While the period from the early 1900s to World War II has been considered
the "golden age" of genetics, scientists still had not determined that
DNA, and not protein, was the hereditary material. However, during this
time a great many genetic discoveries were made and the link between
genetics and evolution was made.
Robert Feulgen, in
1914,
discovered that fuchsine dye stained DNA. DNA was then found in the
nucleus of all eukaryotic cells.
During the 1920s, biochemist P.A. Levine analyzed the components of the
DNA molecule. He found it contained four nitrogenous bases: cytosine,
thiamine, adenine, and guanine; deoxyribose sugar; and a phosphate group.
He concluded that the basic unit (nucleotide) was composed of a base
attached to a sugar and that the phosphate also attached to the sugar. He
(unfortunately) also erroneously concluded that the proportions of bases
were equal and that there was a tetra nucleotide that was the repeating
structure of the molecule. The nucleotide, however, remains as the
fundamental unit (monomer) of the nucleic acid polymer.
In 1944, Oswald Avery, Colin MacLeod, and Macklin McCarty revisited
Griffith's experiment and concluded the transforming factor was DNA. Their
evidence was strong but not totally conclusive. The then-current favorite
for the hereditary material was protein; DNA was not considered by many
scientists to be a strong candidate.
The breakthrough in the quest to determine the hereditary material came
from the work of Max Debunk and Salvador Luria in the 1940s. Bacteriophage
are a type of virus that attacks bacteria, the viruses that Delbruck and
Luria worked with were those attacking Escherichia coli, a bacterium found
in human intestines. Bacteriophages consist of protein coats covering DNA.
Bacteriophages infect a cell by injecting DNA into the host cell. This
viral DNA then "disappears" while taking over the bacterial machinery and
beginning to make new virus instead of new bacteria.
Erwin Chargaff
analyzed the nitrogenous bases in many different forms of life,
concluding that the amount of purines does not always equal the amount
of primitives (as proposed by Levene). DNA had been proven as the
genetic material by the Hershey-Chase experiments, but how DNA served
as genes was not yet certain. DNA must carry information from parent
cell to daughter cell. It must contain information for replicating
itself. It must be chemically stable, relatively unchanging. However,
it must be capable of mutational change. Without mutations there would
be no process of evolution.
Many scientists were interested in deciphering the structure of DNA;
among them were Francis Crick, James Watson, Rosalind Franklin, and
Maurice Wilkens. Watson and Crick gathered all available data in an
attempt to develop a model of DNA structure. Franklin took X-ray
diffraction photomicrographs of crystalline DNA extract, the key to
the puzzle. The data known at the time was that DNA was a long
molecule, proteins were helically coiled (as determined by the work of
Linus Pauling), Chargaff's base data, and the x-ray diffraction data
of Franklin and Wilkens.
DNA is a double helix, with bases to the center (like rungs on a
ladder) and sugar-phosphate units along the sides of the helix (like
the sides of a twisted ladder). The strands are complementary (deduced
by Watson and Crick from Chargaff's data, a pairs with T and C pairs
with G, the pairs held together by hydrogen bonds). Notice that a
double-ringed purine is always bonded to a single ring pyrimidine.
Purines are Adenine (A) and Guanine (G). We have encountered Adenosine
triphosphate (ATP) before, although in that case the sugar was ribose,
whereas in DNA it is deoxyribose.
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