The Human Genome
V. Vemuri, Pleasanton,CA
Humans are made up of trillions of cells. Each cell is so small that you need a microscope to see it. Within each cell there is a nucleus, just like the seed of a mango. Within each nucleus, among other things, there are 23 pairs of chromosomes. These chromosomes are tightly coiled and, when seen through a microscope, appear like tiny strings. When uncoiled and placed end to end, all the chromosomes in a single cell will stretch to a length of about two meters.
Chromosomes are nothing but long filaments of DNA molecules. DNA stands for deoxyribonucleic acid. The DNA molecule has the shape of a double helix, or like a rope ladder twisted around an invisible pillar. The two long "ropes" of the ladder correspond to the two sugar-phosphate backbones. The "steps" of the ladder correspond to the so-called base-pairs. That is, a chromosome is nothing but a DNA molecule and a DNA molecule is nothing but a double stranded helix in which the strands are repeating nucleotide units. Each nucleotide unit consists of a sugar called deoxyribose, a phosphate group and a nitrogenous base. The sugar and phosphate groups together are the sugar-phosphate backbone and the nitrogenous bases are the base-pairs.
Let me explain what I mean by base-pairs. In DNA, the "steps of the ladder" are made out of four types of chemicals, called bases. Their names are adenine (A), thymine (T), guanine (G), and cystosine (C). Each step of the ladder is made out of two distinct bases, that is a base-pair. According to the rules of nature, only guanine and cystosine can form a pair as GC or CG. Similarly, only adenine and thiamine can form a pair as AT or TA.
If you count all the base pairs in all the chromosomes in a cell, you will get a number close to six billion, that is six followed by nine zeros. Let us look at these six billion base-pairs, not as individual steps of the ladder, but as groups of steps, because these groupings, called genes, are responsible for many of the observable attributes we see in people. Indeed, scientists believe that there are about 100, 000 genes in the cells of our bodies. More or less, these genes control our fate, the way we are and the way we live. To see the connection, simply remember that each of these genes is responsible for producing a different protein. To remain healthy, our bodies require about 50,000 different kinds of proteins to be manufactured at the right time, at the right place, and in the right quantity. A single "error" in a single gene is sufficient to alter the nature of the protein it produces.
Scientists are beginning to learn that about 4000 human disorders are known to be caused by defects in genes. There is general agreement that virtually all human afflictions are linked in some way to our genes. The task of the Human Genome Project is to map out the location and function of each of the human genes. What do I mean by mapping? Take color blindness. This is found in about 10% of men, but rarely in women. In 1911 it was found that this deficiency is transmitted to sons by mothers with normal color vision. This led scientists to assign the responsible gene to the sex-linked X chromosome. Recently, just by luck, scientists found out that the gene responsible for Huntington's disease is located on chromosome 4. Where exactly on chromosome 4 is a question that is still eluding the scientists. The monumental task is to associate with each gene all the properties controlled by that gene, and to associate eacg gene with one of the 23 chromosomes in the body. Once the mapping is completed, what we have in front of us a blueprint of the "fate written by Brahma on the forehead of each of us."