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Structure of DNA
Unit 5B.1
AP Biology
2007-2008
 Genetic information is stored and
passed to subsequent generations
through DNA molecules and, in some
cases, RNA molecules.
 Prokaryotes, viruses and eukaryotes
contain plasmids, which are small extra
chromosomal, double stranded circular
DNA molecules
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 Proof that DNA is the carrier of genetic
information involved a number of
important historical experiments
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Hershey & Chase
 Performed an experiment using radioactively
labeled viruses that infect bacteria.
 Viruses were made of only protein and
DNA
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Procedure
 Hershey and Chase
labeled the virus
DNA with a
radioactive isotope
and the virus protein
with a different
isotope.
 They then watched
the virus infect
bacteria cells and
followed the
radioactive isotopes.
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Results
 When the viruses infected bacteria,
only the DNA entered the bacteria cell
Evidence that DNA is the genetic
material of a cell
 DNA caused bacteria to produce more
viruses.

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Franklin & Wilkins
 Used X-ray crystallography to take
pictures of DNA
 helped lead to the discovery of the
shape of DNA
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Watson & Crick
 Double helix shape

DNA is made of two
strands of DNA that twist
together like a spiral
staircase
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Avery-MacLeod-McCarty
 Proved that DNA was the transforming
agent that caused nonpathogenic bacteria
to turn pathogenic and kill mice

Found that enzymes that destroy DNA did
not transform and that enzymes that
destroyed only proteins could transform
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STRUCTURE OF NUCLEIC ACIDS
Arrow from: http://www.harrythecat.com/graphics/b/arrow48d.gif
Built from
NUCLEOTIDE
SUBUNITS
NITROGEN BASES
CAN BE:
ADENINE
GUANINE
CYTOSINE
THYMINE
URACIL
Sugar can be DEOXYRIBOSE (DNA)
RIBOSE (RNA)
Image by: Riedell
DNA has no URACIL
RNA has no THYMINE
PURINES (A & G) have 2 RINGS
PYRIMIDINES (T, C, & U) have 1 RING
http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/genetics/DNA/DNA/fg4.html
http://student.ccbcmd.edu/~gkaiser/biotutorials/dna/fg29.html
Directionality of DNA
 You need to
PO4
nucleotide
number the
carbons!

it matters!
N base
5 CH2
This will be
IMPORTANT!!
O
4
3
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1
ribose
OH
2
The DNA backbone
 Made of phosphates and
deoxyribose sugars
5
PO4
base
5 CH2
O
 Phosphate on 5’ carbon
attaches to 3’ carbon of
next nucleotide
4
1
C
3
O
–O P O
O
5 CH2
2
base
O
4
1
2
3
OH
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3
Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic
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material.”
Watson & Crick
Anti-parallel strands
 Nucleotides in DNA
backbone are bonded from
phosphate to sugar
between 3 & 5 carbons
5
3
3
5
DNA molecule has
“direction”
 complementary strand runs
in opposite direction

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Bonding in DNA
5
hydrogen
bonds
3
covalent
phosphodiester
bonds
3
5
….strong or weak bonds?
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Biology
How
do the bonds fit the mechanism for copying DNA?
Base pairing in DNA
 Purines
adenine (A)
 guanine (G)

 Pyrimidines
thymine (T)
 cytosine (C)

 Pairing

A:T
 2 bonds

C:G
 3 bonds
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CHARGAFF’s RULES
Erwin Chargaff analyzed DNA from
different organisms and found A = T
G=C
Now know its because:
A always bonds with T
G always bonds with C
A Purine always bonds to a
Pyrimidine
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Chromosome Structure in Prokaryotes
Approximately 5 million base pairs
3,000 genes
Chromosome
E. coli bacterium
Bases on the chromosome
DNA molecule in bacteria
single DOUBLE STRANDED circular loop
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