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

2. Overview: Life’s Operating Instructions
In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA

3. 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

4. Evidence That DNA Can Transform Bacteria
The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928
Living S cells
(control)
Living R cells
Heat-killed
S cells (control)
Mixture of heat-killed
S cells and living
R cells
Mouse dies
are found in
blood sample
Mouse healthy
RESULTS

5. Evidence That Viral DNA Can Program Cells
Viruses, called bacteriophages (or phages), are widely used in molecular genetics research
Animation: Phage T2 Reproductive Cycle

6. LE 16-3
Phage
head
Tail
Tail fiber
DNA
100 nm
Bacterial
cell

7. Animation: Hershey-Chase Experiment
The 1952, Alfred Hershey & Martha Chase Experiment
Bacterial cell
Phage
DNA
Radioactive
protein
Empty
protein shell
Radioactivity
(phage protein)
in liquid
Batch 1:
Sulfur (35S)
Centrifuge
Pellet (bacterial
cells and contents)
Pellet
(phage DNA)
in pellet
Batch 2:
Phosphorus (32P)
Animation: Hershey-Chase Experiment

8. Additional Evidence That DNA Is the Genetic Material
Sugar–phosphate
backbone
5 end
Nitrogenous
bases
Thymine (T)
Adenine (A)
Cytosine (C)
DNA nucleotide
Phosphate
3 end
Guanine (G)
Sugar (deoxyribose)
In 1947, Erwin Chargaff reported that DNA composition varies from one species to the next.
Animation: DNA and RNA Structure

9. Building a Structural Model of DNA: Scientific Inquiry
Rosalind Franklin was using a technique called X-ray crystallography and produced a picture of DNA
Franklin’s X-ray diffraction
photograph of DNA
Rosalind Franklin

10. Animation: DNA Double Helix
Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
5 end
3 end
Space-filling model
Partial chemical structure
Hydrogen bond
Key features of DNA structure
0.34 nm
3.4 nm
1 nm

11. Purine + purine: too wide
LE 16-UN298
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data

12. LE 16-8 Sugar Sugar Adenine (A) Thymine (T) Sugar Sugar Guanine (G)
Cytosine (C)

13. Animation: DNA Replication Overview
Concept 16.2: Many proteins work together in DNA replication and repair
Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material
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.
The first step in replication is separation of the two DNA strands.
Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.
The nucleotides are connected to form the sugar-phosphate back-
bones of the new strands. Each “daughter” DNA
molecule consists of one parental strand and one new strand.
Animation: DNA Replication Overview

14. Watson and Crick’s semiconservative model
Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix.
Semiconservative model. The two strands of the parental
molecule
separate, and each functions as a template for synthesis of a new, comple-mentary strand.
Dispersive model. Each strand of both daughter molecules contains
a mixture of
old and newly synthesized
DNA.
Parent cell
First
replication
Second
Watson and Crick’s semiconservative model
Competing models:
conservative model
dispersive model

15. LE 16-11
Bacteria
cultured in
medium
containing
15N
Bacteria
transferred to
medium
containing
14N
Experiments by Meselson and Stahl supported the semiconservative model.
DNA sample
centrifuged
after 20 min
(after first
replication)
DNA sample
centrifuged
after 40 min
(after second
replication)
Less
dense
More
dense
First replication
Second replication
Conservative
model
Semiconservative
model
Dispersive
model

16. DNA Replication: A Closer Look
More than a dozen enzymes and other proteins participate in DNA replication

17. Getting Started: Origins of Replication
Replication begins at special sites called origins of replication
At the end of each replication bubble is a replication fork

18. Animation: Origins of Replication
LE 16-12
Parental (template) strand
0.25 µm
Origin of replication
Daughter (new) strand
Bubble
Replication fork
Two daughter DNA molecules
In eukaryotes, DNA replication begins at may 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).
Animation: Origins of Replication

19. Elongating a New DNA Strand
Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork
New strand
5¢ end
Phosphate
Base
Sugar
Template strand
3¢ end
Nucleoside
triphosphate
DNA polymerase
Pyrophosphate

20. Antiparallel Elongation
Parental DNA 5¢ 3¢
Leading strand
Okazaki
fragments
Lagging strand
DNA pol III
Template
strand
DNA ligase
Overall direction of replication
Terms:
Leading Strand
Lagging Strand
Okazaki Fragments
DNA Polymerase
DNA Ligase
Animation: Leading Strand

21. Priming DNA Synthesis
DNA polymerases cannot initiate synthesis of a polynucleotide
The initial nucleotide strand is a short one called an RNA or DNA primer
An enzyme called primase produces the primer.

