Activating Strategy
• Generate a caption for this
picture
AP Lesson #47
EQ: What is the structure of DNA and
why is it important?
What are the building blocks of DNA?
• Nitrogen Base
• Deoxyribose Sugar
• Phosphate Group
PO4
DNA nucleotide
N base
Which side is up on DNA?
• You need to number
the carbons!
• #1 carbon has the N
base attached to it!
DNA nucleotide
PO4
N base
5′′ CH2
CH2
O
O
4′′
Deoxyribose
Deoxyribose
3′′
OH
How do you build a strand
of DNA?
OH
2′′
DNA has two sides, how
do they line up?
5′′
PO4
• Anti-parallel
• Create a DNA backbone
– refer to the 3′ and 5′ ends of the
DNA
– Phosphate group from the 5’ C
attaches to the 3’ OH
– Covalent Phosophodiester Bond
• Very strong and high energy bond
1′′
base
5′′ CH2
– complementary strand runs
in opposite direction
– Each side has “direction”
O
4′′
1′′
C
3′′
–O
O
P O
O
5′′ CH2
2′′
base
5′′
3′′
3′′
5′′
• Strands are held together by
hydrogen bonds between
nitrogen bases.
O
4′′
1′′
2′′
3′′
OH
3′′
How do nitrogen bases pair in DNA?
Bonds in DNA
All
Get
Cranky
• Purines
Hydrogen
bonds
5′′
Tigers
3′′
– adenine (A)
– guanine (G)
• Pyrimidines
covalent
Phosphodiester
bonds
– thymine (T)
– cytosine (C)
• Pairing
3′′
5′′
–A:T
• 2 bonds
–C:G
….strong or weak bonds?
How do the bonds effect the copying of DNA?
• 3 bonds
So then what does DNA look like?
• Maurice Wilkins and Rosalind Franklin used X-ray
crystallography
– Franklin produced a picture of the DNA molecule
Maurice Wilkins
Rosalind Franklin
(b) Franklin’s X-ray diffraction
photograph of DNA
How is DNA then organized?
Watson and Crick (1953)
• deduced that DNA was
helical
• deduced the width of the
helix and the spacing and
pairing of nitrogenous
bases
• double helix
– in any organism the amount
of A = T and the amount of
G=C
How did this model take shape?
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width
consistent with X-ray data
Double Helix Structure of DNA
Summarizing Strategy
• Let’s Build a Model
– DNA Origami
“It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic material.”
Watson & Crick
Assessment
• HW: Read Chapter 16.1 – 16.2 (pgs. 305 - 319)