair, communication, transportation, policing, waste disposal, administration, food production, temperature control.” Yet that is the case.
If anything, this statement is conservative. The cells of our bodies are more like the population of a planet. But though they are many billions in number, they normally work in harmony, each member of the complex society contributing to the welfare of the others.
Today the exploration of this most minute fragment of life is under way in dozens of laboratories, and there is a growing conviction that cell studies will answer many biological mysteries, including the mystery of cancer.
Look more closely at this remarkable little bundle of life, the human cell. It has three principal parts: nucleus, cytoplasm and outer membrane. The nucleus, the “executive” part of the cell, governs structure, so that cells produce only exact copies of themselves lung cells make only lung cells, kidney cells only kidney cells. Cytoplasm is the jelly like substance in which the all important nucleus floats; it controls cellular respiration, growth, waste disposal. Enclosing the entire cell is the gossamer mem brane, which acts as a molecular sieve. Through it, nourishment passes into the cell to be distributed by a minute system of canals; and through the same membrane wastes are passed outward for disposal.
Each human cell nucleus contains approximately forty-six chromosomes with the exception of egg and sperm cells, which contain half as many. (When these combine in the fertilized egg, the normal cell complement of chromosomes is reached.) Each chromosome contains genes the seeds of inheritance of which there are believed to be thousands in the human body.
Since the mother and the father contribute an approximately equal number of genes for a new life, the number of possible combinations is staggering. Genes are passed directly from parents to children, and have been so passed since man first appeared on earth. Thus in every cell in our bodies we have bits of matter that are directly descended from Dawn Man. “Our ancestors,” says a doctor at the Rockefeller Institute, “are present in our chromosomes and reach down to influence the chemistry of every cell in our bodies.’
There are four general “types” of cell: the nerve cells, which form the body’s communication system; epithelial cells, which line and cover-they line the digestive tract, for example, and make the skin which covers the body; connective tissue cells, which bind the body together; and muscle cells, which move the body’s members.
Cells come in a vast variety of shapes and sizes: globular, disc shaped, elongated. Some are relatively large, a nerve cell, for example, may have a “tail” three feet long. Nerve cells cannot reproduce themselves; we have our lifetime complement at birth and once one is destroyed it is destroyed for good. Other cells usually reproduce by simple division: they narrow in the middle and finally split into two, the “daughter cells” being exact duplicates of the original.
The speed with which cell reproduction occurs varies from place to place in the body according to local need. It is estimated that half of all the body’s protein mainly muscle tissue is replaced every eighty days. The life span of the skin cell is four to five days. New skin is steadily forming from underneath and pushing to the surface, where it dies and is washed away.
One of the most fascinating features of cells is their potential immortality. In 1912 the late Alexis Carrel of the Rockefeller Institute in New York snipped out a piece of chicken heart. Periodically it was fed nourishing broth, and from time to time excess tissue was trimmed away. The cells remained alive for
thirty-four years and were permitted to die only when the experiment had fulfilled its usefulness. In general, cells have two main lines of responsibility: their own housekeeping activities and their community responsibilities. The first includes such functions as eating and waste disposal. The second includes the responsibility of each cell to all others.
Tiny cells in the pancreas, for example, produce minute squirts of the insulin which controls sugar use by all other cells; fat cells store droplets of oil to be used as fuel to warm the rest of the body; stomach cells manufacture enzymes which aid in protein digestion.
Some cells demonstrate an uncanny ability to protect the body from harm. The white blood cells normally float idly in the blood stream. Yet if a finger is cut and bacteria invade the wound, they swarm to the hurt spot and consume the bacteria. If the infection is great they reproduce in enormous numbers to meet the challenge.
Researchers probing the secrets of cells have bumped into a number of problems which have so far proved baffling. For ex- ample, since cells generally show a remarkable specificity lung
cells, and so on why doesn’t the original fertilized ovum duplicate itself instead of going through an amazingly intricate series of divisions and differentiations to produce a mouse, a Whale or a man?
Cell students have found no satisfactory answers, although they have made many interesting observations. They have, for example, studied fertilized frogs’ eggs and watched the division take place which marks the first steps towards a new life: one cell into two, two into four, etc. They have taken the first thirty-two cells from this process cells which would have ultimately made one new frog divided them and produced thirty-two frogs! But after the thirty-two cell stage such interference brings only death to the entire cell mass. Why? No one knows.
