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Название документа The discovery of the structure and function of DNA.doc
The discovery of the structure and function of DNA
Look around you and you will see people of all different shapes and sizes, hair and eye colour. However, despite the differences that seem so striking, if you compare the genes of any two human beings they will be 99.9% the same. This similarity means we belong to the same species while the 0.1% difference makes us individuals and means we are different from each other.
What exactly is a gene? To answer that we have to look at the chromosomes inside the nucleus of a cell. We get 23 of these from our mothers and 23 from our fathers. Found in almost every cell in the body, these chromosomes consist of long strands the chemical called deoxyribonucleic acid which is known as DNA. Each cell contains about two metres and if you put all the strands together they could travel to the Moon and back many times.
Scientists have known that DNA exists since 1869 when the Swiss scientist Johann Friedrich
Miescher noticed something he had not seen before in the nuclei of cells. He called it 'nuclein'. It was more than 70 years later that scientists examined his theory.
It was two researchers at Cambridge University, England, who, in 1953, finally revealed the secret of DNA and its role in the pattern of life. James Watson, an American zoologist, and Francis Crick, an English biologist, had already discovered that DNA was made up of sugar and phosphates in the form of a chain. The whole structure was bound together by four compounds called guanine, adenine, thymine and cytosine. Each of these four nucleotides, as they were known, had a different organic base. They knew that guanine and adenine were the largest of the four, and that thymine and cytosine were the smallest; what they did not know was how they all fitted together.
Returning from London one day, Watson hit on the idea of experimenting with different pairs. The two men made cardboard models of the four nucleotides and then tried systematically fitting them together. Watson later described their breakthrough in this way: 'Suddenly I became aware that an adenine-thymine pair was identical in shape to a guanine-cytosine pair'. Thus the men had discovered the relationship between the organic bases, and could see how each of these pairs of nucleotides formed a single rung on the so-called DNA ladder (DNA is shaped like a long ladder that is twisted into a spiral; this structure is known as the double helix). The discovery was the key to a much better understanding of the process of heredity.
There may be millions of these DNA ladder rungs linked to form hundreds of thousands of coils which in turn make up the structure of a single DNA molecule. In order for a cell to divide, all these coils have to be unwound. Then all the new nucleotides have to be linked in the right order and joined together by enzymes (enzymes are chemicals which we can find in all living organisms; they cause changes to take place). The entire process has to take place at great speed; in fact, in the time it takes for a cell to divide. Since bacteria cells split and form new cells in less than 20 minutes, this would mean that the DNA helix has to unwind very fast (several hundred turns a second) and a new chain has to be formed at the rate of several thousand nucleotides a second. At such a speed, a car engine would blow apart.
It used to be only in science fiction that the existence of a race of identical creatures could be imagined: a group of people with exactly the same hair colour, the same features and the same height. However, now this dream - or nightmare -could actually become reality. In theory, the process of creating replicas of any living being seems quite simple. First, a body cell, which contains the specific genes of a living organism, splits in two. The resulting new cells, each containing the same genes, then grow into two new, identical organisms. This process is known as cloning, and it can be applied to humans, animals, insects and plants.
Early experiments with cloning took place using the tadpoles of frogs. In 1968, Dr J.B. Gurdon of Oxford University, England, took an unfertilised frog's egg from a frog - let us call it frog number 1 - and destroyed its nucleus. This meant that he had removed all the genetic information which related to this frog. He then inserted a new nucleus extracted from a cell from another frog -frog number 2. The tadpole which developed from the egg produced by frog number 1 was identical to frog number 2, not to frog number 1! It was not until 1996, however, in Scotland, that a group of British researchers led by Ian Wilmut achieved the successful cloning of an adult animal. The result was Dolly, who has taken her place in the history books as the first lamb to be cloned from the DNA of an adult sheep.
Following the birth of Dolly, both scientists and ordinary people have begun to think about the possibilities of cloning. The latest technology now means that we can remove body cells from the best of our race, the brilliant scientist, the musical genius, the child prodigy, and ensure that the same genes are reproduced in as many babies as we wish. However, cloning does not mean copying. The process actually takes its name from the Greek word clon which means a twig. A twig has the same genetic information as the tree it comes from, but the two look very different. In the same way, a clone shares the same genes as its donor, but its behaviour and characteristics will be different: personality will always be unique.
Science has provided us with knowledge which seems to have unlimited possibilities. We can not only make designer human beings, but we can also use cloning to improve health. For example, scientists predict that in the future, pigs with organs that could be used in human transplants, could be cloned. Cloning could also enable us to learn more about the embryo and how organisms develop. Cloning could put an end to the risk of extinction of the endangered species on our planet; if animals can be cloned, they need never die out.
However, the process is very controversial. Some people have asked whether a cloned individual would really be a human; would it have a soul? Would there be relationships and responsibilities between donors and clones? What would be the position of the children of donors in relation to clones? These people are concerned that cloning, or genetic engineering, would interfere with the laws of religion or nature. Others are concerned that it might lead to attempts to alter the features of a particular race and result in a new kind of ethnic cleansing. The fact is that the new opportunities offered by science have always meant that we are faced with new ethical questions. These questions need to be discussed and evaluated before we move ahead.
Название документа The discovery of the structure and function of DNA.ppt
Описание презентации по отдельным слайдам:
Shape Size Striking Exactly Long strand Acid DNA Reveal Pattern of life Guanine Adenine Thymine Cytosine Nucleotides Cardboard models Breakthrough Spiral Форма Размер Поразительный Точно, конкретно Длинная цепочка Кислота Дезоксирибонуклеиновая кислота Раскрыть, обнаруживать Принцип жизни Гуанин Аденин Тимин Цитозин Нуклеотиды Картонная модель Достижение, открытие Спираль
Helix Ladder rung Coil Unwound Enzymes Blow apart Assemble Strand Bound Спираль Ступенька Виток размотанный Энзим Разлететься Собираться, созывать Цепь, нить Связывать
Read the text and decide if the following sentences are true or false. A particular species shares almost all its genes. We get equal numbers of chromosomes from our parents. Friederich Miescher immediately revealed the importance of DNA. The four compounds of DNA are not the same size. Watson and Crick did the computer modelling for nucleotide bases. A new helix chain is assembled at a comparatively slow speed.
Striking, substance, breakthrough, compound, coils, strand, bound, revealed, organic base, pattern What is the chemical ……… used in paint? A sprung seat is made up of ……… of strong wire. An important discovery that helps us find a solution is called a ………………… . A long length of something is a …………… . We can find a …………… if we watch something and see if it acts in similar ways over a period of time. It was quite …………… how different the children were. The sheets of paper where …………… together by string. Mendel's discoveries …………… the secret of heredity. A combination of two or more elements or parts is called a …………… . A(n) …………… is an organic compound which acts as base.
Listen to a talk. Then compare the information about DNA. Each strand has about …… billion letters of coding. We inherit the information from our ….. DNA will be useful in the future for……care. The Y chromosome comes from our ……… Archaeologists use DNA found in people’s ………… The police get information from DNA found at a ……
Discuss with your partner: How is DNA useful to different people in different situations? Talk about: archaeologists, doctors, the police
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