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Ch. 16 – (DNA
Discovery and Structure)
By the late 1940’s
scientists knew that
chromosomes carry
hereditary material &
they consist of DNA
and protein.
Most researchers thought PROTEIN
was the genetic material because:
*proteins have functional specificity
*little was known about DNA
*properties of DNA seemed too
uniform to account for the multitude
of inherited traits
 Frederick Griffith (1928) performed
experiments which provided evidence
that the genetic material was DNA
*Griffith was trying to find a
vaccine against Streptococcus
pneumoniae
• 2 strains of pneumococcus:
-smooth, encapsulated strain (S):
PATHOGENIC
-rough, nonencapsulated strain (R):
NONPATHOGENIC
4 experiments (see fig. 16.1)
*Griffith was unable to determine the
chemical nature of the transforming agent
but hinted that it was not protein because
heat denatures protein yet it did not
destroy the transforming ability of the
genetic material in the heat-killed S cells
*Avery, McLean and MacLeod (1944)
discovered that the transforming
agent was DNA (purified chemicals from
heat-killed bacteria and tried to transform
live nonpathogenic bacteria w/each
chemical…only DNA worked!)
 Hershey and Chase
discovered that DNA is
the genetic material of
bacteriophage (T2) in 1952
*these scientists did not know
which viral component (DNA or
protein) was responsible for
reprogramming the host bacterial
cell (see experiment Fig. 16.2)
1953
*Results: viral protein
remains outside the host
cell and viral DNA is
injected into the host
cell; therefore nucleic
acids rather than
proteins are the
hereditary material
-radioactive P-32
found inside host
cell
-radioactive S-35
found outside host
cell
 Experimental evidence for DNA as
the hereditary material in eukaryotes
came from the lab of Erwin Chargaff
(1947); using paper chromatography
to separate nitrogenous bases,
Chargaff reported the following:
...I believe that the double-stranded
model of DNA came about as a
consequence of our conversation; but
such things are only susceptible of a
later judgment...."
1930
Chargaff’s
Results:
*DNA composition is species-specific; the
amounts and ratios of bases vary from one
species to another
*in every species he studied, there was a
regularity in base ratios where the # of A
residues equaled the # of T residues, and
the # of G equaled the # of C
 Watson and Crick
discovered the double
helix by building
models to conform to
X-ray data
*Watson saw an X-ray
photo of DNA
produced by Rosalind
Franklin; Watson and
Crick deduced from
Franklin’s data that:
a) DNA is a helix with a
width of 2 nm
b) purine & pyrimidine bases
are stacked 0.34 nm apart
c) the helix makes 1 full turn
every 3.4 nm along its
length
d) there are 10 layers of
bases in each turn of the
helix
 Watson and Crick’s
proposed structure is a
ladder-like molecule
with sugar-phosphate
backbones as uprights
and pairs of bases as
rungs.
Original DNA
demonstration
model
 the 2 sugar-phosphate
backbones of the helix
are ANTIPARALLEL
(they run in opposite
directions)
 to be consistent with a 2 nm width, a
purine on one strand must pair (by Hbonding) with a pyrimidine on the
other strand
 base structure dictates which pairs
of bases can form hydrogen bonds
 DNA is made up of subunits called
NUCLEOTIDES
 each nucleotide is made up of 3 basic
parts:
*5-carbon sugar: deoxyribose
*nitrogenous base: A, G, C, or T
*phosphate group
IB-HL
DNA is made up of 3 components:
deoxyribose, phosphate and bases. What is
the maximum number of components that
deoxyribose is linked to in a DNA molecule?
A. 1 phosphate & 1 base
C. 1 phosphate & 2 bases
B. 2 phosphates & 1 base
D. 2 phosphates & 2 bases
What did Griffith’s experiments show?
A.
B.
C.
D.
That cytosine and guanine occur in approximately equal
percentages
That bacteria infected with radioactively labeled viruses
became radioactive only when exposed to viruses with
radioactive phosphorus labels
That heat-killed bacteria (denatured protein) could transform
a nonpathogenic bacteria into a pathogenic one
That DNA is antiparallel
What did Hershey & Chase’s experiments show?
A.
B.
C.
D.
That cytosine and guanine occur in approximately equal
percentages
That bacteria infected with radioactively labeled viruses
became radioactive only when exposed to viruses with
radioactive phosphorus labels
That heat-killed bacteria (denatured protein) could transform
a nonpathogenic bacteria into a pathogenic one
That DNA is antiparallel
DNA is antiparallel. What does this mean?
A.
B.
C.
D.
A “C” pairs with a “G” whereas a “T” pairs with an “A”
A “C” pairs with a “T” whereas a “G” pairs with an “A”
The 2 DNA strands run in the same direction
The 2 DNA strands run in opposite directions
Why does a purine have to pair with a
pyrimidine?
A. So that the DNA strand has a consistent width (2 nm).
B. So that the number of C-H bonds remains constant
C. So that the percent of A’s & T’s and C’s & G’s remains
constant
D. Both A and C