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Replication of DNA
CHAPTER 12
SECTION 2
Semi-conservative replication
 DNA replicates by making a strand that is
complementary to each original strand
 Semi-conservative Replication parental strands of
DNA separate, serve as templates, and produce DNA
molecules that have one strand parental DNA and
one strand of new DNA
 Semi-conservative replication occurs in 3 main
stages: unwinding, base pairing, and joining
Unwinding
 DNA replication is carried out by a series of enzymes
 These enzymes break the hydrogen bonds between
the base pairs leaving two separate strands
 As the helix unwinds, another enzyme called an RNA
primer is attached on each DNA strand
 Each strand serves as a template for the attachment
of complementary bases
 A strand that has the bases TACGTT produces a
strand with complementary bases ATGCAA
 The result is 2 DNA molecules identical to each other
and to the original molecule
 The principal enzyme involved in DNA replication is
called DNA Polymerase

DNA polymerase also “proofreads” each new DNA strand to
check for mistakes
Base Pairing
 DNA polymerase adds new DNA nucleotides to the
chain by adding to the 3’ end of the new DNA strand

3’ strand is elongated as the DNA unwinds this is called the
leading strand
 The other strand of DNA elongates away from the
replication fork


This strand is called the lagging strand
It is synthesized discontinuously into small segments called
Okazaki fragments
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Joining
 DNA replication can occur in many areas along the
chromosome
 DNA polymerase come to an RNA primer it removes
the primer and fills in the place with DNA
nucleotides using a special ligase called DNA ligase
DNA ligase repairing chromosomal damage.