KEY CONCEPTS 16.1 DNA is the genetic material 16.2 Many proteins work together in DNA replication and repair 16.3 A chromosome consists of a DNA molecule packed together with proteins

1. Figure 15.UN03b
Testcross
Offspring
Expected
(e)
Observed
(o)
Deviation
(o − e) (o − e)2
(o − e)2
/e
(A−B−)
(aaB−)
(A−bb)
(aabb)
220
210
231
239
χ2
= Sum
Review the Chi-Square Test
Try: 72; 131; 134; 63 for observed

3. Chapter 16
The Molecular Basis
of Inheritance

4. In 1953, James Watson and Francis Crick
introduced an elegant double-helical model for
the structure of deoxyribonucleic acid, or DNA

5. The Search for the Genetic Material:
Scientific Inquiry
• When Morgan’s group showed that genes are
located on chromosomes, the two components of
chromosomes—DNA and protein—became
candidates for the genetic material
• The key factor in determining the genetic material
was choosing appropriate experimental organisms
• The role of DNA in heredity was first discovered by
studying bacteria and the viruses that infect them

6. Evidence That DNA Can Transform
Bacteria
• The discovery of the genetic role of DNA began
with research by Frederick Griffith in 1928
• Griffith worked with two strains of a bacterium, a
pathogenic “S” strain and a harmless “R” strain
• When he mixed heat-killed remains of the
pathogenic strain with living cells of the harmless
strain, some living cells became pathogenic
• He called this phenomenon transformation, now
defined as a change in genotype and phenotype
due to assimilation of foreign DNA

7. Figure 16.2
Living S cells
(pathogenic
control)
Experiment
Results
Living R cells
(nonpathogenic
control)
Heat-killed S cells
(nonpathogenic
control)
Mouse dies Mouse healthy Mouse healthy Mouse dies
Mixture of heat-
killed S cells and
living R cells
Living S cells

8. • In 1944, Oswald Avery, Maclyn McCarty, and
Colin MacLeod announced that the
transforming substance was DNA
• Their conclusion was based on experimental
evidence that only DNA worked in transforming
harmless bacteria into pathogenic bacteria
• Many biologists remained skeptical, mainly
because little was known about DNA

9. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
9-11
 Avery et al also conducted the following experiments
 To further verify that DNA, and not a contaminant (RNA or protein), is the
genetic material

10. 1. Which of the following results from Griffith’s
experiment is an example of transformation?
a. Mouse dies after being injected with living S cells.
b. Mouse is healthy after being injected with living R
cells.
c. Mouse is healthy after being injected with heat-killed
S cells.
d. Mouse dies after being injected with a mixture of
heat-killed S and living R cells.
e. In blood samples from the mouse in “d”, living S cells
were found.

11. 1. Which of the following results from Griffith’s
experiment is an example of transformation?
a. Mouse dies after being injected with living S cells.
b. Mouse is healthy after being injected with living R
cells.
c. Mouse is healthy after being injected with heat-killed
S cells.
d. Mouse dies after being injected with a mixture of
heat-killed S and living R cells.
e. In blood samples from the mouse in “d”, living S cells
were found.

12. • In 1952, Alfred Hershey and Martha Chase
performed experiments showing that DNA is the
genetic material of a phage known as T2
• To determine the source of genetic material in the
phage, they designed an experiment showing that
only one of the two components of T2 (DNA or
protein) enters an E. coli cell during infection
• They concluded that the injected DNA of the phage
provides the genetic information

13. Figure 16.3
Phage
head
DNA
Tail
sheath
Tail fiber
Genetic
material
Bacterial
cell
100nm
Evidence That Viral DNA Can Program Cells

14. Figure 16.4
Experiment
Batch 1: Radioactive sulfur (35
S) in phage protein
Batch 2: Radioactive phosphorus (32
P) in phage DNA
Labeled phages
infect cells.
Agitation frees outside
phage parts from cells.
Centrifuged cells
form a pellet.
Radioactivity
(phage protein)
found in liquid
Pellet
Centrifuge
Radioactive
protein
Radioactive
DNA
Radioactivity (phage
DNA) found in pellet
Centrifuge
Pellet
1 2
4
3
4

15. Additional Evidence That DNA Is the
Genetic Material
• In 1950, Erwin Chargaff reported that DNA
composition varies from one species to the next
A & T same ratio; G & C same ratio, but % GC
varies from species to species (Chargaff’s rule)
• This evidence of diversity made DNA a more
credible candidate for the genetic material
• By the 1950s, it was already known that DNA is a
polymer of nucleotides, each consisting of a
nitrogenous base, a sugar, and a phosphate group

