DNA Replication
Cell DNA is copied
What is meant by DNA REPLICATION:
Replicate means copy, or duplicate.
DNA in cells must be copied exactly.
During the process of DNA replication, a
duplicate of the DNA molecule is made.
Not a good copy
Why must DNA be Copied?
Replication: DNA is duplicated during
interphase so that when cells divide, their
daughter cells will have identical DNA.
During DNA
replication
each strand of the
double
helix is a template
or model for a new
strand
The Replication Process
1) H bonds between bases are broken
(enzyme – helicase)
2) DNA “unzips” between bases and two
strands separate. (Binding proteins attach to
the strands for support)
This happens at a portion of the molecule
called the “replication bubble” The Point
at either end of the bubble, where DNA
unzips is a “replication fork.”
3) The principle enzyme involved in
replication is DNA polymerase.
It allows free nucleotides in the nucleus to
find their place and bond to the DNA
template strand.
Remember Correct Base-Pairing
A bonds to
T
C bonds to
G
4) The DNA polymerase also proofreads the
copied strand.
It can correct any mistakes in the
copied DNA.
The end result of replication is two
duplicate DNA molecules.
Since each of the two new DNA strands contains
one old strand and one new strand, the
process of replication is called
“semi-conservative.”
Okazaki Fragments
DNA is not actually replicated
the same on both strands
of the molecule.
Because of the orientation
of the nucleotides in each
strand (antiparallel),
DNA polymerase makes
one strand in one piece,
and the other
in many
pieces called
Okazaki fragments.
Antiparallel strands
• DNA polymerase can only work from the 3’ – 5’ direction.
Since the strands of DNA are “opposite” one another, the
enzyme must move in a seemingly opposite direction on
the 5’ to 3’ direction.
• The DNA strand that is replicated
in one continuous piece is called
the leading strand.
• The other DNA strand is replicated
in a discontinuous manner at the 3’-5’
direction. This is called the lagging
strand. The fragments of this
strand are called Okazaki fragments.
(named for Reiji Okazaki.
They are usually 1000 – 2000 nucleotides in
length)
• The O. fragments are bonded by the enzyme ligase.
•
RNA primers of about 10 nucleotides are in place before the DNA
polymerase makes a fragment.
It can get complicated.
DNA Replication