Chapter 8
Section 8.2: The Structure of DNA
1
Objectives
• SWBAT describe the interaction o the four
nucleotides that make up DNA.
• SWBAT describe the three-dimensional
structure of DNA.
2
Vocabulary – Section 8.2
• Nucleotide
• Double helix
• What are the four nitrogen-containing bases
of DNA?
– How are they paired up inside of the DNA double
helix?
• Review: Covalent bond
• Review: Hydrogen bond
3
James Watson
• TED Talk by James Watson about his
contribution to the discovery of DNA’s
structure.
4
The Structure of DNA
• DNA is a very long polymer.
phosphate group
– It is made up of monomers
called nucleotides.
– Each nucleotide has three
parts:
1. A phosphate group (one
phosphorus with four oxygens)
2. A ring-shaped sugar called
deoxyribose.
3. A nitrogen-containing base (a
single or double ring built around
nitrogen and carbon atoms
(these form the genes).
nitrogen-containing
base
deoxyribose (sugar)
5
The Structure of DNA
• One molecule of human
DNA (one chromosome)
contains billions of
nucleotides.
– There are, however, only
four types of nucleotides.
– The 4 nucleotides differ
only in their nitrogencontaining bases.
6
The Four Nitrogen Containing Bases
• The bases are Adenine (A) and Thymine (T),
and Guanine (G) and Cytosine (C).
– Adenine and guanine (A and G) have a larger
double-ring structure.
– Cytosine and Thymine (C and T) have a smaller
single-ring structure.
7
The 4 Nitrogen-Containing Bases of DNA
8
Chargaff’s Rule
• Until Edwin Chargaff, many scientists
hypothesized that DNA was made up of the
four nucleotides in equal amounts.
– They believed that DNA was exactly the same in
all organisms and that DNA was not the genetic
material (and thus could not be responsible for
all of the variation seen among all organisms).
9
Chargaff’s Rule
• In 1950, Chargaff analyzed the DNA of several
different organisms.
– Chargaff found that the same four bases are
found in all organisms but the proportions of the
bases was unique to each organism.
– He found that the amount of adenine equals the
amount of thymine; and that the amount of
cytosine equals the amount of guanine.
10
Watson, Crick, and the Double Helix
• As in all science, James Watson (a geneticist from
the US) and Francis Crick (a British physicist)
based their work on the previous work of many
other scientists (this is how science works).
– Others included:
• Erwin Chargaff
• Rosalind Franklin (whose x-rays were key data in the building
of the Watson/Crick DNA model).
• Alfred Hershey and Martha Chase (working with
bacteriophages).
• Linus Pauling (noticed that some proteins were a helix)
• And Maurice Wilkins – shared the Nobel Prize with Watson
and Crick.
11
X-Ray Evidence
• Franklin and Wilkins used x-ray crystallography
to study DNA.
– When hit with x-rays, DNA diffracts the x-rays in a
pattern that can be captured on film.
– Franklin’s x-ray photographs showed an X
surrounded by a circle.
– This gave Watson and Crick the idea that DNA was
a double helix.
12
X-Ray Evidence
This is Franklin’s x-ray
photograph with which
Watson and Crick
worked. They created
wood and metal models
to figure out the DNA’s
structure.
13
Double Helix Model
• Watson and Crick placed the sugar-phosphate
backbones on the outside and the bases inside
(the rungs).
• The reason DNA is a helix is because A and G are
about twice the width of C and T. This causes
the twisting.
• A and G are double ring Purines while C and T are
single ring Pyrimidines.
• When they put A and T together and C and G
together, the helix structure came together in
the double helix model.
14
Double Helix Model
Note the role of hydrogen and
covalent bonds to hold the molecule
together.
15
DNA Structure
• Alternating sugars and phosphates form the
sides of a double helix.
• The nucleotides of a single strand are held
together by covalent bonds.
• The DNA double helix is held together by
hydrogen bonds between the bases in the
middle (the bases are the rungs that hold the
two single strands together).
16
Base Pairing Rules
• Thymine (T)
always pairs
with Adenine (A)
• Cytosine (C)
always pairs
with Guanine
(G).
17
Base Pairing Rules
• The pairings are centered on the sizes of the
bases, note the double versus single rings, and
the ability of the bases to form hydrogen
bonds with each other.
• Of note, A and T form two hydrogen bonds
while C and G form three.
– Which do you think will be stronger and more
difficult (relatively speaking) to break?
18
Question
• If a sequence of bases on one DNA strand is
CTGCTA then what would the sequence be on
the corresponding, other, strand?
19