DNA Structure and Function
Chapter 13
Rosalind Franklin:
A Female Scientist Ahead of her
Time
• When we talk about Watson and Crick,
Rosalind Franklin provided crucial X-ray
diffraction photos that allowed Watson
and Crick to develop the DNA model.
DNA X-Ray Diffraction Photo by
Rosalind Franklin
Rosalind Franklin’s Work
• Was an expert in x-ray crystallography
• Used this technique to examine DNA
fibers
• Concluded that DNA was some sort of
helix
DNA Structure
Mystery of the
Hereditary Material
• Originally believed to be an unknown
class of proteins
• Thinking was
– Heritable traits are diverse
– Molecules encoding traits must be diverse
– Proteins are made of 20 amino acids and
are structurally diverse
Structure of the
Hereditary Material
• Experiments in the 1950s showed that DNA is the
hereditary material
• Scientists raced to determine the structure of DNA
• 1953 - Watson and Crick proposed that DNA is a
double helix
Watson and Crick
Structure of Nucleotides
in DNA
• Each nucleotide consists of
– Deoxyribose sugar (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases
– Adenine, Guanine, Thymine, Cytosine
Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen bonds
between bases
• A binds with T and C with G
• Molecule is a double helix
Watson-Crick
Model
The DNA Molecule
Nucleotide
Single strand
Double-stranded DNA
Nucleotide Bases
sugar
(d e o x yrib o s e )
a d e n in e
A
b a s e w ith a
d o u b le -rin g
s tru c tu re
g u a n in e
(G )
b a s e w ith a
d o u b le -rin g
s tru c tu re
th ym in e
(T )
b a s e w ith a
s in g le -rin g
s tru c tu re
c yto s in e
(C )
b a s e w ith a
s in g le -rin g
s tru c tu re
The DNA Code
•
The information in DNA is encoded in the
sequence of bases
•
A-T (double hydrogen bonded)
•
G-C (triple hydrogen bonded)
A
C
•
(adenine)
(cytosine)
T
G
(thymine)
(guanine)
The DNA sequence in a gene specifies which
amino acids go into making a particular
protein, and in what order
Base Pairing
•
The bases on opposite strands pair up with
each other in very specific ways
•
A large, two-ring base must always pair with a
smaller, one-ring base
•
For this reason alone, A could only pair with
either T
•
Hydrogen-bond compatability restricts pairing
to A with T, and C with G, always
DNA Strands are Complementary
•
If one strand has an “A” at one spot, the other
strand must have a “T” at the same spot
•
Likewise, “C” is matched with “G”
•
This means that if we have one complete
strand, we can predict the sequence of the
other strand
Semiconservative DNA Copying
•
When DNA is
copied, each old
strand functions as
a template to
construct a new,
complementary
strand
•
Each resulting
DNA double-helix
is half old and half
new material
Strands Run in Opposite
Directions (5’ to 3’ direction)
Strands Run in Opposite Directions
Direction of a strand is defined by orientation of the
5’ and 3’ carbon atoms in the sugars in its backbone
Strands Run in Opposite Directions
•
The two strands of DNA run past each other
in opposite directions
•
DNA polymerase always assembles strand
in a particular direction, from the 5’ carbon
toward the 3’ carbon on its nucleotides
Enzymes in Replication
• Enzymes unwind the two strands
• DNA polymerase attaches
complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
Okazaki Fragments
http://www.steve.gb.com/images/science/okazaki_fragments.png
•
The leading strand is
continuously assembled
•
The lagging strand must
be assembled backward
(relative to movement of
the replication fork)
•
DNA Polymerases move
backwards along
lagging strand, making
many short segments,
later attached together
Genes and DNA
•
DNA gives directions for making proteins
•
Differences in the DNA sequence are the basis
of different alleles for genes
•
Sequences may be longer or shorter between
different alleles, or may otherwise differ
•
DNA can be chemically chopped into fragments,
that are then separated by size
•
Different alleles may be cut into different size
fragments, which are then sorted by size
Electrophoresis Separates DNA Fragments
•
DNA found at a crime
scene, as well as DNA
from seven suspects,
is cut into fragments
with special enzymes
•
Electrophoresis
separates DNA
fragments by size,
producing unique
pattern
•
Who did it?
Composition of DNA
• Chargaff showed:
– Amount of adenine relative to guanine
differs among species
– Amount of adenine always equals amount
of thymine and amount of guanine always
equals amount of cytosine
A=T and G=C
Patterns of Base Pairing
DNA Structure Helps
Explain How It Duplicates
• DNA is two nucleotide strands held
together by hydrogen bonds
• Hydrogen bonds between two strands
are easily broken
• Each single strand then serves as
template for new strand
DNA
Replication
• Each parent
strand remains
intact
• Every DNA
molecule is half
“old” and half
“new”
new
old
old
new
Base Pairing
during
Replication
Each old strand
serves as the template
for complementary
new strand
Continuous and Discontinuous
Assembly
Strands can
only be
assembled in
the 5’ to 3’
direction
DNA Repair
• Mistakes can occur during replication
• DNA polymerase can read correct
sequence from complementary strand
and, together with DNA ligase, can
repair mistakes in incorrect strand
What about Cloning?
Moral Dilemma or Technological
Revolution
• Making a genetically identical copy of
an individual
• Researchers have been creating clones
for decades
• These clones were created by embryo
splitting
Dolly:
Cloned from an Adult Cell
• Showed that differentiated cells could
be used to create clones
• Sheep udder cell was combined with
enucleated egg cell
• Dolly is genetically identical to the
sheep that donated the udder cell
More Clones
•
•
•
•
•
Mice
Cows
Pigs
Goats
Guar (endangered species)