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II) The History of DNA
Material of Heredity

Friedrich Misechner (1869)



isolated white substance he called nuclein
contained both protein and DNA/RNA
Joachim Hammerling (1943)

demonstrated that the nucleus contained heredity
material
Phoebus Levene (1910)
 first to isolate the two type of nucleic acid
(DNA and RNA)
 showed chromosomes are made up of DNA and
protein
Frederick Griffith (1920s)
 transforming principle
 showed that pathogenic bacteria can still spread diseases
even when dead.
 you have kill the bacteria and destroy the DNA to stop
the bacteria.
 DNA can transform non pathogenic bacteria in
pathogenic bacteria.
 shows DNA is a hereditary material
Hershey and
Chase (1952)
 gave definitive
proof that DNA
was the
material of
heredity and not
protein.
Chargaff’s Rule (1950)
 discovered that nucleotides are present in varying, but
characteristic, proportions.
 in DNA
 the amount of adenine is always approximately equal to
the amount of thymine (A ~ T)
 the amount of cytosine is always approximately equal to
the amount guanine. (C ~ G)
James Watson and
Francis Crick
(1953)
 worked out the
structure of DNA
using data from:
 Rosalind Franklin
(Maurice Wilkins)x-ray diffraction
 Chargaff’s rule
III) DNA
REPLICATION
I) DNA Structure and Replication
DNA Replication
 for mitosis and meiosis to occur the DNA must make
an exact copy itself first (S Phase of interphase) for
every chromosome

result: 2 identical sister chromatids attached at the
original centromere
this is called DNA replication
the main stages of DNA replication is the same in
prokaryotic and eukaryotic cells.
DNA replication is semiconservative



B) DNA Replication
EXPERIMENT
1 Bacteria
cultured in
medium
containing
15N
2 Bacteria
transferred to
medium
containing 14N
RESULTS
3 DNA sample
centrifuged
after 20 min
(after first
application)
4 DNA sample
centrifuged
after 40 min
(after second
replication)
Less
dense
More
dense
CONCLUSION
First replication
Second replication
Conservative
model
Semiconservative
model
Dispersive
model
B) DNA Replication
EXPERIMENT
1 Bacteria
cultured in
medium
containing
15N
2 Bacteria
transferred to
medium
containing 14N
RESULTS
3 DNA sample
centrifuged
after 20 min
(after first
application)
4 DNA sample
centrifuged
after 20 min
(after second
replication)
Less
dense
More
dense
B) DNA Replication
Fig. 16-11b
CONCLUSION
First replication
Second replication
Conservative
model
Semiconservative
model
Dispersive
model
B) DNA Replication
one sister
chromatid
identical sister
chromatid
centromere
ex. CHROMOSOME #18
not replicated yet
CHROMOSOME #18
now replicated!
#18
#18
I) DNA Structure and Replication
A
T
C
G
T
A
A
T
G
C
(a) Parent molecule
B) DNA Replication
4 steps to DNA SEMI-CONSERVATIVE
REPLICATION
1.

“unzipping”
HELICASE ENZYME breaks the HYDROGEN
BONDS between the NITROGEN BASE PAIRS
ex. chromosome #18
A
T
C
G
Fig. 16-9-2
I) DNA Structure and Replication
A
T
A
T
C
G
C
G
T
A
T
A
A
T
A
T
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
B) DNA Replication
2.



“attaching”
POLYMERASE ENZYME joins free-floating
COMPLEMENTARY NUCLEOTIDES to the
original strand of DNA
A with T, C with G
look! 2 new strands of DNA are being formed from the
original DNA!
AT
TA
C G
G C
RED = original DNA
BLACK = new strand of
DNA being
formed!
I) DNA Structure and Replication
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
T
A
T
A
T
A
T
A
A
T
A
T
A
T
A
T
G
C
G
C
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
(c) “Daughter” DNA molecules,
each consisting of one
parental strand and one
new strand
B) DNA Replication
I) DNA Structure and Replication
B) DNA Replication
3.



“proofreading”
PROOFREADER ENZYMES FIX any
MISMATCHED BASE PAIRS
prevents mutations!
proofreader enzymes are actually other polymerase
enzymes
4.


“final result”
2 IDENTICAL SISTER CHROMATIDS joined at
the ORIGINAL CENTROMERE
note that each sister chromatid is “half-old, half-new”
(SEMI-CONSERVATIVE!)
DNA REPLICATION
Fun Game!

http://nobelprize.org/educational_games/medicine/d
na_double_helix/
Note: There are an
estimated 3 billion
base pairs in the
HUMAN
GENOME.
Trivia: Replication occurs at a
speed of 50 nucleotides per second!