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- Enzymes and Genes
Notes
Some Familiar Proteins The hair on your head is an example of an almost pure protein. So is silk. The protein of hair is called keratin by chemists, and the protein of silk is called fibroin. Both keratin and fibroin are comparatively simple proteins. Their molecules consist of amino acids strung together in more or less a long straight line. Such lines of amino acids are called polypeptides. In the 1940’s chemists learned to manufacture quite long polypeptide chains in the laboratory. They used only one or two different amino acids in doing so, however.
Notes
Enzymes and GenesThe nucleus of the cell is in charge of cell division. Unfortunately, most of the details of the process are as yet unknown. Still we can describe some of them. Inside the nucleus are small patches that can react with certain dyes to become strongly coloured. Biologists noticed them for that reason and called the material in the patches chromatin from the Greek word for colour. In the process of cell division, the chromatin collects into little rods of varying size. The rods are called chromosomes. In the nuclei of human cells are forty-six such chromosomes, existing in pairs. There are twenty-three pairs of chromosomes, in other words. Each kind of creature has its own fixed number of chromosomes. A rat (крыса) has thirty-eight chromosomes, a grasshopper (кузнечик) twenty-four and a housefly (муха) only twelve. A crayfish (рак) on the other hand, has over two hundred chromosomes. Before a cell divides, every chromosome lines up in the centre of the cell and splits in two. The two halves of each chromosome move apart and when the cell divides, each new cell has a duplicate of all the original chromosomes. It is these chromosomes that control a cell’s characteristics. A cell’s nature is determined by the kind of chromosomes it has. Every chromosome is actually a chain of protein molecules which are called genes. Genes are strung along a chromosome as beads are in in a necklace. The genes have a certain chemical resemblance to viruses. Each gene is thought to control a single characteristic of an organism. For instance, there is a gene for blue eyes and one for brown eyes; one for straight hair and one for wavy hair. Every human being has thousands of different genes scattered through1 his various chromosomes. Whenever a chromosome splits in two, during cell division, each gene duplicates itself exactly and both daughter cells get one apiece.How does a gene control a particular characteristic? Many people now think that each gene is in charge of manufacturing one particular enzyme in the cell. But how does a gene manufacture an enzyme? For that matter, how does a gene duplicate itself? This is probably the most important unanswered question in biochemistry today. There are theories, of course. There are enzymes that take proteins apart and separate them into amino acids. These protein splitters can also put amino acids back together again. Apparently, then, the beef (мясо) protein we eat or milk protein, or wheat (пшеница) protein is separated into amino acids and then put together in a different arrangement to make human protein. But how is the arrangement figured out2, when there are so many possibilities? Here is where the gene comes in3. Genes are nucleo-proteins. The non-protein part of the molecule is the nucleic acid. Each gene contains its own variety of nucleic acid. Each different nucleic acid somehow acts as a model for the formation of a particular enzyme. Nucleic acids, therefore, control amino acid arrangements. How? Chemists just began working out the method in the 1950’s. The nucleic acid of the chromosomes forms a "messenger"4 molecule which leaves the nucleus and joins particles in the cytoplasm which are called ribosomes. In the ribosomes are tiny fragments of nucleic acid molecules. There are a number of kinds of these fragments and each will attach its own particular type of amino acid. These nucleic acid fragments carry their amino acids to the "messenger" molecule and use its structure as a guide. They line up to match the structure and each transfers its amino acid. In this way, an entire protein molecule is formed with an exact structure according to the original design of the chromosome’s nucleic acid. You may wonder how enzymes can control characteristics. How can they decide blue or brown eyes, for instance? Well, eyecolour is due to a pigment called melanin. When the eyes contain very little melanin, they appear blue. With more melanin, they are brown. Melanin is formed in the body as a result of a chemical reaction which is catalysed by the enzyme, tyrosinase. The amount of the formed melanin depends upon the amount of tyrosinase present. Possession of a gene producing much tyrosinase will result in brown eyes. A gene that produces less tyrosinase makes for blue eyes. What happens when a cell splits in two without proper duplication of genes? Sometimes the daughter cells just can’t live. At other times, the cells survive, but with a changed chemistry. Some biochemists think that cancer (рак) cells may originate as the result of such imperfect duplications. Notes
