Download DNA Discovery and Structure

NOTES - Ch. 16 (part 1):
DNA Discovery and Structure
By the late 1940’s
scientists knew that
chromosomes carry
hereditary material &
they consist of DNA
and protein.
(Recall Morgan’s fruit fly
research!)
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…it seemed too simple!
● Frederick Griffith (1928) performed
experiments which provided evidence
that the genetic material was DNA
*Griffith (a British medical officer) was
studying the bacterium that causes
pneumonia in mammals:
Streptococcus pneumoniae
● 2 strains of pneumococcus:
-smooth, encapsulated strain (S):
PATHOGENIC
-rough, nonencapsulated strain (R):
NONPATHOGENIC
4 experiments (see fig. 16.2)
DISCOVERY…
**the living R pneumococci
had been TRANSFORMED
into virulent S-strain
organisms!!!
● 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 heatkilled 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
1953
*a virus consists of DNA enclosed by a
protective protein coat;
*to reproduce, a virus must infect a cell and
“take over” (reprogram) the host cell’s
metabolic machinery to make new viruses
*viruses that infect bacteria are called
BACTERIOPHAGES, or just PHAGES.
*these scientists did
not know which viral
component (DNA or
protein) was
responsible for
reprogramming the
host bacterial cell
(see experiment Fig.
16.4)
*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 (1950);
using paper chromatography to
separate nitrogenous bases, Chargaff
reported the following:
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
...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
● 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
PURINES & PYRIMIDINES:
 PURINES = double-ringed nitrogenous
bases;
 adenine (A) and guanine (G)
 PYRIMIDINES = single-ringed
nitrogenous bases;
 thymine (T) and cytosine (C)
Chargaff’s Rules:
 Purines (double-ring) must pair with
pyrimidines (single-ring)
 A pairs with T
 G pairs with C
● 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