DNA: The Making


Lyle Sykes

For more than 50 years after the science of genetics was established and the
patterns of inheritance through genes were clarified, the largest questions
remained unanswered: How are the chromosomes and their genes copied so exactly
from cell to cell, and how do they direct the structure and behavior of living
things? This paper will discuss those questions and the people that answered
them. 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 1996) Each unit (a
polypeptide) is produced by a specific gene. This work launched studies into the
chemical nature of the gene and helped to 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. Partly
because of the close relationship established between genes and enzymes, which
are proteins, protein at first seemed the fundamental substance that determined
heredity.” (Goetinck 1995) “In 1944, however, the Canadian bacteriologist Oswald
Theodore Avery proved 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 subsequent generations. By this time DNA was known to be made up of
substances called nucleotides. 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)."(Caldwell 1996) "In 1953, putting together the accumulated chemical
knowledge, geneticists James Dewey Watson of the U.S. and Francis Harry Compton
Crick of Great Britain worked out the structure of DNA. This knowledge
immediately provided the means of 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, somewhat 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
complementary base on the opposite strand.” (Caldwell 1996) “Adenine always
binds to thymine, and guanine always binds to cytosine.” (Annas 1996) “To make a
new, identical copy of the DNA molecule, the two strands need only 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. 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." (Caldwell
1996) "The DNA backbone is actually a great deal 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, just visible under the most powerful
electron microscope. The DNA is wound around each nucleosome in succession 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,
like the filament of an electric light bulb." (Popper 1996) "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 commonly found
in polypeptides.” (Caldwell 1996) “The number, type, and order of amino acids in
a chain ultimately determine the structure and function of the protein of which
the chain is a part." (Marx 1996) "Since proteins were shown to be products of
genes, and each gene was shown to be composed of sections of DNA strands,
scientists reasoned that a genetic code must exist by which the order of