16. Figure 16.5
5′ end
Thymine (T)
Adenine (A)
Cytosine (C)
Guanine (G)
Nitrogenous
bases
Sugar–
phosphate
backbone
3′ end
Nitrogenous
base
Sugar
(deoxyribose)DNA
nucleotide
Phosphate

17. Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography
to study molecular structure
Franklin’s X-ray diffraction
photograph of DNA
Rosalind Franklin

18. • Franklin’s X-ray crystallographic images of DNA
enabled Watson to deduce that DNA was helical
• The X-ray images also enabled Watson to
deduce the width of the helix and the spacing of
the nitrogenous bases
• The width suggested that the DNA molecule was
made up of two strands, forming a double helix,
anti-parallel in nature
B-DNA
2 nm wide; 3.6 angstrom per unit; 10 units per

19. Figure 16.7
(a) Key features of
DNA structure
(b) Partial chemical structure
0.34 nm
3′ end
5′ endT
T
T
A
A
A
C
C
C
G
G
G
AT
1 nm
TA
C G
CG
AT
3.4 nm
CG
CG
C G
C G
3′ end
5′ end
Hydrogen bond
T A
G C
A T
C G
(c) Space-filling
model

20. • Watson and Crick built models of a double helix to
conform to the X-rays and chemistry of DNA
• Franklin had concluded that there were two
antiparallel sugar-phosphate backbones, with the
nitrogenous bases paired in the molecule’s interior
• At first, Watson and Crick thought the bases paired
like with like (A with A, and so on), but such
pairings did not result in a uniform width
• Instead, pairing a purine with a pyrimidine resulted
in a uniform width consistent with the X-ray

21. Figure 16.UN02
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data

22. Adenine (A) Thymine (T)
Guanine (G) Cytosine (C)
Sugar
Sugar
Sugar
Sugar
•Watson and Crick
reasoned that the
pairing was more
specific, dictated by
the base structures
•They determined
that adenine paired
only with thymine,
and guanine paired
only with cytosine

23. The Basic Principle: Base Pairing to a Template Strand
• Since the two strands of DNA are
complementary, each strand acts as a template
for building a new strand in replication
• In DNA replication, the parent molecule
unwinds, and two new daughter strands are
built based on base-pairing rules
•Watson and Crick noted that the specific base
pairing suggested a possible copying mechanism
for genetic material

24. Figure 16.9-3
(a) Parental
molecule
(b) Separation of parental
strands into templates
A
A
A
T
T
T
G
G C
C
A
A
A
T
T
T
G
G C
C
A
A
A
T
T
T
G
G C
C
A
A
A
T
T
T
G
G C
C
(c) Formation of new strands
complementary to template
strands

25. • Watson and Crick’s semiconservative model of
replication predicts that when a double helix
replicates, each daughter molecule will have
one old strand (derived or “conserved” from the
parent molecule) and one newly made strand
• Competing models were the conservative model
and the dispersive model
• Meselson and Stahl provided the supporting
scientific evidence

26. Figure 16.10
(a) Conservative
model
(b) Semiconserva-
tive model
(c) Dispersive
model
Parent cell
First
replication
Second
replication

27. • Experiments by Meselson and Stahl supported
the semiconservative model
• They labeled the nucleotides of the old strands
with a heavy isotope of nitrogen, while any new
nucleotides were labeled with a lighter isotope
• The first replication produced a band of hybrid
DNA, eliminating the conservative model
• A second replication produced both light and
hybrid DNA, eliminating the dispersive model
and supporting the semiconservative model

28. Figure 16.11
Bacteria cultured
in medium with 15
N
(heavy isotope)
Experiment
Results
Conclusion
Bacteria transferred
to medium with 14
N
(lighter isotope)
DNA sample
centrifuged
after first
replication
DNA sample
centrifuged
after second
replication
Less dense
More dense
Predictions: First replication Second replication
Conservative
model
Semiconservative
model
Dispersive
model
1 2
43

29. DNA Replication: A Closer Look
• The copying of DNA is remarkable in its speed
and accuracy
• More than a dozen enzymes and other proteins
participate in DNA replication
Video: DNA Replication

30. 2. What is the %T in wheat DNA?
a. approximately 22%
b. approximately 23%
c. approximately 28%
d. approximately 45%

31. 2. What is the %T in wheat DNA?
a. approximately 22%
b. approximately 23%
c. approximately 28%
d. approximately 45%

32. Getting Started: Origins of Replication
• Replication begins at special sites called origins
of replication, where the two DNA strands are
separated, opening up a replication “bubble”
• A eukaryotic chromosome may have hundreds
or even thousands of origins of replication
• Replication proceeds in both directions from
each origin, until the entire molecule is copied
• At the end of each replication bubble is a
replication fork, a Y-shaped region where new
DNA strands are elongating

