1. The Molecular Basis of Inheritance
Chapter 16
The Molecular Basis of Inheritance

2. Figure 16.1 Watson and Crick with their DNA model

3. Figure 16.2 Can the genetic trait of pathogenicity be transferred between bacteria?
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.
1922 Griffith
Transformation, a change in genotype and phenotype due to the assimilation of foreign DNA by a cell.
In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming
substance was DNA.

4. Figure 16.3 Viruses infecting a bacterial cell
Phage
head
Tail
Tail fiber
DNA
Bacterial
cell
100 nm

5. Figure 16.4 Is DNA or protein the genetic material of phage T2?
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

6. 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

7. Figure 16.5 The structure of a DNA strand
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

8. Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA
(a) Rosalind Franklin
Franklin’s X-ray diffraction
Photograph of DNA
(b)

9. 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

10. Unnumbered Figure p. 298 Purine + Purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
Consistent with X-ray data
Chargaff’s rules (1947)
In all organisms, the number of adenines was approximately equal to the number of thymines (%T = %A).
Also (%G = %C).
Erwin Chargaff

11. Figure 16.8 Base pairing in DNAH N H O
CH3
Sugar
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)

12. Figure 16.9 A model for DNA replication: the basic concept (layer 1)
(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. A C T G

13. Figure 16.9 A model for DNA replication: the basic concept (layer 2)
(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. A C T G

14. Figure 16.9 A model for DNA replication: the basic concept (layer 3)
(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. A C T G

15. Figure 16.9 A model for DNA replication: the basic concept (layer 4)
(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
The second paper of Watson and Crick

16. 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

17. Figure 16.11 Does DNA replication follow the conservative, semiconservative, or dispersive model?
Late 50s
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

18. 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

19. Figure 16.12 Origins of replication in eukaryotes
Replication begins at specific sites
where the two parental strands
separate and form replication
bubbles.
The bubbles expand laterally, as
DNA replication proceeds in both
directions.
Eventually, the replication
bubbles fuse, and synthesis of
the daughter strands is
complete. 1 2 3
Origin of replication
Bubble
Parental (template) strand
Daughter (new) strand
Replication fork
Two daughter DNA molecules
In eukaryotes, DNA replication begins at many sites along the giant
DNA molecule of each chromosome.
In this micrograph, three replication
bubbles are visible along the DNA of
a cultured Chinese hamster cell (TEM).
(b)
(a)
0.25 µm
E. coli: 5 million/25 min
Human cell: 6 billion/few hours
The rate of elongation is about 500 nucleotides per second in bacteria
and 50 per second in human cells

20. Figure 16.13 Incorporation of a nucleotide into a DNA strand
Why antiparallel?
New strand
Template strand
5’ end
3’ end
Sugar A T
Base C G P OH
Nucleoside
triphosphate
Pyrophosphate
2 P
Phosphate
The exergonic hydrolysis of pyrophosphate to two inorganic phosphate molecules drives the polymerization of the nucleotide to the new strand.

21. 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

22. 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

23. Table 16.1 Bacterial DNA replication proteins and their functions

24. 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

25. Arthur Kornberg (born March 3, 1918) is an American biochemist who won the Nobel Prize in Physiology or Medicine 1959 for his discovery of "the mechanisms in the biological synthesis of deoxyribonucleic acid (DNA)" together with Dr. Severo Ochoa of New York University.
Identified “DNA polymerase”

26. Reiji Okazaki (岡崎令治 Okazaki Reiji, 1930 – 1975) was a Japanese molecular biologist known for his research in the DNA replication and the discovery of Okazaki fragments.
Reiji Okazaki was born in Hiroshima, Japan. He graduated in 1953 from Nagoya University, and worked as a professor there after 1963.
Okazaki died of leukemia only a few years after his discovery. He was exposed to heavy radiation when he was in Hiroshima when the first atomic bomb was dropped.
Tsuneko Okazaki (wife and collaborator)

27. 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
Mutations are caused by
Enzymatic mistakes
High energy radiation (X-ray, UV)
Spontaneous changes
Defects in repair cause cancers (i.e. skin cancer)

28. 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

29. Figure Telomeres
1 µm

30. 4. The ends of DNA molecules are replicated by a special mechanism.
The ends of eukaryotic chromosomal DNA molecules, the telomeres, have special nucleotide sequences.
Multiple repetitions of one short nucleotide sequence. TTAGGG in human
Telomerase catalyzes the lengthening of telomeres in eukaryotic germ cells, restoring their original length.
Telomerase is not present in most cells of multicellular organisms.
Normal shortening of telomeres may protect organisms from cancer by limiting the number of divisions that somatic cells can undergo.
Short telomeres in Werner syndrome
Active telomerase has been found
in some cancerous somatic cells.
Telomerase may provide a useful
target for cancer diagnosis and chemotherapy.