How does all the genetic information contained in the genes get translated into making an individual baby human being? Scientists realized that whatever the nature of the mechanism, it must be remarkably faithful and accurate to pass on so much information from one generation to the next. By the 1950s it was known that genetic information, the blueprint for an individual, was contained in an area of the cell known as the nucleus. Scientists also knew that all animals and plants were composed ofs and that s were themselves made from substances called amino acids, of which there were about 20. The question was then: given 20 basic ingredients where did the instructions necessary to make human beings come from? Whatever the instructions were, they had to be in the form of a language that could replicate itself. They were found in the chemical substance known as DNA (deoxyribonucleic acid).
The story is much easier to explain than it was for the molecular biologists Watson and Crick to unravel, but theirs was undoubtedly one of the most fundamental discoveries in the whole of biological science. They were determined to work out the structure of the DNA molecule. They knew it was made up of sugars, phosphates and four other substances: adenine, thymine, guanine and cytosine. Using a technique called X-ray crystallography to work out how these substances were arranged they found a chemical that resembled a spiral staircase. The spiral, or helix, appeared to be made up of the sugars and the phosphates whereas the steps themselves were made of adenine, thymine, guanine and cytosine, which they noticed seemed to be arranged in a particular pattern. This was the blueprint, the so-called genetic code. The problem then was how this genetic code could exert its control over virtually every process in our bodies. It has now been proven that any hereditary characteristic in essence depends on the different proteins that can be made. The sequence of amino acids in these proteins depends on the DNA molecule of the chromosomes. The specific arrangement of the substances adenine, thymine, cytosine and guanine appears to code for specific amino acids.
When a cell needs to manufacture a particular protein, the double spiral of the DNA-molecule will unravel just like opening a zip fastener. It then exposes just the particular sequence that codes for the manufacture of the required protein. This sequence is copied mirrorwise by a messenger molecule. This messenger molecule known as RNA (ribonucleic acid) has some similarities to DNA. When a sequence is copied mirrorwise it means that at the place DNA has thymine, RNA will have adenine, where DNA has guanine, RNA gets cytosine and so on. The RNA strand is then transported outside the nucleus to the site where proteins are manufactured. The sequence of base-pairs in the DNA molecule is thus reflected in the composition of RNA. In turn the sequence of base-pairs in RNA determines the location of each amino acid.
Protein molecules of all kinds are made every day, most of the time without mistakes. This in abundance shows that in DNA, nature has chosen a very reliable method of data transmission.
The reliability of this method is also shown when one thinks of all the billions of cells in our body that have exactly the same genetic material in their chromosomes. This is copied originally from the DNA in the chromosomes of the fertilized egg. At each cell division DNA replicates itself, to give the daughter cells an identical double helix DNA. This replication is in some way similar to the coding of RNA to produce proteins. The complementary or mirror-image helices also unravel. But this time each strand acts as a template for the manufacture of another complementary DNA strand. In the end each original strand is accompanied by a new strand which is an exact copy of the other original strand. In other words there are now two identical DNA helices, each of which belongs to a daughter cell.
In recent years molecular biologists have been able to manipulate this whole process. For example, the messenger RNA that carries the instructions to make human insulin and the hormone that controls blood sugar metabolism have now been identified. From the messenger RNA, it is possible to make the necessary DNA or the genes for insulin. This piece of DNA can be inserted into the DNA of a fast-growing bacterium, and insulin can be produced in abundance. This is currently being used as a commercial process and is part of a new biologically-based industry called biotechnology.
The same techniques have shown that there are certain genes which cause cells to become cancerous. In the future, it is certain that scientists will be able to pinpoint the function of every gene in human cells, although discovering the manner in which they interrelate may pose a bigger problem. The causes of many inherited disorders may also be found through these techniques but whether science will provide a cure for them remains to be seen.
The effects of certain drugs and forces such as X-rays, which produce abnormal cells in individuals, is now better understood. The abnormalities, or mutations,* are traced back to altered DNA in which the genetic code has been changed. Drugs such as thalidomide, X-rays, chemicals in cigarette smoke and many other `mutagens’, which have the effect of changing the code, are now being identified.