33. Figure 16.12
Origin of
replication
0.5µm
0.25µm
Bacterial
chromosome
Two daughter
DNA molecules
Replication
bubble
Parental (template)
strand
Daughter
(new) strand
Replication
fork
Double-
stranded
DNA molecule
(a) Origin of replication in an E. coli cell (b) Origins of replication in a eukaryotic cell
Origin of
replication Eukaryotic chromosome
Double-stranded
DNA molecule
Parental (template)
strand
Daughter (new)
strand
Replication
fork
Bubble
Two daughter DNA molecules

34. Elongating a New DNA Strand
• Enzymes called DNA polymerases catalyze the
elongation of new DNA at a replication fork
• Each nucleotide that is added to a growing DNA
strand is a nucleoside triphosphate
• The rate of elongation is about 500 nucleotides
per second in bacteria and 50 per second in
human cells

35. LE 16-13
New strand
5′ end
Phosphate
Base
Sugar
Template strand
3′ end 5′ end 3′ end
5′ end
3′ end
5′ end
3′ end
Nucleoside
triphosphate
DNA polymerase
Pyrophosphate

36. Antiparallel Elongation
• The antiparallel structure of the double helix
(two strands oriented in opposite directions)
affects replication
• DNA polymerases add nucleotides only to the
free 3′ end of a growing strand; therefore, a
new DNA strand can elongate only in the
5′ to 3′ direction

37. • Along one template strand of DNA, called the
leading strand, DNA polymerase can synthesize
a complementary strand continuously, moving
toward the replication fork
• To elongate the other new strand, called the
lagging strand, DNA polymerase must work in
the direction away from the replication fork
• The lagging strand is synthesized as a series of
segments called Okazaki fragments, which are
joined together by DNA ligase

38. LE 16-14
Parental DNA
5′
3′
Leading strand
3′
5′
3′
5′
Okazaki
fragments
Lagging strand
DNA pol III
Template
strand
Leading strand
Lagging strand
DNA ligase
Template
strand
Overall direction of replication

39. Priming DNA Synthesis
• DNA polymerases cannot initiate synthesis of a
polynucleotide; they can only add nucleotides
to the 3′ end
• The initial nucleotide strand is a short one called
an RNA or DNA primer
• An enzyme called primase can start an RNA
chain from scratch
• Only one primer is needed to synthesize the
leading strand, but for the lagging strand each
Okazaki fragment must be primed separately

40. Figure 16.16
Overview
1
2
1
1
2
1
2
1
1
2
5′
3′
3′
5′
5′
3′
3′
3′ 3′
5′
5′
5′
5′
3′
3′
5′
3′
5′
3′
5′
3′
5′
3′
5′
3′
5′
3′
1
2 5
6
4
3
Origin of replication
Lagging
strand
Leading
strand
Lagging
strand
Leading
strand
Overall directions
of replication
RNA primer
for fragment 2
Okazaki
fragment 2
DNA pol III
makes Okazaki
fragment 2.
DNA pol I
replaces RNA
with DNA.
DNA ligase
forms bonds
between DNA
fragments.
Overall direction of replication
Okazaki
fragment 1
DNA pol III
detaches.
RNA primer
for fragment 1
Template
strand
DNA pol III
makes Okazaki
fragment 1.
Origin of
replication
Primase makes
RNA primer.
5′

41. Other Proteins That Assist DNA
Replication
• Helicase untwists the double helix and separates
the template DNA strands at the replication fork
• Single-strand binding protein binds to and stabilizes
single-stranded DNA until it can be used as a
template
• Topoisomerase corrects “overwinding” ahead of
replication forks by breaking, swiveling, and
rejoining DNA strands

42. • Primase synthesizes an RNA primer at the 5′ ends
of the leading strand and the Okazaki fragments
• DNA pol III continuously synthesizes the leading
strand and elongates Okazaki fragments
• DNA pol I removes primer from the 5′ ends of the
leading strand and Okazaki fragments, replacing
primer with DNA and adding to adjacent 3′ ends
• DNA ligase joins the 3′ end of the DNA that
replaces the primer to the rest of the leading
strand and also joins the lagging strand fragments

43. Figure 16.17
Overview
5′
3′
Lagging
strand
Leading
strand
Leading
strand
Lagging
strand
Leading strand
Leading strand
template
Origin of
replication
Overall directions
of replication
5′
3′
5′
3′
5′
5′
3′
3′
3′
Single-strand
binding proteins
Helicase
Parental
DNA
DNA pol III
Primer
Primase
Lagging
strand
Lagging strand
template
DNA pol III
DNA pol I
5′
DNA ligase
123
4
5

