PAP Biology
DNA
What are nucleic acids?
Why is DNA important?
Structure of DNA
Scientists
Review
• What are the two types of nucleic acids?
DNA
RNA
_________________
& _______________
• Is a nucleic acid a monomer or polymer?
POLYMER
__________________________
• What is the monomer of DNA and RNA?
nucleotide
_________________________
Understanding DNA video
(6 min)
Summary of Scientists
• Griffith—
• discovered genetic transformation of bacteria
• Avery—
• discovered DNA is genetic material
• Hershey &
Chase—
• confirmed DNA is genetic material
• Chargaff— • discovered nitrogen base pairing rules
• Wilkins &
Franklin—
• developed X-ray pictures of DNA
• Watson &
Crick—
• discovered shape/structure of DNA
DNA The Master Molecule video
(stop at 5:50)
DNA Location
• DNA stands for deoxyribonucleic acid
Location:
• DNA is found in the nucleus of eukaryotic
organisms
– like plants, animals, protists and fungi
• In prokaryotic organisms (like bacteria) that do not
have a nucleus, DNA can be found in a single
circular chromosome within the cell
DNA Function
Function: DNA stores and
carries the information that
is passed from one
organism to its offspring
(from parent to child)
What is DNA?
• The information is coded in the order of
the nucleotide bases (the monomers)
• The monomer of DNA is made of:
– Phosphate group
– 5 carbon sugar (deoxyribose)
– Nitrogen containing base
These three
components
make up the
monomer
called a
nucleotide
Nucleotide
Why is DNA Important?
• DNA is in all living things
• DNA is the informational code that
makes each organism unique.
The Structure of DNA
• double helix- two strands of
nucleotides twisted around each
other, like a winding staircase
• there is a “right side up” strand
and an “upside down” strand.
• this arrangement of 2 strands
running side by side but in
opposite directions is described as
antiparallel
The Structure of DNA
Nucleotides- subunits (monomers) that make up
DNA composed of:
– Phosphate group
– 5- carbon sugar (deoxyribose in DNA)
– Nitrogen containing base:
• The bases pair in the same way in every DNA molecule,
we call this complementary base pairing
• Adenine
• Guanine
Thymine
Cytosine
(A – T)
(G – C)
* Weak hydrogen bonds hold bases together
Base Pairs
• We refer to adenine, thymine, cytosine
and guanine as the nitrogen bases or just
“bases”
• So, when you say “a sugar, a phosphate
and a base” you are implying that the
base is either adenine, thymine, guanine
or cytosine
Base Pairs
The order of the base pairs provides that “unique code” that
makes us all different.
• GTAACTTAG does not contain the same information as
GGACAATTT even though both sequences contain the same
nucleotides
• However, if the nucleotide sequences of nitrogenous bases
(adenine, guanine, cytosine, thymine) are in a similar order, the
two organisms are closely related
– This concept is used in forensic science and is called DNA
fingerprinting
Label complementary
bases and draw the
appropriate number
of hydrogen bonds 
A
T
C
G
A
T
T
A
G
C
T
A
A
T
C
G
The Structure of DNA
*Note about bases:
– Pyrimidines- single ring of carbon
• Thymine and Cytosine
– Purine- two rings of carbon
• Adenine and Guanine
A purine must always hydrogen bond with a
pyrimidine!
P
T
H
A
S
T
H
Label:
-Phosphate (P)
A
S
S
G
H
S
S
H
A
- Correct base pairs
(A, T, C, G)
P
C
P
-5 carbon sugar (S)
P
P
S
P
P
3’
T
S
-Label all hydrogen
S
bonds with an H
P
P
G
S
H
C
-Label the 5’ and 3’
ends
S
P
T
S
H
A
Circle a nucleotide
S
P
P
5’
DNA Review
• What does DNA store that is important?
Genetic information in its sequence of nitrogen bases
• If a DNA strand read 3’ AGT-CCG-GTA 5’ what
would the complementary strand read?
5’ TCA-GGC-CAT 3’
• What holds the nitrogen bases together?
Hydrogen bonds
• What are the 3 components of a nucleotide?
Sugar, phosphate group, and nitrogen base
DNA Replication
• DNA replication- the process of making a copy of
DNA
1 DNA molecule  2 identical DNA molecules
• Why is this important?
– cells need to copy their DNA before the cells divide in
mitosis (growth, repair, and maintenance)
• Where does DNA replication occur?
– in the nucleus (because DNA is found in the nucleus
in eukaryotic cells)
DNA Replication
Summary of what we will learn
There are three steps to DNA replication:
1) helicase unwinds the double helix opens the strand
by breaking the hydrogen bonds between the bases
2) DNA polymerases add nucleotides in a 5’ to 3’
direction, moving TOWARD the replication fork; The
leading strand is built quickly and the lagging strand
is built more slowly in short 5’ to 3’ segments.
3) DNA polymerases detach once replication is complete
and you end with 2 identical molecules of DNA,
each made of one old strand and one new strand
DNA Replication
Step 1:
the double helix must unwind before
replication can begin.
then the DNA “unzips”
DNA helicase is an enzyme that opens the
double helix by breaking the hydrogen bonds
that hold the complementary bases together
(think of a zipper)
enzymes end in (-ase)
DNA Replication
Step 1 con’t:
once separated,
additional proteins hold
the strands apart
the place where the
double helix splits is
called the replication
fork (it’s Y-shaped)
DNA Replication
Step 2:
 at the replication fork, enzymes called DNA
polymerases move along each of the DNA strands
 DNA polymerases add nucleotides to the 3’ end
exposed bases according to the base pairing rules
DNA Replication
• The overall direction of adding nucleotides
is toward the replication fork
– goes from 5’ to 3’
• One strand, called the leading strand, grows
quickly in the 5’ – 3’ direction toward the
replication fork.
• The other strand, called the lagging strandis
created next to the 3’5’ strand.
– It must add nucleotides in short 5’3’ segments
(called Okasaki fragments), but goes slowly since it
has to wait for it to unzip then fill in backwards a little
section at a time making it “grow” in the wrong
direction (away from the replication fork).
– These fragments are joined by ligase.
– Ultimately the overall direction of “growth” is toward
the replication fork, but must be done in small
segments filling in away from the fork.
DNA Replication
Step 3:
step 2 continues until all DNA
has been copied and the
polymerases detach
produces 2 DNA molecules
that are identical to each
other
DNA Replication
-is semiconservative
-semi – half
-conserve – to save
DNA Replication
• Semiconservative:
– This means that every double stranded molecule
of DNA has one strand that is from the “old” DNA
and one strand is “new”
Summary
• DNA unwinds & unzips using helicase
• DNA polymerases add nucleotides to the 3’ end
of the strands, moving toward the replication
fork (5’  3’)
• 2 new strands of DNA are made that are
identical to the original strands and to each
other
• The process is semiconservative