Chapter 12

Chapter 13
13-1 and 13-2 DNA History
Structure and Replication
The Experiment
The disease causing kills
the mice.
When injected separately,
neither the heat-killed
disease-causing bacteria
nor the live, harmless
bacteria killed the mice.
When the 2 types were
injected together, the mice
Griffith’s Results
Some “factor” from the dead bacteria had
transformed the harmless bacteria into
disease-causing ones.
The Conclusion
 From this, biologists inferred that genetic
information could be transformed from
one bacteria to another.
 1944
The History of DNA
Oswald Avery
 discovered
DNA is the nucleic
acid that stores and
transmits genetic
The History of DNA
1950’s Erwin Chargaff
The amount of adenine
in DNA always equals the
amount of thymine and
the amount of cytosine
always equals the
amount of guanine.
At this point, though, no
one knew why
A = T and C = G.
The History of DNA
1952 Alfred Hershey and Martha Chase
determined that DNA (not proteins) is the
hereditary material of living organisms.
The History of DNA
1950’s Rosalind Franklin
crystallized DNA and was able to
see the shape of DNA by taking an
x-ray of the crystals.
This technique is called x-ray
 1953 James Watson
and Francis Crick
Based on Franklin’s xrays, determined the
exact shape of the
DNA molecule.
They described it as a
double helix, in which
two strands were
wound around each
(A twisted ladder)
DNA: (Deoxyribonucleic Acid)
DNA is an example of a nucleic acid. The
basic units that make up nucleic acids are
called nucleotides. What are nucleotides?
DNA: (Deoxyribonucleic Acid)
I. Nucleotides
A. Nucleotides of DNA are made up of three materials:
1. a five-carbon sugar called deoxyribose
2. a phosphate group
3. A nitrogen base
B. Example of one nucleotide:
If nucleotides are made up of the same three
components, then what makes them different?
DNA: (Deoxyribonucleic Acid)
C. There are four types of nitrogen bases:
1. Adenine (A)
2. Thymine (T)
3. Guanine (G)
4. Cytosine (C)
D. A and G are double-ring bases called purines.
E. C and T are smaller, single-ring bases called
So there are four possible nucleotides, each
containing one of these four bases.
F. Nucleotides combine to form long
chains, which combine to form one large
G. The two chains are joined together by
hydrogen bonds between nitrogen bases.
II. DNA Structure
A. Watson and Crick discovered hydrogen
bonds can only form between certain base
1. A can only hydrogen bond with T
2. G can only hydrogen bond with C
B. This principle, called base pairing,
explained the findings of Chargaff years
earlier. For every A there has to be a T
and for every C there has to be a G.
DNA Structure
C. Double helix
(a.k.a. twisted ladder)
 1. It has 2 main sides or
strands. (The sides are
like the upright parts of
a ladder.)
 2. Hydrogen bonds
between the nitrogen
bases connect the two
strands together. (The
nitrogen bases form the
rungs of the ladder)
The Importance of DNA
D. DNA is the molecule
that makes up genes and
determines the traits of
all living things.
E. Genes are made up of
short pieces of DNA with
a certain number of
nitrogen bases.
1. The number and order
of nitrogen bases is what
leads to different traits in
all living things.
13-2 DNA Replication
The double helical
structure of DNA also
explains how DNA
replicates or copies itself.
Each strand of DNA has all
the information needed to
reconstruct the other half
by the rules of base
DNA Replication
It takes about six hours to produce a
complete copy of the genetic code.
This process takes place before prophase
of mitosis.
It occurs during interphase.
DNA Replication
Every time a cell
reproduces, DNA is
copied. This is a process
called replication.
1. The hydrogen bonds
connecting the nitrogen
bases are broken by
enzymes (DNA helicase)
and the 2 strands of the
DNA molecule unwind.
This allows the DNA to
become “unzipped”.
DNA Replication
2. The exposed strands contain the
template of nucleotides that are to be
copied to create an exact replica of the
original DNA. Each strand serves as a
template to make a new DNA molecule.
The DNA molecule unzips
in both directions.
DNA Replication
3. Loose nitrogen bases are present in the
nucleus of the cell. The free nitrogen
bases join with their complimentary
nitrogen bases that are exposed on the
open strand of DNA.
new strand
DNA polymerase
DNA Replication
4. This process continues until the entire
DNA molecule has been unzipped and
replicated. The enzyme DNA polymerase
“proofreads” to make sure that there are
no mistakes.
DNA Replication
5. Replication is complete. Two new DNA
molecules have formed. Each one is
identical to the original and to each other.
original strand
new strand
Two molecules of DNA
When do we need to make a copy of
When cells divide (so each cell will have a
copy of the organism’s DNA to carry out
life functions).