22. LE 16-15_1 Primase joins RNA nucleotides into a primer. 3¢ 5¢ 5¢ 3¢
Template
strand
Overall direction of replication

23. LE 16-15_2 Primase joins RNA nucleotides into a primer. 3¢ 5¢ 5¢ 3¢
Template
strand
DNA pol III adds
DNA nucleotides to
the primer, forming
an Okazaki fragment. 3¢
RNA primer 5¢ 3¢ 5¢
Overall direction of replication

24. LE 16-15_3 Primase joins RNA nucleotides into a primer. 3¢ 5¢ 5¢ 3¢
Template
strand
DNA pol III adds
DNA nucleotides to
the primer, forming
an Okazaki fragment. 3¢
RNA primer 3¢ 5¢ 5¢
After reaching the
next RNA primer (not
shown), DNA pol III
falls off.
Okazaki
fragment 3¢ 3¢ 5¢ 5¢
Overall direction of replication

25. LE 16-15_4 Primase joins RNA nucleotides into a primer. 3¢ 5¢ 5¢ 3¢
Template
strand
DNA pol III adds
DNA nucleotides to
the primer, forming
an Okazaki fragment. 3¢
RNA primer 3¢ 5¢ 5¢
After reaching the
next RNA primer (not
shown), DNA pol III
falls off.
Okazaki
fragment 3¢ 3¢ 5¢ 5¢
After the second fragment is
primed, DNA pol III adds DNA
nucleotides until it reaches the
first primer and falls off. 5¢ 3¢ 3¢ 5¢
Overall direction of replication

26. LE 16-15_5 Primase joins RNA nucleotides into a primer. 3¢ 5¢ 5¢ 3¢
Template
strand
DNA pol III adds
DNA nucleotides to
the primer, forming
an Okazaki fragment. 3¢
RNA primer 3¢ 5¢ 5¢
After reaching the
next RNA primer (not
shown), DNA pol III
falls off.
Okazaki
fragment 3¢ 3¢ 5¢ 5¢
After the second fragment is
primed, DNA pol III adds DNA
nucleotides until it reaches the
first primer and falls off. 5¢ 3¢ 3¢ 5¢
DNA pol I replaces
the RNA with DNA,
adding to the 3¢ end
of fragment 2. 5¢ 3¢ 3¢ 5¢
Overall direction of replication

27. Animation: Lagging Strand
LE 16-15_6
Primase joins RNA
nucleotides into a primer. 3¢ 5¢ 5¢
3¢
Template
strand
DNA pol III adds
DNA nucleotides to
the primer, forming
an Okazaki fragment. 3¢
RNA primer 3¢ 5¢ 5¢
After reaching the
next RNA primer (not
shown), DNA pol III
falls off.
Okazaki
fragment 3¢ 3¢ 5¢ 5¢
After the second fragment is
primed, DNA pol III adds DNA
nucleotides until it reaches the
first primer and falls off. 5¢ 3¢ 3¢ 5¢
DNA pol I replaces
the RNA with DNA,
adding to the 3¢ end
of fragment 2. 5¢ 3¢ 3¢ 5¢
DNA ligase forms a
bond between the newest
DNA and the adjacent DNA
of fragment 1.
The lagging
strand in the region
is now complete. 5¢ 3¢ 3¢ 5¢
Animation: Lagging Strand
Overall direction of replication

28. Other Proteins That Assist DNA Replication
Helicase
Primase
DNA pol III
DNA pol I
DNA ligase

29. Animation: DNA Replication Review
LE 16-16
Overall direction of replication
Leading
strand
Lagging
strand
Origin of replication
Lagging
strand
Leading
strand
OVERVIEW
DNA pol III
Leading
strand
DNA ligase
Replication fork 5¢
DNA pol I 3¢
Primase
Parental DNA
DNA pol III
Lagging
strand
Primer 3¢ 5¢
Animation: DNA Replication Review

30. Proofreading and Repairing DNA
DNA polymerases proofread newly made DNA, correcting most mistakes.
Mismatch repair of DNA - repair enzymes correct errors in base pairing
Nucleotide excision repair - enzymes cut out and replace damaged stretches of DNA
DNA
ligase
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.

31. Replicating the Ends of DNA Molecules
End of parental
DNA strands 5¢ 3¢
Lagging strand
Last fragment
RNA primer
Leading strand
Previous fragment
Primer removed but
cannot be replaced
with DNA because
no 3¢ end available
for DNA polymerase
Removal of primers and
replacement with DNA
where a 3¢ end is available
Second round
of replication
Further rounds
New leading strand
Shorter and shorter
daughter molecules
Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes

32. Telomeres
Do not prevent the shortening of DNA molecules, but postpone the erosion of genes near the ends of DNA molecules

33. LE 16-19
1 µm

34. An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells