Учебно-методическое пособие для студентов I-II курсов заочного отделения неязыковых факультетов
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- Evolution and Heredity
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
Evolution and HeredityMore than a hundred years ago people believed that plants and animals have always been as they are now. They thought that all the different sorts of living things, including men, had been put here by some mysterious (таинственный) power. It was Charles Darwin, born at Shrewsbury in February, 1809, who showed that this was just a legend. As a boy Darwin loved to walk about the countryside collecting insects, flowers and minerals. He enjoyed helping his elder brother at chemical experiments in a shed (сарай) at the far end of their garden. These hobbies interested him much more than Greek and Latin, which were his main lessons at school. His father, Dr. Robert Darwin, sent Charles to Edinburgh University to study medicine. But Charles disliked the medical career. He spent a lot of time with a zoologist friend watching birds and other animals in their natural state and collecting insects in the surrounding countryside. Then his father sent him to Cambridge to become a clergyman1. But Darwin did not care for lectures. He did not want to be a clergyman. At 22 he graduated from Cambridge University and soon was offered an unpaid post as naturalist on the ship "The Beagle". The young naturalist asked himself whether all forms of life always existed just as they are now. This was what everyone believed and what he had been taught, but he doubted it very much. Three and a half years travelling around the world on the British ship "The Beagle" convinced Darwin that his doubts were justified. He returned from his travels convinced that man and all the living creatures on earth today are related. All have grown from earlier types, and those from earlier ones in an unbroken line back to a primitive one-cell creature. More than a thousand million years ago, a small blob of jelly1 floated on the shallow seas of the young earth. It and others like it were the only life on earth. In half a milliard years that blob of jelly had become different kinds of sea worms (червь) and sea scorpions, sea weeds (морские водоросли) and other simple sea plants. During the next half milliard years some of this life crawled (ползти) onto the barren (бесплодный) land. The first land animals were "amphibians", equally at home on land and in the water, like present-day frogs. There were also primitive scorpions, the descendants of which became insects or spiders (паук). From the seaweeds that took root on shore came ferns (папоротник) and mosses (мох). The amphibian became reptiles. For one hundred million years they ruled the earth. Out of them came birds and mammals. Gradually the mammals changed into all the different kinds we have today, including man. Each of these changes was very gradual and took thousands of years. What makes you and your brothers and sisters look somewhat alike? What makes all of you look like your father and mother, and yet also a little different? The answer is to be found in the laws of heredity. Gregor Mendel, son of an Austrian farmer, wanted to be a scientist but couldn't afford the university. He became an Augustinian monk3 and, in the years between 1843 and 1865, he became a great scientist. In the garden of the monastery he raised garden peas – pure tails, pure dwarf (карликовый) and so on. Then, when he was sure he had pure strains, he began crossing them. He did the same with green and yellow peas. In all he raised and studied more than 10,000 specimens. From the way these peas transmitted and inherited various traits such as height or colour, Mendel worked out the laws of heredity. They have been found to be true for all types of plants and animals, including man, and have been widely used in the improvement of flowers and agricultural crops and the breeding of dogs and livestock. Notes
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