DNA STRUCTURE
 What
does DNA look like?
 What
are the important parts of
DNA structure?
 Who
determined this structure and
how did they figure it out?
Background info . . .
– Deoxyribonucleic Acid
 Why is DNA important?
 DNA
Information in DNA determines our traits.
To understand how DNA stores information and
why some DNA leads to genetic disorders,
scientists needed to learn what it looks like.
They had trouble believing that the goop that
you extract somehow determines your traits
and is inheritable. Although, DNA’s structure is
too small to see with any microscope, what does
DNA look like on a molecular level?
=
?
In 1953, James Watson and Francis Crick
discovered the 3-D structure of DNA. What did
people know prior to that and what did Watson and
Crick figure out?
DNA is made of nucleotides
Nucleotides have 3 parts:
 Sugar
 Phosphate
 Base
Deoxyribose
A.
B.
Part of DNA
backbone.
5-sided sugar
1’ – base
3’ and 5’– phosphate
2’ – missing oxygen
4’ – no connection
Phosphate
A.
B.
C.
D.
Gives negative charge
to DNA
Connects to sugar in
DNA backbone
Sugar-phosphate
bonds 
phosphodiester bonds
Connect to 5’ and 3’
ends of deoxyribose
Nitrogenous Base
A.
Four different bases
1. Adenine
2. Guanine
3. Cytosine
4. Thymine
Nitrogenous Base
B. Pyrimidines
- small
- “pyrimidines”? “CUT the Py”
Nitrogenous Base
C. Purines
- large
-”tu rings”
Nitrogenous Base
D. Order of bases “spells out” traits.
A T C G T G T C G A =
A T C G T T T C G A =
E. Only 1% of human base sequences
code for anything. The other 99% is
“junk DNA”; now known as noncoding
DNA
Nucleotide Plus Nucleotide . . .
A.
B.
DNA has 2 strands
A-T pair
C-G pair
basepair (bp)
“Complementary”
Nucleotide Plus Nucleotide . . .
C. Bases are connected using
hydrogen bonds.
A – T  2 H bonds
C – G  3 H bonds
Nucleotide Plus Nucleotide . . .
D. Strands are antiparallel
Top-down
view of you
Make a single
strand of
DNA
Antiparallel
Phosphate
L
Sugar
R
5’
Base
3’
3’
5’
DNA strands are antiparallel
Nucleotide Plus Nucleotide . . .
E. Clockwise or right-handed
twist (follow hand UP
staircase) called alpha helix
F. Twisted so every 10 bp is
one full turn
G. Humans  3 billion bp; 6 ft
or about 2meters long in
every cell; 1/10th width of
hair
Nucleotide Plus Nucleotide . . .
H. In order to fit DNA in
nucleus, DNA is folded
around proteins called
histones and arranged in
packages called
chromosomes
Histones
Who figured this out and how?

James Watson and Francis Crick
solved the structure of DNA in 1953.
Did Watson and Crick do
“experiments”?
What did Watson and Crick know?
 Chargaff’s
Rules
Erwin Chargaff
In all species
% A’s = % T’s
% C’s = % G’s
Who else was in the race?
-Linus Pauling won two
Nobel Prizes (protein
structure; peace)
- Hypothesized that
DNA had 3 strands and
phosphates are on the
inside of triple helix.
- His son Peter shared
his father’s ideas with
Watson and Crick
What else did Watson and Crick know?

X-ray
Crystallography
1. DNA has 2
strands
2. The distance
between
the strands is
constant
But Watson and Crick didn’t do
any chemical experiments!

So whose x-ray data
is it?
Rosalind Franklin 
Why did she give her
data to Watson and
Crick?
She didn’t! He did 
Maurice Wilkins
“The Secret of Life” - What was learned
by looking at DNA Structure?
 DNA
could be copied: unwind helix
and follow base pair rules
 “Script
of life”: sequence of
nucleotides can store information
Nobel Prizes . . . or lack thereof


Watson and Crick awarded
Nobel Prize in 1962
No prize for Franklin
Did
Watson and Crick obtain Franklin’s data
“illegally”?
Should Franklin have shared her scientific
knowledge? (Never knew her info had been
“obtained” by Watson and Crick)
Should Franklin receive credit for her
contributions to solving DNA structure?
Did Watson and Crick really do a lot of work?