Dan Meadows, age 12, of Marmet, West Virginia, for his question:
Why does the human body stop growing?
A person who kept on growing taller and perhaps wider all his life, would feel rather out of things. We are used to the fact a young person inches up to his adult size and then ceases to grow any taller. This is the normal way of growth. But it certainly makes a curious person wonder what makes it stop at that certain point.
The human body grows automatically and we seem to have little or nothing, to say about it. We know that certain diseases may twist its growth and that factors such as poor diet prevent it from doing its best. Scientists now know that growth is stunted when the thyroid or pituitary glands are impaired. The growth of a young person's body is regulated by hormones secreted by these and perhaps other glands. This, of course, does not explain why, at a certain age, these hormones stop instructing a body to continue growing.
Since this is sure to happen, without fail, the limit must be set by some built in biological factor. For almost a century we have known that the features of the body were ordained by genes and chromosomes. These biological dictators are inside the nucleus of each of the body's billions of living cells. The countless, beady little genes carry factors inherited from one parent or the other. Every person inherits a unique genetic pattern selected from countless features on both sides of the family tree. This is why tallness tends to run in some families and members of other families tend to be short even when they are healthy and well fed.
In the 1950s, scientists probed deeper into the biochemical substances in the nucleus of the living cell. They were interested in certain complex acids found there and nowhere else. One was identified as deoxyribonucleic acid DNA. It seemed to be the mysterious substance that carried the blueprint of heredity and ordered the processes of the living cells. Researchers in molecular biology wasted no time in learning more about it. They discovered which atoms and how many are used to build the fabulous DNA molecule and how these atoms are arranged. They also learned a lot about how DNA works to regulate growth and all other features.
The DNA molecule that so quietly governs all these personal factors is an openwork structure of simple atoms. They are arranged in a double spiral staircase linked at intervals with special groups of atoms. Its molecules are strung together in long chains and coiled inside the genes. This fabulous biochemical is the blueprint of the unique genetic pattern inherited from both parents. It is the mastermind of every living cell in the body and every living process, large and small, obeys its instructions. It is DNA that orders the body to grow two arms and legs, the quality of vision and every other operation. It is DNA that sets the time limit on the body's growing period.
A molecule, even a large DNA molecule, is much too small for human eyes to see. But there is enough of this blueprint substance in the body to govern all of its biological processes. If the thin DNA chains in the nucleus of a single cell were unwound, they would measure a yard. There are about a million million cells in your body. And each cell has its quota of DNA with built in instructions to limit your growing years.