AN OVERVIEW OF GENETICS
Should you ever visit the attractive university town of Cambridge, make sure to enjoy some refreshment at a small pub known as The Eagle, for it was in this pub that two brilliant young scientists, James Watson and Francis Crick, discovered the molecular structure of DNA nearly 50 years ago. This discovery has proved to be one of the most profound advances in biology and medicine, as it has led to a far deeper understanding of how the trillions of cells that make up our bodies function. Before looking more closely at DNA we should first examine the chromosomes, which carry this precious cargo of DNA.
With very few exceptions, every cell in our bodies contains a nucleus in which two sets of chromosomes are embedded; one set donated by each parent, providing 24 distinct chromosomes in the human. Human females have a pair of X sex-determining chromosomes, while males have one X and one Y pair of sex chromosomes. The latter pair often being blamed for the more "adventurous" behaviour of males. Chromosomes are relatively large structures and can be examined under an ordinary light microscope. In this way some hereditary defects, such as Down's syndrome, can be detected. The cells of a Down's patient contain a third copy of chromosome 21. Diagnosis of most hereditary diseases, however, requires far more sophisticated analysis, including studying DNA sequences within genes.
DNA (deoxyribonucleic acid) is a large molecule constructed in the shape of a double helix, held together by smaller nitrogenous bases. The analogy of a twisted ladder is often used to explain this structure. Imagine a ladder that has been twisted to form a spiral staircase. The sides of the spiral consist of alternating sugar and phosphate molecules, while the rungs of the ladder consist of the nitrogenous bases adenine, thymine, cytosine and guanine. The sequence of these bases provides the genetic code within our genes that control the development, growth and functioning of all the cells in our bodies.
A gene consists of a specific sequence of nitrogen bases within a DNA molecule and there are about 30,000 genes in the complete human genome. The usual way in which genes control the functioning of our cells is by instructing cells to make new molecules, usually proteins which in turn alter cell function in a programmed way. This usually occurs after a gene has been switched on, either by another gene, or by a new molecule entering the cell. The DNA of the activated gene uncoils, exposing the base code for the synthesis of a new messenger molecule that incorporates the identical code of the exposed DNA. The messenger molecule (messenger RNA) then passes out of the nucleus to enter the body of the cell, where it controls the synthesis of a new molecule based on the code it is carrying. It is difficult to imagine, for example, how many of these intricate and exquisitely timed reactions must take place in a human female to ensure successful ovulation, fertilization, implantation of the fertilized ovum and the development of a fully formed child from a single cell. It is also no surprise that sometimes mistakes occur, including mutations, often with tragic consequences.
The Human Genome Project
On Monday, February 12, 2001, two major research groups published the results of their respective human genome projects &endash; two atlases of all the human genes. These results, known also as The Book of Life, will revolutionize future medical research because they will allow researches to unravel the fundamental genetic bases of diseases such as cancer, heart disease, cystic fibrosis, diabetes and many others. Drugs can now be targeted to alter specific gene function instead of the hit and miss techniques currently employed. The time required to develop new drugs will also be greatly reduced, possibly from an average of 14 to four years. There is, however, still much to be done &endash; although we now know all the letters in this exciting book of life, we still don't understand the words.
Other interesting facts to emerge from these historical studies are that our genes are almost identical to those of all other mammals and that in the case of chimps, we share 99.9% of their genes. We even share 10% of our genes with the lowly roundworms and have only twice as many genes as a fruit fly. The genetic differences between human races are so small that the very concept of race can be questioned. These results give greater credence to the theory of evolution and will advance the philosophy of genetic determinism. They will also stimulate adventurous experiments in cloning which are sure to raise many ethical dilemmas.
The huge amount of data included in the American human genome report is freely available without cost on the website of the U.S. National Institute of Health.