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Ch. 12.2: Replication of DNA
• Section objective:• Summarize DNA
replication
Replication of DNA
• Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes.
• The DNA in the chromosomes is copied in a process called DNA replication.
• Without DNA replication, new cells would have only half the DNA of their parents.
• DNA is copied during interphase prior to mitosis and meiosis.
• It is important that the new copies are exactly like the original molecules.
Replication of DNA• DNA replication depends on specific base pairing• In DNA replication, the strands separate
– Enzymes use each strand as a template to assemble the new strands
Parental moleculeof DNA
Nucleotides
Both parental strands serveas templates
Two identical daughtermolecules of DNA
Replication of DNA
• Semiconservative replication:• Parental strands of DNA separate, serve as
templates and produce 2 molecules of DNA that have one strand of parental DNA and one strand of new DNA
Copying DNA• Matching bases allows DNA
to be easily copied
DNA replication• DNA Helicase: enzyme responsible for
uncoiling the double helix and unzipping the weak hydrogen bonds between the base pairs
DNA replication
DNA polymerase
• Enzyme– DNA polymerase – adds new bases
DNA basesin nucleus
DNAPolymerase
Copying DNA Build daughter DNA strand
use original parent strand as “template”
add new matching bases synthesis enzyme =
DNA polymerase
• Adding bases – can only add
nucleotides to 3 end of a growing DNA strand• need a “starter”
nucleotide to bond to
– strand only grows 53
DNAPolymerase III
DNAPolymerase III
DNAPolymerase III
DNAPolymerase III
energy
energy
energy
Replication energy
3
3
5
5
• Leading strand- elongates as DNA unwinds
• Lagging strand –elongates in opposite direction . Synthesized discontinuously into small segments called Okazaki fragments
• DNA ligase (an enzyme)links these sections
Limits of DNA polymerase III can only build onto 3 end of an
existing DNA strand
Leading & Lagging strands
5
5
5
5
3
3
3
53
53 3
Leading strand
Lagging strand
Okazaki fragments
ligase
Okazaki
Leading strand continuous synthesis
Lagging strand Okazaki fragments joined by ligase
“spot welder” enzyme
DNA polymerase III
3
5
growing replication fork
New copies of DNA• Get 2 exact copies of DNA to split between
new cells
DNA polymerase
DNA polymerase