DNA
Replication
Transcription and Translation
Genes are Made of DNA
• Scientist discovered that DNA
was a chemical inside the
nucleus
Oswald Avery
1944
Frederick Griffith
1928
Genes are Made of DNA
• Further experiments by:
o Alfred Hershey
o Martha Chase
• Worked with viruses to support Avery’s work
Genes are Made of DNA
Virus – a package of nucleic acid wrapped in protein
• Not made of cells
• They can only reproduce by infecting a living cell
Bacteriophage – a virus that infects bacteria
The Double Helix
• At more or less the same time scientist:
o James Watson
o Francis Crick
• Using Franklin’s X-ray crystalography to provide clues
to their discovery
Double Helix – Model created by Watson and
Crick; in which two strands of
nucleotides wound about each
other
• Sugar phosphate backbones on the outside;
Nitrogenous base on the inside.
Remember the monomer of DNA is a
nucleotide
What is DNA and RNA??
• DNA and RNA are a class of macromolecules called
Nucleic Acids
• DNA (Deoxyribonucleic Acid)
o where genetic information is stored
o Composed of
1. Deoxyribose sugar
2. Phosphate group
3. Nucleotide base
o Double stranded
• RNA (Ribonucleic Acid)- involved in protein synthesis
o Composed of the sugar ribose, a phosphate group,
and nitrogenous base
o Has the nitrogen base uracil in place of thiamine
DNA - Nucleotides
• Each DNA and RNA monomer has three parts:
DNA - Nucleotides
Nitrogenous Base – 4 nucleotides found in DNA
differ only in their nitrogenous
bases
Pyrimidines: single ring structure
• Thymine (T)
• Cytosine (C)
Purines: double ring structure
• Adenine (A)
• Guanine (G)
DNA structure and
bonding
• DNA is composed of
o the sugar-phosphate
“backbone”
o The nitrogenous bases
“rungs”
• The sugar-phosphate
molecules are joined by
covalent bonds
• The nitrogen bases are
joined by hydrogen bonds
Complementary Base Pairs
Pyrimidines
always pair
with purines
Base Pairing Rules
A with T
C with G
• AT CG (Coral Gables)
Chargaff’s rule
If….
• Adenine (A) only pairs with Thiamine (T)
• Guanine (G) only pairs with Cytosine (C)
Then for every DNA molecule,
• the amount of A=T
• The amount of C=G
• C% + G% = 100%- (A% + T%)
Practice Problem
A molecule of DNA is composed of 20%
Adenine.
• What % is Thiamine?
• What % is Guanine?
• What percent is Cytosine?
• A= 20%
• A=T; T=20%
• A+T= 40%
• (C% + G%) = 100% - 40%= 60%
• C = G, then
o C = ½(60%)=30%
o G = ½(60%)=30%
You Practice
• Given the following problems, figure out the
% of the rest of the nucleotides
1. 10 % guanine
2. 15% thiamine
3. 30% cytosine
You Do: Draw a DNA
Molecule
• Draw a molecule of DNA
• Make sure to color code
o Phosphate Group
o Deoxyribose sugar
o Nitrogenous Bases
•
•
•
•
Adenine
Guanine
Thymine
Cytosine
• Show which bonds are hydrogen bonds and
which are covalent bonds
DNA Replication
• When a cell divides; a complete set of genetic instructions
are copied for each cell
• Genetic material uses the template principle to make
more DNA
• Watson and Crick’s model for DNA allowed for the
hypothesis of specific base pairing
o A–T
oC-G
o If you know one strand of DNA you can determine
the sequence on the other
What is the essential vocabulary?

Replication – the process of DNA duplication.

Replication fork – Site where DNA strands
separate and replication occurs.

Enzyme – a protein that
facilitates a specific chemical
reaction in the body.

Leading strand – DNA strand
that forms as a continuous
strand during DNA replication.

Lagging strand – DNA strand that forms as a series
of short segments, called Okazaki fragments,
which are then connected together.
What is DNA replication?
 During DNA replication, the
DNA molecule produces two
new complementary strands.
 Each strand of DNA serves as
a template for the new
strand.
 Described as
semiconservative because
each DNA molecule has
one new strand and one
original strand.
 New nucleotides are always
added in the 3’ to 5’ direction
How does DNA replication work?
 A very complex process requiring the assistance of
several enzymes and regulatory molecules.
 DNA helicases unwind parent strand.
 Primase initiates replication by adding a short RNA
fragment called a primer to the old strands
 DNA polymerases join individual nucleotides to
produce a new strand of DNA using the old strand as
a template. They also carry out DNA proofreading
and repair.
 Ligase binds the DNA fragments together by addition
of phosphates in the gaps that remain in the
phosphate-ribose sugar backbone.
DNA Replication Overview
A
T
C
G
T
A
A
T
G
C
(a) Parent molecule
 Each strand of the DNA
double helix has all the
information needed to
reconstruct the other half
by the mechanism of
base pairing.
 Because each strand
can be used to make the
other strand, they are
said to be
complementary.
DNA Replication Overview
A
T
A
T
C
G
C
G
T
A
T
A
A
T
A
T
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
1. DNA helicase begins
replication by
separating the strands
and exposing the
nucleotide sequence.
2. Primase adds a primer
to the DNA molecule to
initiate replication
3. DNA polimerases
move along the two
strands, pairing
complementary bases
to the exposed
nucleotides and
proofreads the added
segments
DNA Replication Overview
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
T
A
T
A
T
A
T
A
A
T
A
T
A
T
A
T
G
C
G
C
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
(c) “Daughter” DNA molecules,
each consisting of one
parental strand and one
new strand
4. DNA ligase seals up the fragments into one long continuous
strand
5. Each original strand joins its complementary strand to form a
DNA molecule, resulting in two identical DNA molecules.
DNA Replication Overview