What Is a Mutation?

The body is like a Chinese puzzle¹ box. It consists of organs, such as liver, legs, eyes. The organs consist of tissues, such as bone, muscle, nerve. The tissues consist of cells. The cell contains a nucleus. The nucleus contains chromosomes. The chromosomes carry the genes. Mutations are changes in chromosomes and genes.

The cell and the nucleus can be seen under the microscope, but the chromosomes cannot always be seen. They become visible only at certain stages in the life of the cell, namely², when the cell divides to give two daughter cells. They then appear as rod-like or dot-like structures which, in thin tissue slices (слой, срез), can be stained with certain dyes which they take up more readily than the rest of the cell. The genes are too small to be seen even with a high-power microscope. The genes are arranged linearly along the chromosomes. Some particularly big chromosomes show a visible subdivision into smaller units, so that they look like strings of beads, or like ribbons with a pattern of cross-bands? These beads and bands are much too big to be the genes themselves, but they indicate the position of the genes on the chromosomes.

The number of chromosomes in the nucleus is characteristic for each species. Man has 46, the mouse (мышь) 40, the broad bean plant (боб) 12, maize (кукуруза) 20. Each chromosome carries hundreds of thousands of genes. It has been estimated⁴ that the chromosomes in a human cell carry at least 40,000 genes, possibly twice as many. This seems a large number, but it is not so large when we consider that the genes between them are responsible for all that is inborn and inherited in us. Genes determine whether we belong to blood-group A or О, whether we are born with normal vision or not, whether we have brown, blue or hazel eyes, whether on a rich diet we grow fat⁵ or remain slim (стройный), whether musical education makes virtuosi of us or we are unable to distinguish one tune (мелодия) from another, and so on through the thousands of details which together make up our physical and mental personalities.

Every time, before a cell divides, each chromosome makes another chromosome just like itself with the same genes in the same order. Then, when two cells arise from one, the old chromosomes separate from their new-formed duplicates and both "daughter cells" receive exactly the same numbers and types of chromosomes and genes.

The human body develops from a single cell, the fertilized egg, which contains 46 chromosomes. The egg divides to form two cells; these divide again to form four cells, and so it goes on until the whole body with its billions of cells has been formed. Before every cell di­vision, chromosomes and genes are duplicated. Every cell therefore contains the same 46 chromosomes carrying the same genes.

The process by which chromosomes and genes are duplicated is remarkably accurate. It results in millions and billions of cells with exactly the same genes. But sometimes, perhaps once in a million times, something goes wrong⁶. A gene undergoes a chemical change, or the new gene is not exactly like the old one, or the order of the genes in the chromosome has been changed. This process of change in a gene or chromosome is called a mutation. Its result, the altered gene or chromosome, is also often called mutation, but to avoid confusion⁷ it is better to speak of a mutated gene and a re-arranged chromosome, and reserve the term mutation for the process which produced them. The individual, which shows the effect of a mutated gene or re-arranged chromosome, is called a mutant.

When a chromosome on which a mutation has occurred makes a duplicate of itself in preparation for the next cell division, it copies the mutated gene or the new gene arrangement as accurately as it copies the unaltered portions. In this way a mutation is inherited and becomes perpetuated⁹ exactly like the original gene from which it arose. The enormous variety of genes which are found in every living species results from mutations, many of which may have occurred millions of years ago.

Notes

1. Chinese puzzle – неразрешимая загадка

2. namely – а именно

3. ribbons with a pattern of cross-bands – ленты с поперечными полосами

4. estimate – подсчитывать

5. grow fat – толстеть, полнеть

6. go wrong – разладиться, испортиться

7. to avoid confusion – чтобы избежать путаницы

8. perpetuate – сохранять навсегда, увековечивать