Download Sect 12.2

Section 12.2
Replication of DNA
Objectives
•
•
Summarize the role of the enzymes involved in the replication of DNA.
Explain how leading and lagging strand are synthesized differently.
Main Idea
• DNA replicates by making a strand that is complementary to each original strand.
Semiconservative Replication
• In the Watson-Crick model
◦ a replication method was suggested
◦ called semiconservative replication
◦
• In semiconservative replication
◦ DNA strands separate
 serve as templates
 produce DNA that has
□ one strand of parental DNA
□ one strand of new DNA
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Unwinding
◦ responsibility of an enzyme
 DNA helicase
◦
•
Unzipping
◦ breaking the hydrogen bonds
 between the base pairs
 leaving single strands of DNA
◦
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occurs at a structure called
 the replication fork
□ location where the strands separate
one strand is called the leading strand
 other strand is called the lagging strand
Base pairing
◦ DNA polymerase
 catalyzes the addition of complementary nucleotides to the DNA strands
• remember
• A to T and C to G

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Base pairing
◦ on the lagging strand
 nucleotides are added away fm the rep fork
◦
nucleotides are added
 discontinually in small segments
• called Okazaki fragments

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the leading strand
• adds nucleotides to its end
in the opposite direction
Base pairing
◦ on the lagging strand
 the Okazaki fragment are joined by
□ DNA ligase
Comparing Replication with Eukaryotes and Prokaryotes
• Eukaryotic DNA
◦ unwinds in multiple places as the DNA is replicated
◦ these replication sites look like bubbles
 along the DNA strand
•
Prokaryotic DNA
◦ is circular
◦ it unwinds at a single spot
◦ replication occurs in both directions
 until it meets