Chapter 13
DNA Structure and Function
DNA Scientist
• Johann Friedrich Miescher discovered
nucleic acid in 1868
• Linus Pauling discovered the helical
structure of proteins in 1951
• 1953 Watson and Crick discovered the
structure of the master molecule of life–
DNA
Fred Griffith
• 1928 working with the pneumonia-causing
bacterium (S strain - pathogenic and R strain –
nonpathogenic)
– Found that nucleic acid was the causes the person to
be sick
– Performed four experiments:
• Injected mice with R Cells – mice lived
• Injected mice with S Cells – mice died
• S Cells were heat-killed then injected then into mice; live S
cells were found in the blood
– Griffith explanation – heated S strain did not have
hereditary information it was destroyed
– Transformation: process of changing the genetic
material from one source to another
Conclusion
• Living bacteria acquired genetic
information from dead bacteria particularly the instructions for making
capsules, thus transforming the naked
bacteria into incapsulated bacteria.
• The Transforming agent was discovered to
be DNA. DNA was isolated and added to
live naked bacteria, and they were
transformed into the incapsulated kind.
Oswald Avery
• Took Griffith experiment one step further
and found that both proteins and nucleic
acid was DNA
Hershey and Chase
• Used the bacteriophage (a virus) to show that
viruses are composed of DNA or RNA
• 1. Hershey and Chase forced one population of phages to
synthesize DNA using radioactive phosphorous.
• 2. The radioactive phosphorous "labeled" the DNA.
• 3. They forced another group of phages to synthesize protein
using radioactive sulfur.
• 4. The radioactive sulfur "labeled" the protein.
• 5. Bacteria infected by phages containing radioactive protein
did not show any radioactivity.
• 6. Bacteria infected by phages containing radioactive DNA
became radioactive.
• 7. This showed that it was the DNA, not the protein, that was
the molecule of heredity.
Experiment A
Experiment B
DNA contains
phosphorous but
not sulfur.
Proteins contain
sulfur but not
phosphorus
DNA Structure
• DNA is composed of four kinds of
nucleotides which consist of:
– Five carbon sugar– deoxyribose
– Phosphate group
– One of four bases: adenine (A), guanine (G),
Thymine (T) and Cytosine (C)
• Nucleotides are similar, but thymine and
cytosine are single-ring pyrimidines; A and
G are double ring purines
Chargaff
• In 1949 found that the four kinds of
nucleotide bases making up DNA
molecule differ in relative amounts from
species and species
• Adenine =Thymine
• Cytosine=Guanine
Rosalind Franklin
• Used X-ray diffraction techniques to
produce images of DNA molecules
– DNA exist as a long, thin, molecule of uniform
diameter
– Structure is highly reptitive
– DNA is helical (twisted ladder)
Watson and Crick
• Used numerous sources of data to build models of
DNA
• Following features were
– Single-ringed thymine was hydrogen bond with double
ringed adenine and single-ringed cytosine with double
ringed guanine, along the entire length of the molecule
– Backbone was made of chains of sugar-phosphate
linkages
– The molecule was double stranded and looked like a
ladder with a twist to form a double helix
DNA Structure
• The sugar and phosphates make up the
"backbone" of the DNA molecule.
• The phosphate is attached to the 5' carbon
(the 5 is a number given to sugar
molecules). The DNA strand has a free
phosphate on th 5' end, and a free sugar
on the 3' end - these numbers will become
important later.
Continue…
• DNA is composed of subunits called
nucleotides, strung together in a long
chain -- Each nucleotide consists of: a
phosphate, a sugar (deoxyribose), and a
base
• The two sides of the helix are held
together by Hydrogen bonds
Hydrogen
Bond
Nitrogen Bases
DNA is composed of subunits called
nucleotides, strung together in a long chain
-- Each nucleotide consists of: a phosphate,
a sugar (deoxyribose), and a base
Bases come in two types: pyrimidines
(cytosine and thymine) and purines
(guanine and adenine)
DNA Replication and Repair
• Steps of DNA replication
• 1. DNA helicase (enzyme) unwinds the DNA.
The junction between the unwound part and the
open part is called a replication fork.
• 2. DNA polymerase adds the complementary
nucleotides and binds the sugars and
phosphates. DNA polymerase travels from the
3' to the 5' end.
• 3. DNA polymerase adds complementary
nucleotides on the other side of the ladder.
Traveling in the opposite direction.
• 4. One side is the leading strand - it follows the
helicase as it unwinds.
• 5. The other side is the lagging strand - its
moving away from the helicase
• Problem: it reaches the replication fork, but the
helicase is moving in the opposite direction. It
stops, and another polymerase binds farther
down the chain.
• This process creates several fragments, called
Okazaki Fragments, that are bound together by
DNA ligase.
• 6. During replication, there are many
points along the DNA that are synthesized
at the same time (multiple replication
forks). It would take forever to go from one
end to the other, it is more efficient to open
up several points at one time.
• Hyperlink\DNAReplication.swf
Monitoring and Fixing the DNA
• DNA polymerase, DNA ligases and other
enzymes engage in DNA repair
• DNA polymerase “proofread” the new
bases for mismatched pairs, which are
replaced with correct bases