8.1. Identifying DNA as the Genetic Material
Griffith finds a ‘transforming principle.’
• Griffith experimented with the bacteria that cause
pneumonia.
• He used two forms: the S form (deadly) and the R form (not
deadly).
• Dead S made R deadly
Avery identified DNA as the transforming principle.
• Avery performed three tests on the transforming
principle.
– Showed DNA was present.
– Matched chemical makeup to DNA
– No DNA = no transformation
Hershey and Chase confirm that DNA is the genetic
material.
• Hershey and Chase studied viruses that infect bacteria, or
bacteriophages.
– Used radioactive DNA and proteins
–DNA found in bacteria, proteins not
8.2. Structure of DNA
DNA is composed of four types of nucleotides.
• DNA is made up of a long chain of nucleotides.
• Each nucleotide has three parts.
– a phosphate group
– a deoxyribose sugar
– a nitrogen-containing base
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
• The nitrogen containing bases are the only difference in
the four nucleotides.
Watson and Crick determined the three-dimensional
structure of DNA by building models.
• Double helix
• Sugar-phosphate
backbone
• Nitrogen base “rungs”
 Double Helix is often referred to as a ladder. Why?
Compare the structures. What would make up the sides
of the ladder? What would make up the rungs or steps
of the ladder?
• Watson and Crick’s discovery built on the work of Rosalind
Franklin and Erwin Chargaff.
– Franklin: even width
– Chargaff’s rules stated that A=T and C=G.
Nucleotides always pair in the same way.
• The base-pairing rules show
how nucleotides always pair
up in DNA.
– A pairs with T
– C pairs with G
• Because a pyrimidine
(single ring) pairs with a
purine (double ring), the
helix has a uniform width.
G
C
A T
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
8.3. DNA Replication
Replication copies the genetic information.
• A single strand of DNA serves as a template for a
new strand.
• Directed by base pairing
• Each body cell gets a
complete set of
identical DNA.
Review: During which
phase does the DNA
replicate?
Proteins carry out the process of replication.
• DNA serves only as a template.
• Enzymes and other proteins do all work
1. DNA unwinds; enzymes (DNA helicase) unzip the
double helix.
2. Free-floating nucleotides form hydrogen bonds
with the template strand.
nucleotide
The DNA molecule unzips
in both directions.
3. DNA polymerase enzymes bond the nucleotides
 What kind of bond forms?
4. Polymerase enzymes form covalent bonds between
nucleotides in the new strand.
new strand
nucleotide
DNA polymerase
• Two new molecules of DNA are formed, each with an
original strand and a newly formed strand.
• DNA replication is semiconservative.
original strand
Two molecules of DNA
new strand
Replication is fast and accurate.
• DNA replication starts at many points in eukaryotic
chromosomes.
 Why does replication need to happen at multiple points?
There are many origins of replication in eukaryotic chromosomes.
• DNA polymerases can find and correct errors.
Modeling DNA Replication
1. Below is one side of a DNA strand.
2. First, using the base pairing rules, fill in the
other side of the strand you have a segment of
DNA that is Nitrogen base pairs in length.
T TAA C G G C C C A G
AAT T G C C G G G T C
3. Now that you have a strand of DNA,
replicate the strand of DNA you just created.
Describe each step as you complete it.
4. When completed, how are the new strands
compare to the original stand?
Replicating the strand
• Make the other side of the DNA strand and then use the
steps of replication to make two new molecules of DNA
TTAACGGCCCAG
AAT T G C C G G G T C
T G
T
A
C A
Your two new strands should look like this: one old (in
black) and one new (in red)
T T AA C G G C C C A G
AA T T G C C G G G T C
T T AA C G G C C C A G
AA T T G C C G G G T C