12.2
Replication of DNA
Textbook pages 333-335
Semiconservative Replication
• Parent strands of DNA separate, serve as
templates, and produce DNA molecules with 1
strand of parent DNA and 1 strand of new DNA.
• Three phases:
▫ Unwinding
▫ Base pairing
▫ Joining
Unwinding
• DNA helicase (enzyme) unwinds and unzips the
double helix
• Hydrogen bonds break between bases, making
a single strand of DNA
• Single-stranded binding proteins help keep
strands apart during replication
• RNA primase (enzyme) adds short piece of RNA
(RNA primer) on each DNA strand
Base pairing
• DNA polymerase (enzyme) catalyzes addition of
nucleotides to new DNA strand
• Nucleotides added to 3' end of new strand and
follow Chargaff’s rule
• Leading strand – elongates as DNA unwinds,
built continuously on 3' end
• Lagging strand – elongates away from
replication fork
• Okazaki fragments
▫ Small segments of synthesized DNA
▫ Later connected by DNA ligase
▫ 100-200 nucleotides long in eukaryotes
• DNA replication is
▫ Semi-discontinuous and semi-conservative
Joining
• Eukaryotic DNA replication – many areas on
chromosome where replication begins
• When DNA polymerase comes to an RNA primer
on DNA, primer is removed and filled with DNA
nucleotides
• RNA primer replaced, DNA ligase links the 2
sections
Comparing DNA Replication in
Eukaryotes and Prokaryotes
• Eukaryotes
▫ DNA unwinds in
multiple areas during
replication
▫ Lengths vary from
10,000 to 1 million
base pairs
▫ Multiple replication
areas look like
bubbles in DNA strand
• Prokaryotes
▫ Circular DNA strand is
opened at origin of
replication
▫ DNA replication occurs in
2 directions
▫ Shorter DNA than in
eukaryotes
▫ Remains in cytoplasm not
packaged in nucleus