1. Chapter 16: The Molecular Basis of Inheritance
Test corrections – due Friday
Test stats….
avg: 22.5……nice 
range: (7) 12 – 30….nice 
Chapter 14 – traits carried in gametes
Chapter 15 – genes found on chromosomes
Chapter 16 – DNA is the molecule of inheritance
How did we learn this?

2. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
Transformation – external DNA assimilated by a cell which changes the cell’s genotype and phenotype
Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they
have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a
capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below:
Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria
by an unknown, heritable substance from the dead S cells.
EXPERIMENT
RESULTS
CONCLUSION
Living S
(control) cells
Living R
Heat-killed
(control) S cells
Mixture of heat-killed S cells
and living R cells
Mouse dies
Mouse healthy
Living S cells
are found in
blood sample.

3. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
- Bacteriophage – viruses that infect bacteria
Phage
head
Tail
Tail fiber
DNA
Bacterial
cell
100 nm

4. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur
and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells.
EXPERIMENT
Radioactivity
(phage protein)
in liquid
Phage
Bacterial cell
Radioactive
protein
Empty
protein shell
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Pellet
(phage DNA)
in pellet
Batch 1: Phages were
grown with radioactive
sulfur (35S), which was
incorporated into phage
protein (pink).
Batch 2: Phages were
phosphorus (32P), which
was incorporated into
phage DNA (blue). 1 2 3 4
Agitated in a blender to
separate phages outside
the bacteria from the
bacterial cells.
Mixed radioactively
labeled phages with
bacteria. The phages
infected the bacterial cells.
Centrifuged the mixture
so that bacteria formed
a pellet at the bottom of
the test tube.
Measured the
radioactivity in
the pellet and
the liquid

5. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
Phage proteins remained outside the bacterial cells during infection, while
phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA
released new phages with some radioactive phosphorus.
Hershey and Chase concluded that DNA, not protein, functions as the
T2 phage’s genetic material.
RESULTS
CONCLUSION

6. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
Nucleotide composition
Human
A – 30.3%
T – 30.3%
C – 19.9%
G – 19.5%
Sugar-phosphate
backbone
Nitrogenous
bases
5 end O– O P
CH2 5 4 H 3 1
CH3 N
Thymine (T)
Adenine (A)
Cytosine (C) OH 2
Sugar (deoxyribose)
3 end
Phosphate
Guanine (G)
DNA nucleotide
Chargaff’s rules
A=T
C=G
Note:
5’ end – phosphate group
3’ end - -OH

7. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
- NONE – compiled everyone else’s data
(a) Rosalind Franklin
Franklin’s X-ray diffraction
Photograph of DNA
(b)
Double helix

8. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
NONE – compiled everyone else’s data
Determined double helix from Rosalind Franklin’s data
Purine + Purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
Consistent with X-ray data

9. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
NONE – compiled everyone else’s data
Determined double helix from Rosalind Franklin’s data N H O
CH3
Sugar
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)

10. Figure 16.7 The double helixOH P
H2C T A C G
CH2 O–
5 end
Hydrogen bond
3 end
0.34 nm
3.4 nm
(a) Key features of DNA structure
(b) Partial chemical structure
(c) Space-filling model
1 nm
DNA is antiparallel
Hydrogen bonds hold double helix together
Now that we know how DNA is shaped & how it fits together…..

11. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
How does DNA replicate itself? The basic concept
(a) The parent molecule has two complementary strands of DNA Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.
(b) The first step in replication is separation of the two DNA strands.
(c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.
(d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands Each “daughter” DNA molecule consists of one parental strand and one new strand. A C T G
But is replication done in a conservative, semi-conservative or dispersive manner?

12. Figure 16.10 Three alternative models of DNA replication
Conservative
model. The two
parental strands
reassociate
after acting as
templates for
new strands,
thus restoring
the parental
double helix.
(a)
Semiconserva-
tive model.
The two strands
of the parental
molecule
separate,
and each functions
as a template
for synthesis of
a new, comple-
mentary strand.
(b)
Dispersive
model. Each
strand of both
daughter mol-
ecules contains
a mixture of
old and newly
synthesized
DNA.
(c)
Parent cell
First
replication
Second
Conservative
Original strands
return together
Semi-conservative
One original &
one new strand
Dispersive
Original strands
are dispersed
through new
strands
The Meselson – Stahl Experiment showed how DNA replicates.

