Dna

DNA is a term that has been used in science as well as in many parts of daily

life. The general public knows that DNA is a part of our bodies but they may

wonder what exactly is DNA? DNA is a term used for deoxyribonucleic acid and it

is the genetic material of all organisms, it is the molecule of life and it

determines all of our physical characteristics. DNA is present in every single

form of life. More than 50 years after the science of genetics was established

and the patterns of inheritance were clarified, the largest questions remained

unanswered: How are the chromosomes and their genes copied exactly from cell to

cell, and how do they direct the structure and behavior of living things?

Two American geneticists, George Wells Beadle and Edward Lawrie Tatum, provided

one of the first important clues in the early 1940s. Working with the fungi

Neurospora and Penicillium, they found that "genes direct the formation of

enzymes through the units of which they are composed." (Annas) Each unit, a

polypeptide is produced by a specific gene. This establish the field of

molecular genetics.

The fact that chromosomes were almost entirely composed of two kinds of chemical

substances, protein and nucleic acids, had long been known. In 1944, however,

the Canadian bacteriologist Oswald Theodore Avery showed that deoxyribonucleic

acid (DNA) performed this role. He extracted DNA from one strain of bacteria

and introduced it into another strain. The second strain not only acquired

characteristics of the first but passed them on to the next generation. Each

nucleotide consists of a phosphate, a sugar known as deoxyribose, and any one of

four nitrogen-containing bases. The four nitrogen bases are adenine (A),

thymine (T), guanine (G), and cytosine (C).

In 1953 James Dewey Watson of the United States and Francis Harry Crick of

England worked out the structure of DNA. In 1962, both men earned the Nobel

Prize in physiology for their discovery. This knowledge provided understanding

how hereditary information is copied. Watson and Crick found that the DNA

molecule is composed of two long strands in the form of a double helix,

resembling a long, spiral ladder. The strands, or sides of the ladder, are made

up of alternating phosphate and sugar molecules. The nitrogen bases, joining in

pairs, act as the rungs. Each base is attached to a sugar molecule and is

linked by a hydrogen bond to a base on the opposite strand. Adenine always

binds to thymine, and guanine always binds to cytosine. "To make a new,

identical copy of the DNA molecule, the two strands unwind and separate at the

bases which are weakly bound; with more nucleotides available in the cell, new

complementary bases can link with each separated strand, and two double helixes

result." (Caldwell) Since the "backbone" of every chromosome is a single long,

double-stranded molecule of DNA, the production of two identical double helixes

will result in the production of two identical chromosomes.

The DNA backbone is actually longer than the chromosome but is tightly coiled up

within it. This packing is now known to be based on minute particles of protein

known as nucleosomes, that can only be seen under the most powerful electron

microscope. The DNA is wound around each nucleosome to form a beaded structure.

The structure is then further folded so that the beads associate in regular

coils. Thus, the DNA has a "coiled-coil" configuration.

After the discoveries of Watson and Crick, the question that remained was how

the DNA directs the formation of proteins, compounds central to all the

processes of life. Proteins are not only the major components of most cell

structures, they also control virtually all the chemical reactions that occur in

living matter. The ability of a protein to act as part of a structure, or as an

enzyme affecting the rate of a particular chemical reaction, depends on its

molecular shape. This shape, in turn, depends on its composition. Every

protein is made up of one or more components called polypeptides, and each

polypeptide is a chain of subunits called amino acids. Twenty different amino

acids are found in polypeptides. The number, type, and order of amino acids in

a chain ultimately determine the structure and function of the protein.

Since proteins were shown to be products of genes, and each gene was shown to be

composed of sections of DNA strands, scientists found that a genetic code must

exist by which the order of the four nucleotide bases in the DNA could direct

the sequence of amino acids in the formation of polypeptides. Because only four

different