Harvey Shuster, age 14, of Montreal, Quebec, Canada, for his question:
What are human chromosomes?
Our earliest ancestors noticed that children tend to take after their parents. In the past century or so, biologists coined the term heredity to cover the physical features relayed from generation to generation. But the inner secret of heredity remained a mystery until the last decade. When it was solved, it also revealed the inner secrets of all living cells.
The early microscopes of the 1600s inspired great excitement. Scientists saw that wood and animal tissues were made from tiny cells and a whole new miniature world became visible. Later, more powerful microscopes revealed details of the living cell and microbiologists devised ingenious techniques to trace its miraculous chemical operations. Most of the cell tissue is the cytoplasm of membranes and jellyfied protoplasm. But the most fascinating item is the tiny nucleus inside every living cell.
Finally dyes were found to stain cell specimens for microscope slides. This revealed that the nucleus is a bundle of tight red wads. They were called chromosomes, meaning colored bodies. But even our best microscopes cannot define them clearly when the cell is in its normal phase. However, the chromosomes are revealed with all their miraculous talents when a cell divides to become a pair of twins. Then the 46 chromosomes in the nucleus of a human cell uncoil and become fuzzy, stringy little rods. Each rod duplicates itself and splits lengthwise. The two copies move apart, the cell divides into two cells, each with its own duplication of the original 46 chromosomes.
Improved microscopes revealed that the chromosomes are bumpy strings of beads. By then it had been deduced that chromosomes held the secrets of genetic heredity and the bumpy beads were named the genes. Now we know that each chromosome is a string of maybe 2,250 genes. And each gene carries an hereditary trait from the family tree.
The final and most exciting part of the story was revealed in the 1950s. Biochemists investigated strange nucleic acids found in the cell nucleus. They identified dyoxyribonucleic acid and the complex structure of its long molecules, twisted in double strands like spiral staircases. Here, in this fabulous DNA, they found the innermost secrets of heredity. They also found that this same DNA is the basic blueprint that gave the multitude of chemical activities that go on in every living cell.
The cells of plants and animals also contain chromosomes, though some species have more and others less than human cells. Every human cell has 46, 23 inherited from each parent. Each chromosome is a string of 1,000 or more genes, each bearing an inherited trait.
An adult human body has about 60,000 million cells. The nucleus is but a small part of each and every cell. But its set of 46 infinitesimal chromosomes is identical to every other cell in the body. What's more, every person's set of chromosomes is unique and different from the blueprint inherited by anyone else in the world.