Chapter 16 RQ
1.
2.
3.
4.
5.
What is a virus that infects
bacteria called?
Who actually took the X-ray
diffraction photo of DNA’s
structure?
What are the bonds between
nitrogenous bases?
What does the “semiconservative
model” describe?
What does “ligase” do?
1. Explain why researchers originally thought
protein was the genetic material.
• Proteins are macromolecules with great
heterogeneity and functional specificity
• Little was known about nucleic acids
• The physical and chemical properties of
DNA seemed too uniform to account for
the multitude of inherited traits 
2. Describe the experiment that led to the
discovery that DNA was the genetic material
in cells.
• Frederick Griffith in 1928
• Trying to find a vaccine to fight pneumonia
• Experimented with the two strains of
pneumococcus; smooth & rough
• Smooth caused the disease, rough did not
• When dead S strain was mixed with live R,
the mice DID die, indicating an acquired
ability 
3. Define ‘transformation’ and briefly discuss
viruses and their effects on bacteria.
• Change in
phenotype due to
the assimilation of
external genetic
material by a cell
• Viruses can inject
their information
into cells and cause
drastic changes in
behavior 
4. List the three components of a nucleotide.
1. Pentose
(5-C sugar)
2. Phosphate
3. Nitrogenous
base 
5. List the nitrogenous bases found in DNA,
and distinguish between pyrimidine and
purine.
•
•
•
•
Pyrimidines
Purines
6 membered ring of • 5 membered ring
carbon and
with 6 membered
nitrogen
ring
C – cytosine
• A – adenine
T – thymine (DNA) • G – guanine 
U – uracil (RNA)
6. Explain how Watson and Crick deduced the
structure of DNA, and describe what
evidence they used.
• Built models to
conform to x-ray
data
- sugar phosphate
backbone
- nitrogenous base
interior 
7. Explain the “base-pairing rule” and
describe it’s significance.
• A – T : 2 hydrogen
bonds
• G – C : 3 hydrogen
bonds
• Suggests the
mechanisms for DNA
replication
• Dictates combination
of complementary
pairs 
8. Describe the structure of DNA, and explain what
kind of chemical bond connects the nucleotides of
each strand and what type of bond holds the two
strands together.
• Hydrogen bonds hold the nucleotides
together
• Van der Waals forces help keep helix
spiral shape 
9. Explain semiconservative replication and
the Meselson-Stahl experiment.
10. Describe the process of DNA replication, and
explain the role of helicase, single strand binding
protein, DNA polymerase, ligase, and primase.
1.
2.
3.




The helical molecule untwists while it copies its 2
antiparallel strands simultaneously
Very rapid – only a few hours to copy 6 billion bases
of a human cell
Very accurate – one in a billion nucleotides are
incorrect
Helicase  catalyzes the unwinding of the parental
double helix to expose the template
Single strand binding protein  keeps the separated
strands apart and stabilizes the unwound DNA
Polymerase and ligase  catalyze the filling-in
process
Primase  the enzymes that polymerize the short
segments of RNA (primers) 
11. Explain what energy source drives
endergonic synthesis of DNA.
• It is the hydrolysis of nucleoside
triphosphates, which are nucleotides with a
triphosphate covalently linked to the 5’
carbon of the pentose
• Exergonic hydrolysis of this phosphate
bond drives the endergonic synthesis of
DNA  it provides the required energy to
form the new covalent linkages between
nucleotides 
12. Define antiparallel, and explain why
continuous synthesis of both DNA strands is
not possible.
• Antiparallel  the
sugar-phosphate
backbones of the 2
complementary DNA
strands run in opposite
directions
• DNA can only elongate
in the 5’ to 3’ direction
due to polarity issues
- 3’ end has a hydroxyl
group
- 5’ end has a
phosphate 
13. Distinguish between the leading strand
and the lagging strand.
• Leading  continuous
DNA synthesis, it is
synthesized as a single
polymer in the 5’ to 3’
direction towards the
replication fork
• Lagging  the DNA
strand that is
discontinuously
synthesized against
the overall direction
of replication 
14. Explain how the lagging strand is
synthesized when DNA polymerase can add
nucleotides only to the 3’ end.
• The lagging strand is
produced as a series
of Okazaki fragments
in the 5’  3’ direction
• Fragments are ligated
by DNA ligase which
catalyzes the
formation of a
covalent bond between
the 3’ end of each
fragment to the 5’ end
of the chain 
15. Explain the role of DNA polymerase,
ligase, and repair enzymes in DNA
proofreading and repair.
• DNA polymerase and ligase catalyze
the filling-in process of the new DNA
strands
• Repair enzymes excise ( remove) the
damaged segments and the gap is
filled in by the correct nucleotides 
Pictures 