THE FACULTY OF GEOGRAPHY A Country Across the Channel The British Isles, which include Great Britain, Ireland and a lot of smaller islands, are situated off the north western coast of Europe and once formed part of that continent. They became islands when they were separated from it. The separation took place thousands of years ago, after the last Ice Age, when the ice melted, the level of the oceans rose and drowned the low-lying coastlands. Politically the British Isles are divided into two countries —the United Kingdom of Great Britain and Northern Ireland, and the Irish Republic or Eire. All in all there are over 5,000 islands in the system of the British Isles which lie on the continental shelf, the zone of shallow water surrounding at present the continent and resembling a shelf above the deep water of the oceans. From the European continent the British Isles are separated by the English Channel and the North Sea. The English Channel, in its widest part in the west is 220 km wide, and in the narrowest, what is called the Strait of Dover, only 32 km. So, the islands have had an easy and mainly profitable contact with mainland Europe. In the past there were a number of schemes how to connect the two coasts. In 1994 the dream came true: the construction of the two-rail tunnel was completed and it was opened for public use. The most important sea routes pass through the English Channel and the North Sea linking Europe with the Americas and other continents. The advantageous geographical position of Great Britain created favourable conditions for the development of shipping, trade and the economy as a whole. However, the true value of Britain's geographical position has not always been obvious. Indeed, it clearly emerged in the late 15th and 16th centuries, a period which saw the discovery of America and the opening of the sea route round the Cape of Good Hope to the Far East. Before this time European civilization had been centred in the-Mediterranean lands. The British Isles, although developing slowly, were on the margins of this civilization. With the discovery of the Americas the British Isles became an intermediary between Europe and the New World. From the 16th century onwards, the wealth and influence of Great Britain increased rapidly. With the acquisition of overseas colonies and the establishment of an empire she attained the status of a world power. Her position as such was emphasized by the Industrial Revolution of the 18th and 19th centuries, which was based on her resources of coal and iron and on the markets she had established throughout the world. By Victorian times (1837-1901) Great Britain had become the richest country in the world, the first great modern industrial and capitalist society. During the 20th century Britain has lost this position and her economy has faced increasing problems, especially with the collapse of the empire. The problems of supporting her population (57 million) on such a small land area (244,100 sq km) are also obvious. At the same time, however, it is important to remember that Britain, with the benefits of North Sea oil production, is still one of the leading industrial and trading countries in the world. The British Isles in general, but especially England, form one of the most densely peopled areas in the world. Archaeologists and historians have demonstrated that the present-day inhabitants of Britain and Ireland are largely the descendants of settlers and traders from western Europe, who came to these islands in a series of invasions, from about 2500 B.C. down to the Norman Conquest of 1066. The growth of population in Britain reflects, to a large extent, the economic changes. The basic population distribution of the 20th century had been established by the Industrial Revolution and the increase in population of the 19th century. The British Isles, apart from Great Britain and Ireland, the two largest islands, include several other important islands and islands known as the Hebrides. They are groups of islands. Off the northwestern divided into the Inner and Outer Hebrides, coast of Great Britain there is a group of They are separated from each other by the Sea of the Hebrides and the Little Minch. The main occupation of people here is farming combined with fishing. Off the northern coast of Scotland, separated from the mainland by the stormy Pentland Firth are the Orkney Islands, comprising about a hundred islands. Most of the people (about 20,000) are engaged in dairy and poultry farming. Bacon, cheese and eggs are exported to Central Scotland. The Shetland Islands are situated about 70 miles north of the Orkneys. They provide thin poor soils suitable only for rough pasture. The population (18,000) is actively engaged in herring-fishing. Apart from fish, the only exports from the islands are Shetland ponies and lace knitted from the wool of local sheep. In the middle of the Irish Sea there is the Isle of Man (571 sq km). The island is administered by its own Manx Parliament and has a population of about 50,000 chiefly engaged in farming, fishing and tourist trade. The largest settlement is the holiday resort of Douglas (23,000). Another important island in the Irish Sea is Anglesey, situated off the north coast of Wales. Anglesey contains only 52,000 people, and more of the working population are now engaged in industry than in fishing and agriculture. This is due partly to an increase in the tourist trade and partly to the introduction of several new industries, for example, the construction and operation of the nuclear power station at Wylfa. The Isle of Wight is in the English Channel. It is diamond-shaped, 40 km from west to east, and about half as much from north to south. The Isle of Wight lies across the southern end of Southampton Water, and is separated from the mainland by the Solent. With its sunny beaches and pleasant varied countryside, the island forms one of the most important tourist resorts. It is linked to London by ferry and rail services. Also lying in the English Channel off the extreme south-western coast of Great Britain is a tiny group of the Isles of Scilly, another resort area. The Channel Islands lie to the southwest on the French side of the English Channel. They are known to the French as the Isles Normandes. The Channel Islands form an archipelago, separated by shallow waters from northern France. As part of the Duchy of Normandy, they have been attached to the English Crown since the Norman Conquest (1066). The total area of the islands is only 194 sq km, but the population is over 130,000 what results in high density of population — 686 per sq km. In summer the population increases greatly by holiday-makers. The chief islands of the group are Jersey and Guernsey. In rural areas many of the people speak a French-Norman dialect, but the official languages are English and French. The British Isles arc known for their greatly indented coastline. Therefore there are many bays and harbours, peninsulas and capes on the coast, which were formed as a result of the raising and submerging of the land surface in the process of the geological development of the islands. Due to its extreme indentity the coastline of Great Britain, despite its relatively modest size, is 8,000 km long. Very much indented is the western coast, especially the coasts of Scotland and Wales. The east coast is less lofty and more regular than the west coast, and the coastal lowlands are flooded frequently. Hardly has anything been more important in British history than the fact that Great Britain is an island. Living on islands, and therefore near the- sea, the inhabitants naturally grew into a nation of sailors. Their love of the sea led them to become navigators and discoverers of new lands in many parts of the globe. The capital of the country, London, is an enormous city. Its name is probably derived from the Celtic Llyn, a pool or lake (the River Thames at an earlier period expanded into a considerable lake — the part immediately below London Bridge is still “the Pool”), and din wdun, a hill, fort, or place of strength. The “hill” may have been that on which St.Paul's now stands, or Cornhill. When the Romans conquered Llyndun they Latinised the name as Londinium. Great military roads radiated from the city to various parts of Britain, and distances were measured from the lapis milliaris (mile-stone) in the Forum of Agricola, in the heart of the town. The stone, now known as the London Stone, may still be seen in the wall of St.Swithin’s Church, Cannon Street. Under the Saxons London became the metropolis of the kingdom of Essex. The city was constituted by Alfred the Great the capital of England, York and Winchester having previously enjoyed that dignity in succession — the former under the Romans, the latter under the Saxons. In 994, the first bridge accross the Thames was built. The White Tower, in the Tower of London, was erected by William I in 1078, on the site of the Roman fort already noticed. The same king granted a charter to the city confirming the burghers in the rights enjoyed by them under Edward the Confessor. King John granted the citizens several charters, and in Magna Charta (1215) it was expressly provided that London should have all its ancient privileges and customs. About 7 million people live in Greater London. The oldest part of London is the “City”. Centuries ago, there was a high wall all round the City of London. Places like Soho and Chelsea were small villages outside the City. Now they are part of Central London. There are always crowds of tourists in London. They visit London’s many sights. Buckingham Palace and Westminster Abbey are two of the favourite ones. London is great for shopping. There are lots of big department stores, like the famous Harrods and Selfridges. People from many different countries live in London today, and their way of life has given London a new “face”. If you want to see the latest ideas in fashion, go and look at the shops in the King’s Road. жүктеу/скачать 0.55 Mb. Достарыңызбен бөлісу:
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