There is another great, and pressing, mystery. Normally, cellular division is a perfectly orderly process. But at times cells go on a wild, uncontrolled, reproductive spree. This is cancer. Why do cells suddenly go berserk? Again, no one knows.
In some of the most portentous research of our day, scientists are seeking basic information about the chemical composition of cells in effect, how we are put together.
The focus of many of these studies is an exquisite and astonishing structure called DNA, a handy nickname for deoxyribonucleic acid (translated: “an acid, in the nuclei of cells, made of deoxidized sugar”). This single molecule is the marvellous inheritance machine. But it is more than that.
DNA is life itself.
DNA has a vivid memory which time does not dim. It stores a vast number of directions and blue-prints which it issues at the right time and place to trigger the building of all the cells and structures of a body, make them grow and synchronize their operations at every second during all their allotted life. It exists not only in sex cells but in every living cell of every animal and plant on earth!
Your personal DNA is peppered throughout your body in millions of specks, corresponding to the number of living cells in your body. And in every cell whether in the beating heart, the hair, the liver or any part of your body-it carries the full information about you.
Your DNA specks have the same chemical composition, are about the same size, and look like the DNA in your dog, or in a house fly, a bread mould, a blade of grass or a beech tree. Yet somehow DNA makes every living thing different from every other living thing. It makes dogs different from fish or birds, bread moulds different from apple trees, and elephants different from mosquitoes.
The coded directions in your DNA were compiled by chance selection from those of your mother and father at the conception of your egg cell. That first cell was a complete you, your first person singular. Its DNA was prepared to generate on a prearranged schedule your heart, lungs and kidneys, your eleven pints of blood, twenty-five feet of intestines all of you. It held advance reservations for all the body functions of your life span, bestowed all the equipment and talents you will ever have.
The number of jobs DNA controls varies according to the organism. A one celled amoeba has little growth to do, it does not think about anything and it has no heart, liver, lungs or limbs to operate, so its DNA has few responsibilities. On the other hand, the number of DNA jobs in a human being is estimated at 700,000.
In decades of remarkable research, much had been learned about the workings of inheritance. But the astonishing physical nature of genes was not revealed to scientists until after the advent in the 1940’s of such spectacular new laboratory tools as electron microscopes that can magnify a shilling into an image which would make the shilling bigger than the area covered by Regent’s Park in London, and X-ray devices which can “see” the actual patterns of molecules.
Surprisingly, DNA has a basically simple form. It consists of two intertwined, tape-like coils, connected by cross-pieces at regular intervals like a spiral staircase. Thus it can shorten and elongate, compress and open up, by coiling and uncoiling.
Since ordinary molecules are apt to be chunky, scientists marvelled at the extreme slenderness and length of the spiralling tapes. Dr. John Kendrew, a Nobel Prize winner and an authority on DNA, estimates that if the DNA tapes inside the nucleus of a single human cell were uncoiled and laid end to end they would extend three or four feet. What utter fineness of tapes and tightness of coils, where all this is neatly packed inside an ultra- microscopic speck! There is surprising logic in the long, slender form of DNA this gives it a capacity, like magnetic recording tape, for storing all the vast quantity of information needed in a lifetime.
Countless atoms make up the internal structure of the giant DNA molecule. The tapes themselves are made up of sugar and phosphate groups. Embossed on the sugar groups are nubs of four kinds of nitrogen compounds-the steps of the spiral staircase.
The nitrogen together spell out a mysterious code. For it is the sequence of these four nitrogen nubs on the DNA tapes that spells out the events which make bodies grow and turns out wood or muscles, leaves or lungs, fins, wings or legs, as the case may be much as the electronic specks on magnetic tapes produce, according to their order, the sounds of music or the voice of a lecturer or actor.
Does that alphabet seem too simple for all the information and instructions it must carry? It has been estimated that if we were to put the DNA code of a single human cell into English letters and a typist copied them, they would fill a 1000 volume encyclopaedia.
What the final results will be from these exciting studies now under way in virtually every country in the world, no one will venture to say. Besides contributing new knowledge to the problem of cancer and other diseases, they will almost certainly shed more light on the mechanics of inheritance.
In the words of E. B. Wilson in The Cell in Development and Heredity: “The key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.”
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