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What is DNA?
• DNA: Deoxyribonucleic Acid
carries genetic information
(genetic code) and controls
cellular functions
• All living things have DNA
that only differs in the
sequence of nucleotides.
- In eukaryotes,
DNA is located
in the nucleus.
- In prokaryotes,
the DNA is a
loop, which is not
contained in a
nucleus but
found in an area
called the
nucleoid region
DNA
History of DNA
Discovery of DNA
- In 1928 Fredrick Griffith did
experiments trying to find a
vaccine for pneumonia; he
discovered that when harmless
bacteria and a virulent
(disease-causing) bacteria were
mixed together, some of the
harmless bacteria became
virulent.
- He called this process
transformation
History of DNA
Transformation: the ability of one organism to
be permanently changed by another through
the exchange of DNA
Discovery of DNA
- In 1944 Oswald Avery repeated
Griffith’s experiments but he used enzymes
enzymes to remove proteins, lipids,
RNA & DNA separately.
- Transformation occurred each time except when
DNA was removed, so DNA must be the
transformation factor.
- DNA is the genetic material that stores and
transfers genetic information from one
generation of an organism to another
Discovery of DNA
- During the 1950’s, most scientists still thought
that proteins carried genes.
- In 1952, Alfred Hershey and Martha Chase did a
series of experiments with bacteriophages &
discovered that DNA is the source of genetic
information that proved Avery’s transformation
principle.
History of DNA
• Bacteriophage: viruses
that attack bacteria,
composed of DNA or RNA
core and a protein coat
• Hershey and Chase
confirmed that DNA was
the genetic molecule of
inheritance in organisms
and not proteins
History of DNA
• When Hersey & Chase used radio active
markers to mark the protein coats of
the bacteriophages, the infected
bacteria didn’t become radio active.
• Repeated with radioactive markers on
the DNA of bacteriophages, the
infected bacteria became radioactive.
Discovery of DNA
- In 1952, Rosalind Franklin
developed special X-ray
diffraction experiments to
study the structure of the
DNA molecule
- These X-ray patterns
showed an X-pattern with
2 strands of DNA twisted
around each other. DNA
resembled the coils of a
spring.
Discovery of DNA
- In 1953, James Watson and
Francis Crick described the
shape and structure of DNA
(with the help of previous Xray experiments from
Franklin)
- Through the construction of
a model, they proposed the
structure of DNA that is
known as double helix.
History of DNA
Nitrogen Bases
• [A] & [G] – belong to a
group of compounds
called purines (double
ring of carbon)
* [T] & [C] – belong to a
group of compounds
called pyrimidines
(single ring of carbon)
• Watson & Crick discovered that it was a
double helix shape, with a sugar phosphate
backbone, connected in the middle with
nitrogen bases.
• T binds with A, & G binds with C: base
paring
Structure
of
DNA
- The monomer of DNA is a nucleotide
- A nucleotide consists of 3 main parts:
1. Phosphate group
2. 5 carbon sugar (deoxyribose)
3. Nitrogen base
- Adenine [A]
- Guanine [G]
- Thymine [T]
- Cytosine [C]
Structure of DNA
- Phosphate and sugar
make up the
backbone or sides of
the DNA molecule
- Nitrogen bases pair
up to form the
center
- These base pairs are
held together by
weak hydrogen bonds
Discovery of DNA
- Erwin Chargaff experimented
with DNA and observed that
in each organism the amount
of the 4 nitrogen bases was
equal. Although, the amount
of DNA was different
between different types of
organisms
- His experiments resulted in
the base pairing rule :
[A] = [T] and [G] = [C]
• Antiparallel
(see how
one side is upside down)
• Phosphate-sugar
backbone linked
with covalent
bonds
• Nitrogen bases
linked with
hydrogen bonds
DNA
- DNA can hold a great deal of
information because it is a very long
molecule.
- Living things are different because the
sequence (order) of the nucleotides is
limitless.
- The more similar the sequence, the
more closely related the organisms.
