Chapter 13
Section 2:
DNA Replication
Opening Activity
DNA is considered to be a relatively stable molecule.
What gives it this stability, even though the hydrogen
bonds between the nitrogen bases are easily broken?
Write out your answer, then share and compare your
ideas with other students.
Content Objectives
I will be able to identify:
• How DNA replicates, or makes a copy of itself.
• What the roles of proteins are in DNA replication.
• How DNA replication is different in prokaryotes and
eukaryotes.
Chapter 13 Section 2:
DNA Replication
Key Vocabulary
Terms
Adapted from Holt Biology 2008
DNA Replication
The process of making a copy of DNA
DNA Helicase
An enzyme that unwinds the DNA double
helix during DNA replication
DNA Polymerase
An enzyme that
catalyzes the
formation of the DNA
molecule.
Replication Forks
A replication fork is the
mechanism by which a
strand of DNA is
synthesized. Unzip the
DNA and it looks like a
fork, ie fork in road, not
eating fork.
Chapter 13 Section 2:
DNA Replication
Supplementary Words
Adapted from Holt Biology 2008
Histones
Remember:
Long molecules
of DNA are
tightly wound
around
proteins called
histones
Replication “bubbles”
By starting replication at many sites along the
chromosome. Two distinct replication forks
form at each start site, and replication occurs in
opposite directions. This process forms
replication “bubbles” along the DNA molecule.
Chapter 13
Section 2:
DNA Replication
Notes
Adapted from Holt Biology 2008
Your Turn Activity
DNA Replication
Because DNA is made of two strands of
complementary base pairs, if the strands are separated
then each strand can serve as a pattern to make a new
complementary strand.
DNA Replication
The process of
making a copy of
DNA is called DNA
replication.
DNA Replication
In DNA replication, the
DNA molecule
unwinds, and the two
sides split. Then, new
bases are added to
each side until two
identical sequences
result.
DNA Replication, continued
As the double helix unwinds, the two
complementary strands of DNA separate from
each other and form Y shapes. These Y-shaped
areas are called replication forks.
DNA Replication, continued
At the replication fork, new nucleotides are
added to each side and new base pairs are
formed according to the base-pairing rules.
DNA Replication, continued
Each double-stranded DNA helix is made of one
new strand of DNA and one original strand of
DNA.
Three steps in replication:
1. Unwinding and separating the DNA
strands
2. Adding complimentary bases
3 Formation of two identical molecules
Replication Proteins
The replication of
DNA involves many
proteins that form a
machinelike complex
of moving parts. Each
protein has a specific
function.
Replication Proteins
Proteins called DNA
helicases unwind the
DNA double helix during
DNA replication. These
proteins wedge
themselves between the
two strands of the double
helix and break the
hydrogen bonds between
the base pairs.
Replication Proteins
Proteins called DNA
polymerases catalyze
the formation of the
DNA molecule by
moving along each
strand and adding
nucleotides that pair
with each base.
Replication Proteins, continued
DNA polymerases also
have a “proofreading”
function.
During DNA replication,
errors sometime occur
and the wrong
nucleotide is added to
the new strand.
Replication Proteins, continued
If a mismatch occurs,
the DNA polymerase
can backtrack, remove
the incorrect
nucleotide, and
replace it with the
correct one.
Function of enzymes (proteins) of
replication
DNA Helicase –
unwinds the helix
Function of enzymes (proteins) of
replication
DNA polymerase:
add new
nucleotides to the
open DNA strand
proofread to
prevent errors
Review Questions
1.
What is the first step in DNA
replication?
–Which enzyme performs this step?
Unwinding and separating DNA
strands
DNA Helicase
Review Questions
2. When in the cell cycle does DNA
replication occur?
Interphase (S)
Review Questions
3. What would happen if DNA
polymerase didn’t correct errors?
It would change the DNA code
causing mutations or a change
in the function of the cell.
Prokaryotic and Eukaryotic
Replication
Prokaryotic cells usually
have a single chromosome
which is a closed loop
attached to the inner cell
membrane.
Replication in prokaryotes
begins at one place along
the loop. This site is called
the origin of replication.
Eukaryotes and prokaryotes replicate
their chromosomes differently.
Eukaryotic cells often have
several chromosomes which are
linear and contain both DNA and
protein.
In eukaryotic cells, replication
starts at many sites along the
chromosome.
This process allows eukaryotic
cells to replicate their DNA faster
than prokaryotes.
Prokaryotic and Eukaryotic Replication
Click to animate
Prokaryotic and Eukaryotic Replication
Two distinct replication
forks form at each start
site, and replication occurs
in opposite directions.
This process forms
replication “bubbles” along
the DNA molecule.
Replication bubbles
continue to get larger as
more of the DNA is copied.
The smallest eukaryotic chromosomes are often 10
times the size of a prokaryotic chromosome.
Eukaryotic chromosomes are so long that it would
take 33 days to replicate a typical human
chromosome if there were only one origin of
replication.
Some prokaryotes replicate their DNA and form two
new cells in 20 minutes.
Prokaryotic and Eukaryotic Replication
Human chromosomes are replicated in about
100 sections that are 100,000 nucleotides long,
each section with its own starting point.
Because eukaryotic cells have multiple
replication forks working at the same time, an
entire human chromosome can be replicated in
about 8 hours.
Summary
• In DNA replication, the DNA molecule unwinds, and
the two sides split. Then, new bases are added to
each side until two identical sequences result.
• The replication of DNA involves many proteins that
form a machinelike complex of moving parts.
• In prokaryotic cells, replication starts at a single site.
In eukaryotic cells, replication starts at many sites
along the chromosome.