45. The DNA Replication Machine as a
Stationary Complex
• The proteins that participate in DNA replication
form a large complex, a DNA replication “machine”
Called the Replisome
• The DNA replication machine is probably
stationary during the replication process
• Recent studies support a model in which DNA
polymerase molecules “reel in” parental DNA and
“extrude” newly made daughter DNA molecules

46. Table 16.1

47. Proofreading and Repairing DNA
• DNA polymerases proofread newly made DNA,
replacing any incorrect nucleotides
• In mismatch repair of DNA, repair enzymes
correct errors in base pairing
• In nucleotide excision repair, enzymes cut out
and replace damaged stretches of DNA

48. LE 16-17
DNA
ligase
DNA
polymerase
DNA ligase seals the
free end of the new DNA
to the old DNA, making the
strand complete.
Repair synthesis by
a DNA polymerase
fills in the missing
nucleotides.
A nuclease enzyme cuts
the damaged DNA strand
at two points and the
damaged section is
removed.
Nuclease
A thymine dimer
distorts the DNA molecule.

49. Replicating the Ends of DNA Molecules
• Limitations of DNA polymerase create problems for
the linear DNA of eukaryotic chromosomes
• The usual replication machinery provides no way to
complete the 5′ ends, so repeated rounds of
replication produce shorter DNA molecules

50. Figure 16.20
Ends of parental
DNA strands
Lagging strand
Parental strand
RNA primer
Last fragment
Next-to-last
fragment
Lagging strand
Leading strand
Removal of primers and
replacement with DNA
where a 3′ end is available
3′
5′
5′
Second round
of replication
Further rounds
of replication
New lagging strand
New leading strand
Shorter and shorter daughter molecules
5′
3′
3′
5′
3′
5′
3′

51. • Eukaryotic chromosomal DNA molecules have at
their ends nucleotide sequences called
telomeres
• Telomeres do not prevent the shortening of
DNA molecules, but they do postpone the
erosion of genes near the ends of DNA
molecules
• It has been proposed that the shortening of
telomeres is connected to aging

52. Figure 16.21
1 µm

53. • If chromosomes of germ cells became shorter in
every cell cycle, essential genes would
eventually be missing from the gametes they
produce
• An enzyme called telomerase catalyzes the
lengthening of telomeres in germ cells using an
enzyme with an RNA template

54. Concept 16.3: A chromosome
consists of a DNA molecule packed
together with proteins
• The bacterial chromosome is a double-
stranded, circular DNA molecule associated
with a small amount of protein
• Eukaryotic chromosomes have linear DNA
molecules associated with a large amount
of protein
• In a bacterium, the DNA is “supercoiled” and
found in a region of the cell called the
nucleoid

55. • In the eukaryotic cell, DNA is precisely
combined with proteins in a complex called
chromatin
• Chromosomes fit into the nucleus through an
elaborate, multilevel system of packing

56. Figure 16.22
DNA, the
double helix
Histones Nucleosomes,
or “beads on
a string”
(10-nm fiber)
30-nm fiber
Looped
domains
(300-nm fiber)
Metaphase
chromosome
DNA
double helix
(2 nm in diameter)
Nucleosome
(10 nm in diameter)
30-nm fiber
Loops Scaffold
Histones
Histone tail
H1
300-nm
fiber
Chromatid
(700 nm)
Replicated
chromosome
(1,400 nm)

57. Figure 16.23
5 µm

58. • Most chromatin is loosely packed in the
nucleus during interphase and condenses
prior to mitosis
• Loosely packed chromatin is called
euchromatin
• During interphase a few regions of chromatin
(centromeres and telomeres) are highly
condensed into heterochromatin
• Dense packing of the heterochromatin makes
it difficult for the cell to express genetic
information coded in these regions

59. 3. Which of the following statements is false when
comparing prokaryotes with eukaryotes?
A) The prokaryotic chromosome is circular,
whereas eukaryotic chromosomes are linea
B) Prokaryotic chromosomes have a single origin
of replication, whereas eukaryotic
chromosomes have many.
C) The rate of elongation during DNA replication
is higher in prokaryotes than in eukaryotes.
D) Prokaryotes produce Okazaki fragments
during DNA replication, but eukaryotes do not.
E) Eukaryotes have telomeres, and prokaryotes
do not.

60. 3. Which of the following statements is false when
comparing prokaryotes with eukaryotes?
A) The prokaryotic chromosome is circular,
whereas eukaryotic chromosomes are linea
B) Prokaryotic chromosomes have a single origin
of replication, whereas eukaryotic
chromosomes have many.
C) The rate of elongation during DNA replication
is higher in prokaryotes than in eukaryotes.
D) Prokaryotes produce Okazaki fragments
during DNA replication, but eukaryotes do
not.
E) Eukaryotes have telomeres, and prokaryotes
do not.