In the latter part of the 19th century, with improvements in microscopes, biologists began to look more deeply into the contents of the cells that make all parts of the animal and plant body. A central entity known as the nucleus of the cell was given particular attention, and it was noted that within the nucleus there were a certain number of bodies that readily picked up certain organic dyes. These structures were given the name of chromosomes, which means colored bodies.
When Gregor Mendels basic laws of inheritance were rediscovered in 1900, some scientists postulated that the chromosomes carried what Mendel called determiners, which we know today as genes. Thus was formulated the chromosome theory of inheritance, the foremost advocate being Nobel Prize winner Thomas Hunt Morgan of Columbia University. The evidence for this theory was strong, as one could readily observe that when cells divided via a process known as mitosis, each new daughter cell gained an exact set of chromosomes as the parent cell had. Other discoveries in the early 1900s reinforced the theory.
As the science of genetics exploded, it was proven beyond a doubt that the chromosomes carried Mendels determiners or genes, and it was pictured that the genes were strung out along a chromosome similar to a string of beads. It was determined that a chromosome was made up of a material known as Dioxyribonucleic Acid (DNA) in combination with a series of complex proteins. It was not until 1953 when the monumental discovery of the actual structure of the DNA molecule was discovered by the American James Watson and the Englishman Francis Crick, that it was understood that segments of DNA on a chromosome constituted the actual individual gene.
It was soon recognized that each species had its own number and type of chromosome complement, occurring in pairs. Finding suitable tissue to study the human number proved to be difficult, but seemed to be settled when a University of Texas zoologist claimed to have counted 48 in testicular cells. This number was accepted until about 50 years ago, when it was accurately determined that humans had 46 chromosomes, or 23 pairs. With chromosomes and their DNA, it is quality and not quantity that counts. For example, a crayfish has 104, fruit fly eight, cat 38, dog 78, gorilla 48, and a goat 60.
There are two basic types of cells in the body of an organism, namely somatic or body cells, and germ or reproductive cells. When somatic cells divide, the chromosomes exactly duplicate themselves with each daughter cell receiving the same number and type as the mother cell.
In reproductive or germ cells, the process is different. In this type of cell division, that occurs in the formation of sperm and egg cells; the chromosomes do not duplicate themselves at this time, but line up in pairs with one chromosome going to each of the daughter cells. Therefore, each human sperm and egg cell contains 23 of the colored bodies, and at fertilization the normal number of 46 is restored.
The randomness of the arrangement of the male and female chromosome at the time of fertilization creates an untold number of variations in the offspring, and it is safe to say there are no two organisms exactly alike genetically. Variations are a central part of the theory of the evolution of species by means of natural selection.
An error rarely occurs during the formation of sex cells, and the resulting offspring will get an extra or one less chromosome from one of its parents. Downs syndrome in humans, for example, results when the child has 47 chromosomes in each cell and not the normal 46.
Now that it has been possible to map or pinpoint the exact location of an individual gene on a human chromosome, genetic surgery will be used to alter the genes that cause serious inherited diseases in man such as hemophilia, Tay-Sachs, and sickle-cell anemia.
Who would have thought that those mysterious bodies in the nucleus of cells that puzzled biologists in the 19th century would turn out to hold the secret to life itself?
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.
From the Dec. 28, 2005, -Jan. 3, 2006, issue