1 Review List the chemical components of DNA
Relate Cause and Effect Why are hydrogen bonds so
essential to the structure of DNA
2 Review Describe the discoveries that led to the
modeling of DNA
Infer Why did scientists have to use tools other than
microscopes to solve the structure of DNA
3 Review Describe Watson and Crick’s model of the
DNA model
CH 12 DNA
12.2 The Structure of DNA
Nucleic Acids and Nucleotides

DNA nucleotides are made up of
 Deoxyribose
 Phosphate
 Nitrogen

base
DNA has four kinds of nitrogenous bases
 Adenine (A)
 Thymine (T)
 Guanine
(G)
 Cytosine (C).


Nucleic acids are a chain of nucleotides
Nucleotides can be joined in any order.
Solving the Structure of DNA
Chargaff’s Rules
Percentages of adenine [A] and thymine [T] bases
are almost equal in any sample of DNA
 Same with guanine [G]
and cytosine [C]



A=T
C=G.
Franklin’s X-Rays




X-ray diffraction
X-shaped pattern showing
that the strands in DNA are
twisted
Angle of the X-shaped
pattern suggested two
strands
Width of X suggested width.
Watson and Crick



Used clues from
Franklin’s X-ray
Used Chargaff’s rules
Proposed structure for
double helix.
The Double-Helix Model



Antiparallel Strands
Hydrogen Bonding
Base Pairing.
Antiparallel Strands


Two strands of DNA run in opposite directions
A-T-C-G-C-G-A-T
T-A-G-C-G-C-T-A
Hydrogen Bonding


Relatively weak chemical
bonds
Allows for two strands to
separate.
Base Pairing


A binds with T
C binds with G.
1.
2.
3.
Interpret Tables Which organism has the highest
percentage of adenine
Calculate If a species has 35% adenine in its DNA
what is the percentage of the other three bases
What did the fact that A and T, and C and G,
occurred in equal amounts suggest about the
relationship among these bases