DNA
Replication
How DNA Replication Occurs
• DNA replication is the process by which DNA is
copied in a cell before a cell divides by mitosis,
meiosis, or binary fission
– One copy moves to each new cell
– Since the two strands are
complementary, each strand
serves as a template to make a
new strand
Steps of DNA Replication
1. Helicases separate the DNA
Strands
-
These move along the DNA
molecule breaking the hydrogen
bonds between the complementary
bases
-
This allows the two strands to
separate
-
The Y shaped region where the
strands are separating is called the
replication fork (not spoon)
Steps of DNA Replication
2. DNA polymerases add
complementary nucleotides
to each of the original
strands
-
-
Nucleotides float freely in the
nucleus
As nucleotides are added,
covalent bonds form between
the sugar of one nucleotide and
the phosphate group of the
next
Hydrogen bonds form between
the bases
Steps of DNA Replication
3. DNA polymerases are finished
and fall off
- Result is two molecules of
DNA identical to the original
- Molecules are ready to move
into new cells during division
- Called semi-conservative
replication because each of
the new DNA molecules has
kept (or conserved) one of the
two (or semi) original DNA
strands.
Steps of DNA Replication
Click picture for
video
Action at the Replication Fork
• DNA synthesis occurs
in different directions
on each strand
• As the replication fork
moves along the
original DNA,
synthesis follows the
movement of the
replication fork
• Synthesis on the other
strand moves in the
opposite direction,
away from the
replication fork
• This leaves gaps in
the newly synthesized
strand, which are later
joined by an enzyme
DNA ligase
Prokaryotic and Eukaryotic Replication
Prokaryotic
• Circular chromosome
• Replication begins at one
place along the
chromosomes
• Two replication forks are
formed and proceed in
opposite directions
• Replication continues along
each fork until they meet an
the whole molecule is copied
Prokaryotic and Eukaryotic Replication
Eukaryotic
• Each chromosome is long, but not circular
• At the rate a polymerase adds nucleotides
(50/sec), it would take 53 days to replicate the
largest human chromosome
• Instead, replication begins at many points or
origins along the DNA, and two forks move in
opposite directions
• In a fruit fly, replication begins simultaneously
at about 3500 sites in a DNA molecule
Errors in DNA Replication
• DNA replication usually occurs
with great accuracy (only one error
for every billion nucleotides added)
• How is this done???
• Polymerases have repair or
“proofreading” functions in the
same way the computer checks for
spelling errors
• For example, if A pairs with C, the
polymerase removes the C and
replaces it with T
Click picture for
video
Errors in DNA Replication
• When mistakes do occur,
the base sequence of the
newly formed DNA differs
from the original
• A change in the nucleotide
sequence of a DNA
molecule is called a
mutation
• These can have serious
effects on the function of a
gene – therefore disrupting
cell function
DNA Replication and Cancer
• Mutations can arise
and lead to altered
cells and organisms
• The changes allow
individuals to survive
and reproduce better
• Sometimes nonrepaired mutations
can cause cancer
• Tumors are caused by
mutations that affect
cell division
How is the exact replication
of DNA ensured?
By complementary base pairing and
proofreading by DNA polymerases
What are replication forks?
Areas of DNA where the double helix
separates prior to replication