DNA replication in
7 easy steps
1.
Helicase enzyme breaks
hydrogen bonds
between bases,
unzips and unwinds the
helix
A protein that
catalyzes
chemical
reactions.
–Helicase begins to unwind the DNA at
the ORIGIN OF REPLICATION (a
specific DNA nucleotide sequence)
It’s
common to
only show
one strands
sequence of
bases, since
the other
can be
inferred.
Helicase enzymes move in both directions
from the Point of Origin, forming a
REPLICATION BUBBLE.
At either end of the replication bubble is a
REPLICATION FORK, a Y-shaped region
where the new strands of DNA are elongating.
In prokaryotic cel
there is one
origin
In eukaryotic cells
there are 100’s to
1000’s of origins
What does it really look like?
1
2
3
4
BILL: define terms related to step 1
• Origin of replication
• Helicase
• Single strand binding
proteins
• DNA polymerase III
• Enzyme
• 3’ end
• Nucleotides
• Primase
•
•
•
•
•
•
•
•
•
Primer
Deoxyribonoucleotides
Antiparallel
Replication fork
Leading strand
Lagging strand
Okazaki fragment
DNA polymerase I
DNA ligase
2. Single stranded binding proteins hold the DNA
strands apart
– Keeps the
separated
strands
apart and
stabilize
the
unwound
DNA
single-stranded binding proteins
helicase
replication fork
BILL: define terms related to step 2
• Origin of replication
• Helicase
• Single strand binding
proteins
• DNA polymerase III
• Enzyme
• 3’ end
• Nucleotides
• Primase
•
•
•
•
•
•
•
•
•
Primer
Deoxyribonoucleotides
Antiparallel
Replication fork
Leading strand
Lagging strand
Okazaki fragment
DNA polymerase I
DNA ligase
3. RNA nucleotides bind with complementary base
sequences under the direction of the enzyme
RNA
primase.
These RNA nucleotides act as a primer
for DNA nucleotides.
– Primers are short segments of RNA, about 10
nucleotides long
– Must have a primer because DNA polymerase can only
add nucleotides to another nucleotide
REVIEW BILL:
List three ways RNA is
different than DNA.
RNA
Primer
Primase
DNA
BILL: define terms related to step 3
• Origin of replication
• Helicase
• Single strand binding
proteins
• DNA polymerase III
• Enzyme
• 3’ end
• Nucleotides
• Primase
•
•
•
•
•
•
•
•
•
Primer
Deoxyribonoucleotides
Antiparallel
Replication fork
Leading strand
Lagging strand
Okazaki fragment
DNA polymerase I
DNA ligase
4. DNA polymerase III adds DNA nucleoside
triphosphates to the RNA primer sequence
in a 5’ to 3’ direction.
Two of the
phosphates
are stripped
off in the
bonding
Energy of Replication
Where does energy for bonding usually come from?
We come
with our own
energy!
energy
energy
ATP
CTP
TTP
GTP
modified nucleotide
And we
leave behind a
nucleotide!
CMP
TMP
GMP
AMP
ADP
Energy of Replication
• The nucleotides arrive as nucleosides
– DNA bases with P–P–P
• P-P-P = energy for bonding
– DNA bases arrive with their own energy source
for bonding
– bonded by enzyme: DNA polymerase III
ATP
GTP
TTP
CTP
Adding Bases
5
3
• DNA polymerasre III
can only add
nucleotides to
3 end of a growing
DNA strand
– need a “starter”
nucleotide to
bond to
• strand only grows
53
5