DNA (Deoxyribonucleic Acid)

Deoxyribonucleic acid, or DNA, is the genetic material that carries the code for all living things. This code determines the form, development, and behavior patterns of an organism and is part of the chromosomes that exist within the nucleus of cells. DNA consists of two long chains joined together by chemicals called bases and coiled together into a twisted-ladder shape.

DNA is a large molecule found in almost all organisms and contains codes for the making and using of proteins. Since proteins carry out the work of all cells, it is DNA that ultimately controls and directs all the activities of a living cell. Biologists have known about DNA for a very long time. Even before they discovered that genes control heredity, they were aware that the cell's chromosomes were made up of protein and a special chemical they called deoxyribonucleic acid (DNA). Although DNA was discovered in 1869, more than fifty years passed before biologists believed that genes were composed of DNA. As a nucleic acid, DNA was considered too simple a chemical to contain the huge amount of complex information needed to determine heredity. Since it is made of only four or sometimes five chemical bases, called nucleotides, no one thought that DNA was complex enough. However, as more experimental evidence began to point toward DNA as key in the transmission of hereditary characteristics, more scientists began to turn their attention to DNA.

WATSON AND CRICK DISCOVER THE STRUCTURE OF DNA

By the early 1950s, an unusual pair of scientists teamed their efforts in the passionate belief that the structure of DNA held the key to understanding how genetic information is stored and transmitted. In 1951, the twenty-four-year-old American geneticist James Watson met the thirty-six-year-old English physicist (and self-trained chemist) Francis Crick and the two decided to try to solve the puzzle of how such a simple material as DNA could store so much complicated information. Both knew they did not have to make any new discoveries, but instead, had to solve what might be called the "molecular architecture" of DNA.

The key to solving that problem lay in a technique known as x-ray crystallography. When x rays are directed at a crystal of some material, such as DNA, they are reflected and refracted by the atoms that make up the crystal. A refraction pattern is produced from which a skilled observer can determine how the atoms of the crystal are arranged. However, this is more difficult than it sounds, and Watson and Crick were trying to solve a very difficult puzzle. It was the chemical composition of DNA itself that led them to the correct model. They knew that DNA was composed of four chemicals—adenine (A), guanine (G), cytosine (C), and thymine (T)—and that A was always paired with T, and C always with G. Knowing this and studying the x rays led them to realize that this alignment could only happen if DNA was made up of two strands that were twisted together to form what is called a "double helix." (A double helix is the correct name for a corkscrew-like spiral shape.) In March 1953 Watson and Crick built a wire model showing that the DNA molecule could be thought of as a ladder where the nucleotide bases (A, T, G, C) form the "rungs" connecting the two side rails. The sides were then twisted to make the double helix. In Watson and Crick's Nobel Prize-winning work, the base pairs were the critical part that allowed them to explain how nature stores and uses a genetic code. Each DNA base is like a letter of the alphabet, and a sequence of nucleotide bases can be thought of as forming a message. Put another way, each "rung" (base pair) of the twisted ladder consists of some combination of the four chemicals (A, T, G, C) that form the coded message.

THE CELL'S INSTRUCTION MANUAL

DNA has been called the instruction manual for the cell. It has also been called the chemical language in which "gene recipes" are written. This is because genes can be considered recipes for making proteins, and proteins control the characteristics of all organisms. These codes or recipes are written with the four nucleotide building blocks (the bases A, T, G, and C). Each gene has several thousand bases joined together in a precise

JAMES DEWEY WATSON AND FRANCIS HARRY COMPTON CRICK

The team of Watson and Crick discovered the structure of deoxyribonucleic acid (DNA), one of the most important discoveries of modern science. Their model explained how genetic information is coded and how DNA makes copies of itself. This discovery formed the basis for all the genetic developments that have followed.

English molecular biologist Francis Crick (1916– ) was born in Northampton, England. As a boy, he was very interested in chemistry, although he eventually obtained a degree in physics from University College in London. During World War II (1939–45) he worked on the development of radar (an instrument used to determine an object's position, speed, or other characteristics) and new weapon design. After the war, Crick decided he wanted to concentrate on biology and did so on his own, eventually taking a job at the Cavendish Laboratory where he would meet the younger Watson.

