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.
Adapted from Holt Biology 2008
Chapter 13 Section 2: DNA Replication
Key Vocabulary Terms
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.
Adapted from Holt Biology 2008
Chapter 13 Section 2: DNA Replication
Supplementary Words
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.
Adapted from Holt Biology 2008
Chapter 13 Section 2:
DNA Replication
Notes
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.
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
Function of enzymes (proteins) of
replication
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.