Chromosomes &
Replication
DNA Length
• Very long
– E. coli: over 4.6 million
nucleotides, but must fit in a
cell 1/1000 in length
– Eukaryotes: varies by species,
but can be 1000 times longer
than prokaryotes
• Humans over 1 m of DNA in each
cell
Chromosome Structure
• Prokaryotes: DNA is folded into a single
loop chromosome
• Eukaryotes: DNA is folded around
proteins (histones) and coiled to form
multiple chromosomes
– Humans have 46 chromosomes in each cell
Replication
• Every cell must contain an identical copy
of DNA so it must be replicated before
new cells form.
– Prokaryotes: starts at 1 “point of origin”
and continues around the loop chromosome
– Eukaryotes: hundreds of points of origin
DNA Replication
- DNA needs to
replicate itself,
which means it
needs to make an
exact copy of itself
to pass along to the
daughter cell during cell division
- 1 DNA molecule -------> 2 identical DNA
molecules
- Occurs in the nucleus prior to cell division (in the
cytoplasm of a prokaryote)
Replication
1. Separate the DNA Stands
– The enzyme helicase breaks the hydrogen
bonds to open the DNA strand like a zipper
DNA Replication
The split where it unzips
is called the
replication fork
DNA Replication
• Replication happens in the 5’---> 3’ direction
• It is semiconservative,
meaning that every doublestranded molecule of DNA has
one strand that is “old” and
one strand that is “new”
• Replication can occur at
hundreds of different
replication forks all at the
same time on the same
molecule
DNA Replication
2. Another enzyme called
DNA polymerase pairs
“free” complimentary
nucleotides found in the
nucleus to each side of
the original unzipped
strand.
New hydrogen bonds
form between the
nitrogen bases.
Replication
Nucleotides are added
– When the enzyme called DNA polymerase adds
nucleotides, it uses the original DNA strand as
a template.
– DNA polymerase can only add in the 5’ to 3’
direction
• Leading strand: (5’ to 3’) continuously adds
nucleotides
• Lagging strand: (3’ to 5’) creates small
sections called Okazaki Fragments as the
DNA opens
– Enzyme DNA ligase joins Okazaki
fragments together
DNA Replication
• This process takes time as the lagging strand
has to wait for the DNA to unzip and then fill in
backwards a little section at a time making it
“grow” in the wrong direction (away from the
replication fork).
• The Okasaki fragments are joined into a single
strand by an enzyme called DNA ligase
• Ultimately, the overall direction of “growth” is
toward the replication fork but it must be done
in small segments that require the lagging strand
to fill in away from the fork
DNA Replication
3. This continues until the
entire strand has been
copied. The results are
two identical strands of
DNA.
Overall Direction of DNA Replication is
Towards the Replication Fork
• One strand, called the leading strand,
grows in the 5’ to 3’ direction towards the
replication fork
DNA Replication
• Other strand called the lagging strand, is
created next to the 3’--->5’ strand
• this lagging strand must add nucleotides
in short 5’ --->3’ away from the
replicating fork in segments called
Okazaki fragments
DNA Replication
DNA Replication
Replication fork opens in both directions 
Gene Regulation
• Not all genes are used in every cell.
Genes can be turned off (silenced)
• In prokaryotic cells groups of genes
that are turned on/off together are
called operons
Gene Regulation
Eukaryotic Cells
• Genes can be turned on by enhancer and
promoter sequences
• Genes can be turned off by repressor
proteins
• Cell Differentiation: cells in multicellular
organisms have to change in order to do
different jobs. All cells have all of the
exact DNA, but only parts of the DNA are
expressed.
DNA Replication
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http://www.youtube.com/watch?v=hfZ8o9D1tus
http://www.youtube.com/watch?v=teV62zrm2P
http://www.youtube.com/watch?v=4jtmOZaIvS0
http://www.johnkyrk.com/DNAreplication.html
http://highered.mcgrawhill.com/sites/0072437316/student_view0/cha
pter14/animations.html#
• http://www.wiley.com/legacy/college/boyer/047
0003790/animations/replication/replication.htm