Unit 25 At the conference

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DNA

　　ACID, organic chemical of complex molecular structure that is found in all prokaryotic and eukaryotic cells and in many viruses. DNA codes genetic information for the transmission of inherited traits.

　　DNA was first discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943. In 1953. James Watson and Francis Crick determined that the structure of DNA is a double-helix polymer, a spiral consisting of two DNA strands wound of a long chain of monomer nucleotides. The nucleotide of DNA consists of a deoxyribose sugar molecule to which is attached a phosphate group and one of four nitrogenous bases: two purines (adenine and guanine)and two pyramidines(cytosine and thymine). The nucleotides are joined together by covalent bonds between the phosphate of one nucleotide and the sugar of the next, forming a phosphate-sugar backbone from which the nitrogenous bases protrude. One strand is held to another by hydrogen bonds between the bases; the sequencing of this bonding is specific—i. e. , adenine bonds only with thymine, and cytosine only with guanine.

　　The configuration of the DNA molecule is highly stable, allowing it to act as a template for the replication of new DNA molecules, as well as for the production (transcription) of the related RNA (ribonucleic acid) molecule. A segment of DNA that codes for the cell's synthesis of a specific protein is called a gene.

　　DNA replicates by separating into two single strands, each of which serves as a template for a new strand. The new strands are copied by the same principle of hydrogen-bond pairing between bases that exists in the double helix. Two new double-stranded molecules of DNA are produced, each containing one of the original strands and one new strand. This “semiconservative” replication is the key to the stable inheritance of genetic traits.

　　Within a cell, DNA is organized into dense protein—DNA complexes called chromosomes. In eukaryotes, the chromosomes are located in the nucleus, although DNA also is found in mitochondria and chloroplasts. In prokaryotes, which do not have a membrane-bound nucleus, the DNA is found as a single circular chromosome in the cytoplasm. Some prokaryotes, such as bacteria, and a few eukaryotes have extrachromosomal DNA known as plasmids, which are autonomous, self-replicating genetic material. Plasmids have been used extensively in recombinant DNA technology to study gene expression.

　　The genetic material of viruses may be single or double-stranded DNA or RNA. Retroviruses carry their genetic material as single-stranded RNA and produce the enzyme reverse transcriptase, which can generate DNA from the RNA strand.

II. DNA fingerprinting

　　DNA fingerprinting, also called DNA TYPING, in genetics, method of isolating and making images of sequences of DNA (deoxyribonucleic acid). The technique was developed in 1984 by the British geneticist Alee Jeffreys, after he noticed the existence of certain sequences of DNA (called minisatellites) that do not contribute to the function of a gene but are repeated within the gene and in other genes of a DNA sample. Jeffreys also determined that each organism has a unique pattern of these minisatellites, the only exception being multiple individuals from a single zygote(e. g. , identical twins) .

　　The procedure for creating a DNA fingerprint consists of first obtaining a sample of cells containing DNA (e. g. , from skin, blood, or hair), extracting the DNA, and purifying it. The DNA is then cut at specific points along the strand with substances called restriction enzymes. This produces fragments of varying lengths that are sorted by placing them on a gel and then subjecting the gel to an electric current (electrophoresis): the shorter the fragment the more quickly it will move toward the positive pole (anode). The sorted, double-stranded DNA fragments are then subjected to a blotting technique in which they are split into single strands and transferred to a nylon sheet. The fragments undergo auto radiography in which they are exposed to DNA probe species of synthetic DNA that have been made radioactive and that bind to the minisatellites. A piece of X-ray film is then exposed to the fragments, and a dark mark is produced at any point where a radioactive probe has become attached. The resultant pattern of these marks can then be analyzed.

　　If only a small amount of DNA is available for fingerprinting, a polymerase chain reaction (PCR) may be used to create thousands of copies of a DNA segment. PCR is an auto mated procedure in which certain oligonucleotide primers are used to repeatedly duplicate specific segments of DNA. Once an adequate amount of DNA has been produced, the exact sequence of nucieotide pairs in a segment of DNA can be determined using one of several biomolecular sequencing methods. New automated equipment has greatly increased the speed of DNA sequencing and made available many new practical applications, including pinpointing segments of genes that cause genetic diseases, mapping the human genome, engineering drought-resistant plants and producing biological drugs from genetically altered bacteria.

III. GENE

　　GENE, a natural unit of the hereditary material, which is the physical basis for the transmission of the characteristics of living organisms from one generation to the next.

　　At first glance, the hereditary material does not seem to lend itself to rational dissection. The progeny of a pair of cats is a kitten, which develops into a cat; dogs engender pups; humans engender children . These are the most primitive observations concerning heredity, and they give no hint of the possible analysis of the hereditary material into unit components, or genes.

　　Initial Concept of the Gene. Modern genetics is based on the study of discrete differences within a single species capable of interbreeding. Gregor Mendel, a 19th century Austrian botanist-monk, studied heredity in garden peas. He foretold the modern concept of the gene by hypothesizing that the appearance of alternative characters in the offspring was determined by the transmission of invisible “anlagen” —the concept denoted by the Danish geneticist Wilhelm Johannsen in 1909 as “the genes”.

　　Mendel’s finding have now been extended to the proposition that every character of a plant or animal is controlled by a corresponding gene. For example, the height of a pea plant (tall or dwarf) is determined by the inheritance of a specific gene. Later it became clear that many different genes could cooperate in determining a given character, perhaps by influencing its development at different critical stages. Thus we could speak of “the genes” collectively, meaning the totality of genetic, or hereditary, information in a cell or in an organism.

　　More detailed analysis of what a single gene is, and does, was beclouded for many years by residuals of the vitalistic concept of protoplasm as a “living substance” responsible for the intrinsic properties of the cell. Today, the overwhelming weight of scientific evidence points to the genes as the core of the cell, the seat of its biological specificity; nevertheless, the cytoplasm( the content of the cell outside the nucleus) is the arena of the genes’ ultimate action and in a few cases may add its own input to the entirety of hereditary information passed from parental to daughter cells.
Questions About the Gene. The concept of the gene, while in itself a great advance. left many unanswered questions. For example, how large is a gene? How many genes are there in a single cell? Are all genes alike in the different cells of the body? Can a gene be seen under a light microscope? Do genes change, or mutate, independently of one another? How closely do human genes resemble genes of other species? Is a virus a gene? How do we define the boundaries of one gene that set it off from another gene? What is the connection between a gene and the character it controls? Are all genes in the chromosome of cells? Do genes vary, and if so, does any rational classification emerge? Above ail, what is the chemical nature of the gene? The last question elucidated in the late 20th century, opened the way to understanding the others.

　　Search for Answers. Before the introduction of biochemical analysis, breeding experiments gave brilliant, if tentative, answers to many questions about the gene. Indeed, between 1915 and 1945 many geneticists took pride in how far they could push genetic analysis without direct chemical information. Mapping studies, based on the findings of breeding experiments, showed that genes were arranged in linear order on chromosomes. The studies failed, however, to reveal very much about the nature of a particular gene. With the advent of biochemical analysis, tremendous advances in the field of genetics were made, and in the 1950’s and 1960 s all of the questions listed above were illuminated. Nevertheless, as with all important scientific advances, these have opened many new challenges and opportunities.

　　Scope of This Article. The origin and historical development of the concept of the gene are presented fully in the article GENETICS. In this article, we shall proceed directly to contemporary views of the concept of the gene from the perspective of molecular genetics.

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