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DNA
The molecule that carries the genetic information in all living things
Notes: The Structure of DNA
Before the early 1900’s,
scientists did not know the
chemical make-up of genes
Griffith’s Experiment
 Griffith was studying what
caused pneumonia. He
isolated two types of bacteria
from mice (R strain & S strain)
 R Strain- harmless bacteria the
had rough edges
 S Strain- pneumonia causing
bacteria that had smooth
cultures
Bacterial Transformation
Griffith hypothesized that some chemical
compound in the heat killed S-strain was
transferred into the living R-strain. He
called this process transformation
What causes Transformation?
Oswald Avery built on Griffith's experiment
and treated heat-killed bacteria with enzymes
that killed proteins, lipids & RNA and
transformation occurred.
When bacteria was treated with enzyme that
killed DNA, transformation no longer occurred.
Hershey-Chase Experiment Continued
 DNA contains phosphorus &
protein contains sulfur.
 Hershey & Chase used that
information to determine which
macromolecule carried genetic
material.
 When the bacteriophage
injected its genetic material into
the bacteria, phosphorus was
found, indicating that DNA was
transferred, not protein!
DNA is genetic material found in genes
DNA made of nucleotides
Deoxyribonucleic
Acid (DNA) is a
nucleic acid
Nucleic acids are
made of subunits
called nucleotides
Nitrogenous bases
DNA bases have
nitrogen in them
DNA has 4 different
types of bases
1)Adenine (A)
2)Guanine (G)
3)Cytosine (C)
4)Thymine ( T)
Chargaff’s Rule
Biologist Erwin Chargaff found
that the percentage of Adenine
and Thymine in any sample of
DNA was always equivalent. The
same with Guanine & Cytosine.
A=T and G=C known as
Chargaff’s Rule
Rosalind Franklin’s X-ray
Franklin used a
technique called Xray diffraction to
obtain the
arrangement of the
molecules in DNA
The pattern showed
a pattern consistent
with a helix
James Watson & Francis Crick
Watson identified
Rosalind's X-ray picture as
a double helix.
Watson & Crick model
showed that the DNA
strands had to run
opposite of each other.
They were antiparallel.
Hydrogen Bonds & Base Pairing
The strands are
held together by
weak hydrogen
bonds. Bonds can
only for between
Adenine & Thymine
and Guanine &
Cytosine. This is
called base pairing
Exit Ticket
Explain how Chargaff’s rule & Rosalind
Franklin’s X-ray contributed to the
discovery of DNA’s structure.
DNA
The molecule that carries the genetic information in all living things
DNA Replication
Before the cell is
able to divide, the
DNA has to be
copied. This process
is called replication.
This happens during
S-phase in the cell
cycle.
DNA Replication Process
The hydrogen bonds
between base pairs are
broken by an enzyme-DNA Helicase.
 Two template strands will
be created.
DNA Replication Process
Free-Floating nucleotides
will be added to the
template strand.
DNA polymerase joins
nucleotides to the
template strand to create
the complementary strand
DNA Replication Process
DNA Polymerase will also
proofreads each strand and
check for any errors.
At the end of DNA Replication,
two semiconservative strands are
created.
Telomeres
Repetitive sequences
added
Prevents the cells from
losing genes.
DNA Replication in Prokaryotes
Prokaryotes DNA is
circular.
Replication has one
starting point and it
replicates in both
directions until it is
complete.
DNA Replication in Eukaryotes
Replication can begin
in hundreds of places on
the DNA molecule in
Eukaryotes
 The strand will replicate
in both directions until
replication is complete.
Exit Ticket
Describe the function of each
enzyme during DNA replication
1) DNA Helicase
2) DNA Polymerase