American molecular biologist James Watson (1928– ) was a former "Quiz Kid," which was the name of a popular radio show in the 1940s. Born in Chicago, Illinois, Watson was a child prodigy (an exceptionally smart person) who graduated from the University of Chicago when he was nineteen and who obtained his Ph.D. from the University of Indiana at twenty-two. After receiving a fellowship to study in Copenhagen, Denmark, he joined the Cavendish Laboratory at Cambridge, England, where he met Crick.

Although Crick was twelve years older than Watson, both shared what was described as "youthful arrogance" and found that they got along extremely well both personally and professionally. Crick had been studying protein structure, and Watson came to Cambridge highly interested in discovering the basic substance of genes. With these complementary goals, they teamed up to try to unravel the structure of DNA, the carrier of genetic information at the molecular level.

Beginning in 1951, they worked to create a DNA model that would explain how it could copy and pass on its instructions to every new cell in a living thing. After a great deal of experimentation, they recognized the importance of DNA x rays being done by the English biochemist Rosalind Franklin (1920–1958), and soon built an accurate spiral-shaped model, called a double helix, in which they said two parallel chains of alternate sugar and phosphate groups were linked by pairs of organic bases. Their model looked like a twisted, spiral staircase. They then theorized that replication (the process by which DNA molecules copy themselves) occurs by a parting, or unwinding, of the two strands, or bases, of the staircase that then unite with newly created strands to form new DNA molecules (made up of one old strand and one new strand). Watson and Crick then published their findings in the journal Nature, which appeared on April 25, 1953 (with Watson's name appearing first due to a coin toss). Other researchers soon confirmed their hypothesis and the Watson-Crick model was accepted as correct.

Watson went on to write several highly regarded books, one of which became the first widely used textbook on molecular biology. He taught at Harvard University, became director of Cold Spring Harbor Laboratory of Quantitative Biology in Massachusetts, and served as the first director of the United States Human Genome Project. Crick joined the Salk Institute for Biological Studies in San Diego, California, in 1977. In 1962, Watson and Crick shared the Nobel Prize in Physiology and Medicine with their colleague, English physicist, Maurice H. F. Wilkins, for their discovery of the structure of DNA. Rosalind Franklin worked on Wilkins's team, and she would have received the award had she lived. The Watson-Crick discovery of DNA structure ushered in the modern era of molecular biology and made possible all that has happened since. There would be no understanding of human genetics without their discovery.

code that is different from the code for any other gene. For example, a sequence such as A-T-T-C-G-C-T… etc. might tell a cell to make one type of protein (for red hair), while another sequence such as G-C-T-C-T-CG… etc. might code for a different type of protein (for blonde hair). When cells reproduce by division (a process called mitosis), each parent cell must make sure that its daughter cell (its exact duplicate) gets a complete copy of its DNA. This is accomplished by a process called "replication" in which the two strands or rails of the DNA ladder "unzip" themselves down the middle of the bases (or rungs of the ladder). Since A always links to T and G always to C, each separate rail of the ladder becomes a template or model for free-floating bases to link up to. The result is the existence of two identical double helixes where there was just one.

The other important processes to understand are called "transcription" and "translation," which are the two stages of making a protein. In transcription, DNA "unzips" again and another type of nucleic acid called ribonucleic acid (or messenger RNA) uses one strand of DNA as a template to make an exact, single-strand copy. The RNA then leaves the nucleus with its message and becomes a template for the production of protein (by a ribosome in the cell) in what is known as translation. This process is going on all the time in our bodies, since cells are constantly called upon to make protein molecules.

Breakthroughs in understanding DNA have also led to the use of DNA in forensic science where "DNA fingerprinting" or "DNA profiling" is conducted. This use of DNA is based on the fact that repetitive sequences of DNA vary greatly among individuals, since each person has his or her own unique code. It has also led to the beginnings of treatment for hereditary disease by genetic engineering. In theory, doctors will be able to replace a defective (inherited) gene that causes a certain disease with a normal gene, thus preventing the patient from getting the disease in the first place.

[See alsoCell; Double Helix; Gene; Nucleic Acid ]