The chemical secret of life resides in the DNA molecule that composes the chromosomes. As noted earlier, special segments of this molecule -the genes - determine our heredity. Specifically, each gene, with the assistance of ribonucleic acid (RNA), directs the synthesis of a particular protein.

Since proteins serve as protoplasmic building blocks and enzymes (catalytic agents that spark metabolic activities), we can well appreciate that a given cell is what it is because of its particular DNA blueprint. What is more, when a cell divides, this blueprint or genetic code is passed along to its daughter cells.

This explains:

1. Why daughter cells are identical to the parent cell (for example, the offspring of liver cells are liver cells, and the offspring of kidney cells are kidney cells
2. Why children resemble their parents. In the latter instance, the offspring inherits one half of its genetic code from the mother and one half from the father

Genetic Disease

Hundreds of hereditary disease are known to be caused by faulty genes. The first such disease to be spelled out in pure chemical terms- right down to the last atom- was sickle cell anemia. The faulty gene in this case is the one that directs the synthesis of hemoglobin, the protein pigment of red blood cells. The result of the disease, of course, is faulty hemoglobin and sickling of the red cells.

Since these distorted cells are unable to pass through the capillaries, they plug these vessels and trigger blood clots. Further, sickled cells and ultimately undergo dissolution (hence, the anemia). Genes, however, come hn pairs (one from each parent), and in most genetic diseases the normal gene overshadows its faulty partner and is referred to as dominant, whereas the faulty gene is called recessive. This is true in sickle cell anemia, that is, the full blown disease occurs only in those who have inherited two faulty genes. Few of these victims live beyond age 40.