Essay/Term paper: Dna: the making

Essay, term paper, research paper:  Science Reports

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 the
four nucleotide bases in the DNA could direct the sequence of amino acids in the
formation of polypeptides." (Barinaga 1995) "In other words, a process must
exist by which the nucleotide bases transmit information that dictates protein
synthesis. This process would explain how the genes control the forms and
functions of cells, tissues, and organisms. Because only four different kinds of
nucleotides occur in DNA, but 20 different kinds of amino acids occur in
proteins, the genetic code could not be based on one nucleotide specifying one
amino acid. Combinations of two nucleotides could only specify 16 amino acids (4²
= 16), so the code must be made up of combinations of three or more successive
nucleotides. The order of the triplets—or, as they came to be called, codons—
could define the order of the amino acids in the polypeptide." (Snaz 1996) "Ten
years after Watson and Crick reported the DNA structure, the genetic code was
worked out and proved biologically. Its solution depended on a great deal of
research involving another group of nucleic acids, the ribonucleic acids (RNA).
The specification of a polypeptide by the DNA was found to take place indirectly,
through an intermediate molecule known as messenger RNA (mRNA). Part of the DNA
somehow uncoils from its chromosome packing, and the two strands become
separated for a portion of their length. One of them serves as a template upon
which the mRNA is formed (with the aid of an enzyme called RNA polymerase). The
process is very similar to the formation of a complementary strand of DNA during
the division of the double helix, except that RNA contains uracil (U) instead of
thymine as one of its four nucleotide bases, and the uracil (which is similar to
thymine) joins with the adenine in the formation of complementary pairs. Thus, a
sequence adenine-guanine-adenine-thymine-cytosine (AGATC) in the coding strand
of the DNA produces a sequence uracil-cytosine-uracil-adenine-guanine (UCUAG) in
the mRNA." (Witten 1996) "The production of a strand of messenger RNA by a
particular sequence of DNA is called transcription. While the transcription is
still taking place, the mRNA begins to detach from the DNA. Eventually one end
of the new mRNA molecule, which is now a long, thin strand, becomes inserted
into a small structure called a ribosome, in a manner much like the insertion of
a thread into a bead. As the ribosome bead moves along the mRNA thread, the end
of the thread may be inserted into a second ribosome, and so on." (Lemonick
1996) Using a very high-powered microscope and special staining techniques,
scientists can photograph mRNA molecules with their associated ribosome beads.
"Ribosomes are made up of protein and RNA. A group of ribosomes linked by mRNA
is called a polyribosome or polysome. As each ribosome passes along the mRNA
molecule, it "reads" the code, that is, the sequence of nucleotide bases on the
mRNA. The reading, called translation, takes place by means of a third type of
RNA molecule called transfer RNA (tRNA), which is produced on another segment of
the DNA. On one side of the tRNA molecule is a triplet of nucleotides. On the
other side is a region to which one specific amino acid can become attached
(with the aid of a specific enzyme). The triplet on each tRNA is complementary
to one particular sequence of three nucleotides—the codon—on the mRNA strand.
Because of this complementary, the triplet is able to "recognize" and adhere to
the codon. For example, the sequence uracil-cytosine-uracil (UCU) on the strand
of mRNA attracts the triplet adenine-guanine-adenine (AGA) of the tRNA. The tRNA
triplet is known as the anticodon." (Witten 1995) "As tRNA molecules move up to
the strand of mRNA in the ribosome beads, each bears an amino acid. The sequence
of codons on the mRNA therefore determines the order in which the amino acids
are brought by the tRNA to the ribosome. In association with the ribosome, the
amino acids are then chemically bonded together into a chain, forming a
polypeptide. The new chain of polypeptide is released from the ribosome and
folds up into a characteristic shape that is determined by the sequence of amino
acids. The shape of a polypeptide and its electrical properties, which are also
determined by the amino acid sequence, dictate whether it remains single or
becomes joined to other polypeptides, as well as what chemical function it
subsequently fulfills within the organism." (Witten 1996) "In bacteria, viruses,
and blue-green algae, the chromosome lies free in the cytoplasm, and the process
of translation may start even before the process of transcription (mRNA
formation) is completed. In higher organisms, however, the chromosomes are
isolated in the nucleus and the ribosomes are contained only in the cytoplasm.
Thus, translation of mRNA into protein can occur only after the mRNA has become
detached from the DNA and has moved out of the nucleus." (O"Brien 1996) As
funding for research becomes available for scientist, they continue to study the
DNA molecule with hopes of find the secrets that are hidden with in our own
bodies. Their findings continue to aid us in cures and the prevention of many
illnesses that years ago we couldn"t solve. Hopefully the research will soon
pay off, with the cure for cancer or Alzheimer"s Disease, for instance. Only
time will tell what discoveries will be made to help those that are ill. The
sad thing is, most that are ill have very little time to spare. That is why the
DNA research is important now, to save the ones that aren"t in need.

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