Biology Reference
In-Depth Information
of Segregation and Independent Assortment, which are discussed further in
the description of meiosis and mitosis in Chapter 3. His work, however, was
not widely known until 1900, when Hugo de Vries, Carl Correns, and Erich von
Tschermak rediscovered these laws of inheritance. Mendel described traits in
peas that were “dominant” or “recessive,” showed that peas could be selected
for different traits, and showed that the traits were inherited in a stable
manner.
The discovery that DNA is the hereditary material was first determined using
a bacterium that causes pneumonia, Streptococcus pneumoniae ( (Griffiths 1928 ).
Before this discovery, scientists speculated that the hereditary material might be
composed of proteins or RNA. Proteins were considered the most likely heredi-
tary material because they were known to be more variable (having 20 amino
acids that could serve as the genetic code) than DNA. Furthermore, proteins
are present in the nucleus in amounts nearly equal to DNA. DNA, by contrast,
seemed to have only four types of structure (consisting of A, T, C, or G) that
could serve as the genetic code. Griffiths (1928) found that nonvirulent forms
of S. pneumoniae could be “transformed” to virulent forms by combining
heat-treated virulent bacteria with nonvirulent bacteria. The reverse was true
and led to the conclusion that the virulence traits were heritable and that the
heritable material was capable of surviving mild heat treatment. Subsequently,
Avery et al. (1944) conducted experiments in which the “transforming principle”
was found to have the characteristics of DNA, and the transforming factors did
not test positive for proteins or RNA. Avery et al. (1944) showed that enzymes
that degrade proteins or RNA did not degrade the transforming principle but
that enzymes that could degrade DNA did degrade the transforming principle.
Hershey and Chase (1952) conducted experiments to further resolve whether
protein or DNA was the hereditary material. They labeled DNA and protein from
viruses that infect bacteria (bacteriophages) with different radioactive markers
and monitored whether labeled DNA or labeled protein entered the bacterial
host. Only labeled DNA entered the bacteria, confirming that the transforming
principle, or genetic information, was contained in DNA.
The next big questions were how the DNA was structured, how the genetic
information was encoded, and how the genetic information was replicated in a
reliable manner. Answers to these questions were hotly pursued by several scien-
tists, including Francis Crick, James Watson, Rosalind Franklin, Maurice Wilkins,
Linus Pauling, and others. Rosalind Franklin and Maurice Wilkins provided criti-
cal information relevant to the solution of the structure of DNA with their X-ray
diffraction pictures of purified DNA. The X-ray diffraction photographs provided
an essential clue that allowed Watson and Crick to propose the correct structure
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