Enzymes, Genes and Consistent Chemical Changes

The Genetic Effects of Radiation by Isaac Asimov is part of HackerNoon’s Book Blog Post Series. The table of Links for this book can be found here. The Machinery of Inheritance: Enzymes and Genes

Enzymes and Genes

Each cell is a tiny chemical factory in which several thousand different kinds of chemical changes are constantly taking place among the numerous sorts of molecules that move about in its fluid or that are pinned to its solid structures. These chemical changes are guided and controlled by the existence of as many thousands of different enzymes within the cell.

Enzymes possess large molecules built up of some 20 different, but chemically related, units called amino acids. A particular enzyme molecule may contain a single amino acid of one type, five of another, several dozen of still another and so on. All the units are strung together in some specific pattern in one long chain, or in a small number of closely connected chains.

Every different pattern of amino acids forms a molecule with its own set of properties, and there are an enormous number of patterns possible. In an enzyme molecule made up of 500 amino acids, the number of possible patterns can be expressed by a 1 followed by 1100 zeroes (10¹¹⁰⁰).

Every cell has the capacity of choosing among this unimaginable number of possible patterns and selecting those characteristic of itself. It therefore ends with a complement of specific enzymes that guide its own chemical changes and, consequently, its properties and its behavior. The “instructions” that enable a fertilized ovum to develop in the proper manner are essentially “instructions” for choosing a particular set of enzyme patterns out of all those possible.

The differences in the enzyme-guided behavior of the cells making up different species show themselves in differences in body structure. We cannot completely follow the long and intricate chain of cause-and-effect that leads from one set of enzymes to the long neck of a giraffe and from another set of enzymes to the large brain of a man, but we are sure that the chain is there. Even within a species, different individuals will have slight distinctions among their sets of enzymes and this accounts for the fact that no two human beings are exactly alike (leaving identical twins out of consideration).

Each chromosome can be considered as being composed of small sections called genes, usually pictured as being strung along the length of the chromosome. Each gene is considered to be responsible for the formation of a chain of amino acids in a fixed pattern. The formation is guided by the details of the gene’s own structure (which are the “instructions” earlier referred to). This gene structure, which can be translated into an enzyme’s structure, is now called the genetic code.

Stained section of one cell from salivary gland of Drosophila, or fruit flies, reveals dark bands that may be genes controlling specific traits.

If a particular enzyme (or group of enzymes) is, for any reason, formed imperfectly or not at all, this may show up as some visible abnormality of the body—an inability to see color, for instance, or the possession of two joints in each finger rather than three. It is much easier to observe physical differences than some delicate change in the enzyme pattern of the cells. Genes are therefore usually referred to by the body change they bring about, and one can, for instance, speak of a “gene for color blindness”.

A gene may exist in two or more varieties, each producing a slightly different enzyme, a situation that is reflected, in turn, in slight changes in body characteristics. Thus, there are genes governing eye color, one of which is sufficiently important to be considered a “gene for blue eyes” and another a “gene for brown eyes”. One or the other, but not both, will be found in a specific place on a specific chromosome.

The two chromosomes of a particular pair govern identical sets of characteristics. Both, for instance, will have a place for genes governing eye color. If we consider only the most important of the varieties involved, those on each chromosome of the pair may be identical; both may be for blue eyes or both may be for brown eyes. In that case, the individual is homozygous for that characteristic and may be referred to as a homozygote. The chromosomes of the pair may carry different varieties: A gene for blue eyes on one chromosome and one for brown eyes on the other. The individual is then heterozygous for that characteristic and may be referred to as a heterozygote. Naturally, particular individuals may be homozygous for some types of characteristics and heterozygous for others.

When an individual is heterozygous for a particular characteristic, it frequently happens that he shows the effect associated with only one of the gene varieties. If he possesses both a gene for brown eyes and one for blue eyes, his eyes are just as brown as though he had carried two genes for brown eyes. The gene for brown eyes is dominant in this case while the gene for blue eyes is recessive.

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This book is part of the public domain. Asimov, Isaac. (October 13, 2017). THE GENETIC EFFECTS OF RADIATION. Urbana, Illinois: Project Gutenberg. Retrieved June 2022, from https://www.gutenberg.org/files/55738/55738-h/55738-h.htm#c4

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