UNIT 3: Molecular Genetics
Chapter 6: DNA: Hereditary Molecules of Life pg. 268 6.2: DNA Structure and Function: A History
pg. 273 - 279
150 years ago Mendel hypothesized the existence of hereditary material
(factors).
70 years ago it was determined that the hereditary material was DNA,
Watson and Crick.
Genome sequencing of species has occurred in humans and plants but still
continues for other organism.
Once this information is mapped an understood, science can understand how
diseases occur through genetic mutations, and possibly develop gene therapy
technologies and trail vaccines to combat these diseases.
Establishing DNA as the Hereditary Molecule
Frederick Meischer (1868 – Swiss) studied pus from the wounds of
soldiers; to determine the composition of the nucleus. He determined that the
pus, mainly white blood cells had a large quantity of an unknown acidic
substance in the nucleus, this he called nuclein. The make up and function
of nuclein was not determined until 50 years later. The actual structure was
determined later.
Frederick Griffith (1928 – British) study the pneumonia epidemic in
Europe. Through experimentation he determined the function of DNA in
inheritance.
He studied two different strains of pneumonia bacterium, a capsulated
(smooth) version known as the S-strain and an un-capsulated (rough) version
known as the R-strain. Griffith experimented on mice, injecting them with
the different versions of pneumonia.
Experiment 1, he injected mice with the S-strain, and the mice died within
days. He determined that the S-strain was highly virulent (pathogenic). The
mice he injected with the R-strain remained healthy and survived.
Experiment 2, heated the S-strain, destroying the capsule killing the bacteria.
Injecting mice with the dead S-strain no longer killed the mice, they
remained healthy similar to those mice injected with the R-strain.
Experiment 3, Griffith mixed the dead S-strain version with the R-strain
version prior to injection into the mice. After injecting the mice with the
combination of strains, the mice contracted pneumonia and died. He isolate
the bacteria from the dead mice, and found that the R-strain had become Sstrain. Some how the R-strain acquired some factor from the heated S-strain.
Griffith was not able to determine the actual material (DNA or Protein)
responsible for this change, he understood that hereditary substance had
passed from the dead S-strain cells to the R-strain cells, transforming them
into S-strain cells. He called this the Transforming Principle.
Transformation – is a change in a genotype or phenotype caused by the
direct uptake of genetic material by a cell.
Figure 2: Mixing heat killed S-strain cells with live R-strain cells made the R-strain cells virulent.
Avery, McLeod, and McCarty (1944), followed up on Griffith’s
experiment to confirming the transformation principle. They did determine
that the material responsible for this transformation was DNA. But they
were still not sure of their result because of a possible experimental error.
Hershey and Chase (1952) confirmed that DNA was the hereditary material.
Hershey and Chase wanted to determine whether it was the DNA or the
protein that was the genetic material. They studied a virus called a
bacteriophage that killed bacteria cells.
Bacteriophage – is a virus that infects bacteria.
The bacteriophage had both a protein coat and DNA. When a virus attacks a
cell it injects its genetic material into the cell, which then instructs the
internal organelles to build new virus.
The experiment set up, was to determine whether it was the protein or the
DNA that did the transforming. Using radio-isotopes to mark the molecules,
DNA and protein, they would be able to follow the transformation process.
DNA is made up of phosphate, therefore radioactive phosphate was used to
mark the DNA molecule, 32P.
Protein contains sulfur, therefore radioactive sulfur was used to mark the
protein, 35S.
Figure 3: Hershey and Chase used progeny bacteriophages labeled with radioisotopes of either a)
phosphorus, 32P, or b) sulfur, 35S. Their experiments indicated that DNA is the material transmitted from
virus to bacteria.
Keeping the samples separate, allowing the versions of radio-isotope viruses
to infect bacteria cells, the cells were then separated from their viruses, using
a blender.
The results determined that the radioactive DNA had entered the bacteria
cell causing the infection of the cell, where the radioactive protein did not.
Hershey-Chase concluded that DNA must be the hereditary material.
The Chemical Composition of DNA
Phoebus Levene (1920’s) determined that each DNA molecule
consisted of three parts, phosphate group, deoxyribose sugar group
and one of four nitrogenous bases. By 1949 it had been determined
that there were four different nitrogenous bases; adenine, guanine,
thymine, and cytosine.
These nitrogenous bases were subdivided into two types; purines
(double ringed) and pyrimidines (single ringed).
Purines: adenine and guanine.
Pyrimidines: thymine and cytosine.
Edward Chargaff (1950) determined the nitrogenous base ratio, which was
different from what was originally thought, equal ratio. He determined that
the number of adenine bases equaled the number of thymine base, while
guanine equaled cytosine. Yet there ware always an equal number of purines
to pyrimidines. This led to the understanding of complementary base
pairings.
Figure 4: A Nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
Figure 5: The four nitrogenous bases of DNA are classified as either a) a purines or b) pyrimidines.
Purine – is a class of nitrogenous bases with a double-ring structure, adenine
and guanine are purines.
Pyrimidine – is a class of nitrogenous bases with a single-ring structure;
thymine and cytosine are pyrimidines.
Complementary Base Pairing – is the chemical tendency of adenine to form
hydrogen bonds with thymine and cytosine to form hydrogen bonds with
guanine.
Wilkins and Franklin used new technology to determine the shape of the
DNA molecule. The development of X-ray Crystallography was used to
determine the pattern of the DNA molecule by deflecting X-rays off the
molecule onto a photographic plate. Wilkins determined the helical pattern
while Franklin determined the X-shape.
From these results it was determined that the DNA molecule was a double
helix, which rotated clockwise. Also the diameter was 2.0 nanometers, and a
complete helix was 3.4 nanometers (ten nucleotides).
Watson and Crick (1952) built a model of the DNA molecule based on
previous research.
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four different nitrogenous bases (A, T, G, and C)
Chargaff’s ratios of nitrogenous bases
Phosphate and deoxyribose sugar backbone
Franklins Double helix and inward bases, and dimensions
The model had two strands consisting of a phosphate and deoxyribose sugar
backbone, with the nitrogenous bases attached to the backbone facing
inward. The nitrogenous bases on one strand were attached to nitrogenous
bases on another strand by hydrogen bonds. Watson and Crick showed that
thee stability of the molecule was dependent on the fact the strands had to be
running antiparallel, running in opposite directions. One end of the stand
was 3′ containing a hydroxyl group and the other end was 5′ containing a
carbon.
It was also determined that hydrogen bonds held the complementary
nitrogenous bases together. Adenine bonded to thymine, forming two
hydrogen bonds, while guanine bonded with cytosine, forming three
hydrogen bonds. This created symmetry within the DNA molecule, and its
ability to divide to convey genetic information.
Figure 7: DNA is in the shape of a double helix.