13. Figure 16.11 Does DNA replication follow the conservative, semiconservative, or dispersive model?
Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations
on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15N. The bacteria
incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with
only 14N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be
lighter than the parental DNA made in the 15N medium. Meselson and Stahl could distinguish DNA of different
densities by centrifuging DNA extracted from the bacteria.
EXPERIMENT
The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask
in step 2, one sample taken after 20 minutes and one after 40 minutes.
RESULTS
Bacteria
cultured in
medium
containing
15N
transferred to
14N 2 1
DNA sample
centrifuged
after 20 min
(after first
replication) 3
after 40 min
(after second 4
Less
dense
More

14. CONCLUSION
Meselson and Stahl concluded that DNA replication follows the semiconservative
model by comparing their result to the results predicted by each of the three models in Figure
The first replication in the 14N medium produced a band of hybrid (15N–14N) DNA. This result eliminated
the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated
the dispersive model and supported the semiconservative model.
First replication
Second replication
Conservative
model
Semiconservative
Dispersive

15. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
How does DNA replicate itself?
How is DNA replicated?
Origin

16. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
How does DNA replicate itself?
How is DNA replicated?
Origin
Elongation
DNA polymerase
500 bp/sec bacteria, 50 bp/sec in us
New base added to 3’end (-OH)
dNTPs provide energy

17. Figure 16.13 Incorporation of a nucleotide into a DNA strand
New strand
Template strand
5’ end
3’ end
Sugar A T
Base C G P OH
Nucleoside
triphosphate
Pyrophosphate
2 P
Phosphate OH
DNA polymerase

18. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
How does DNA replicate itself?
How is DNA replicated?
Origin
Elongation
Dealing with antiparallel strands

19. Overall direction of replication
Figure 16.14 Synthesis of leading and lagging strands during DNA replication
Parental DNA
DNA pol Ill elongates
DNA strands only in the
5  direction. 3 5 2 1
Okazaki
fragments
DNA pol III
Template
strand
Leading strand
Lagging strand 3
DNA ligase
Overall direction of replication
One new strand, the leading strand,
can elongate continuously 5 
as the replication fork progresses.
The other new strand, the
lagging strand must grow in an overall
3  direction by addition of short
segments, Okazaki fragments, that grow
5  (numbered here in the order
they were made).
DNA ligase joins Okazaki
fragments by forming a bond between
their free ends. This results in a
continuous strand. 4

20. Figure 16.15 Synthesis of the lagging strand
Overall direction of replication 3 5 1 2
DNA ligase forms a bond between the newest DNA and the adjacent DNA of fragment 1. 6
The lagging strand in this region is now complete. 7
DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2. 5
After the second fragment is primed. DNA pol III adds DNA nucleotides until it reaches the first primer and falls off. 4
After reaching the next RNA primer (not shown), DNA pol III falls off. 3
DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.
Primase joins RNA nucleotides into a primer.
Template strand
RNA primer
Okazaki fragment

21. Figure 16.16 A summary of bacterial DNA replication
Overall direction of replication
Helicase unwinds the
parental double helix.
Molecules of single-
strand binding protein
stabilize the unwound
template strands.
The leading strand is
synthesized continuously in the
5 3 direction by DNA pol III.
Leading
strand
Origin of replication
Lagging
OVERVIEW
Replication fork
DNA pol III
Primase
Primer
DNA pol I
DNA ligase 1 2 3
Primase begins synthesis
of RNA primer for fifth
Okazaki fragment. 4
DNA pol III is completing synthesis of
the fourth fragment, when it reaches the
RNA primer on the third fragment, it will
dissociate, move to the replication fork,
and add DNA nucleotides to the 3 end of the fifth fragment primer. 5
DNA pol I removes the primer from the 5 end
of the second fragment, replacing it with DNA
nucleotides that it adds one by one to the 3’ end
of the third fragment. The replacement of the
last RNA nucleotide with DNA leaves the sugar-
phosphate backbone with a free 3 end. 6
DNA ligase bonds
the 3 end of the
second fragment to
the 5 end of the first
fragment. 7
Parental DNA 5 3

22. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
How does DNA replicate itself?
How is DNA replicated?
How is DNA repaired?
Proofreading – mismatch repair – DNA polymerase fixes mistakes made
during copying
Excision repair

23. Figure 16.17 Nucleotide excision repair of DNA damage
Nuclease
DNA
polymerase
ligase
A thymine dimer
distorts the DNA molecule. 1
Repair synthesis by
a DNA polymerase
fills in the missing
nucleotides. 3
DNA ligase seals the
Free end of the new DNA
To the old DNA, making the
strand complete. 4
A nuclease enzyme cuts
the damaged DNA strand
at two points and the
damaged section is
removed. 2

24. Chapter 16: The Molecular Basis of Inheritance
What evidence did Frederick Griffith have that DNA was the genetic material?
How did Hershey & Chase determine that DNA was the genetic material?
What was Chargaff’s contribution?
What experiment did Watson & Crick do?
How does DNA replicate itself?
How is DNA replicated?
How is DNA repaired?
What happens at the end of chromosomes (telomere)?
Recall DNA polymerase works 5’ → 3’ direction
After RNA primer is removed, chromosome is shortened

25. Figure 16.18 Shortening of the ends of linear DNA molecules
End of parental
DNA strands
Leading strand
Lagging strand
Last fragment
Previous fragment
RNA primer
Removal of primers and
replacement with DNA
where a 3 end is available
Primer removed but
cannot be replaced
with DNA because
no 3 end available
for DNA polymerase
Second round
of replication
New leading strand
New lagging strand 5
Further rounds
Shorter and shorter
daughter molecules 5 3
Telomerase – active in germ line cells & new borns prevents shortening
2009 Nobel Prize in Medicine - for the discovery of how chromosomes are protected
by telomeres and the enzyme telomerase