Part 3
DNA Replication
24-1
Learning Targets
5. Understand base pairing and its role in replication,
transcription and translation.
6. Understand that replication is semi-conservative and
occurs on a lagging and leading strand.
7. Know the role of enzymes in replication including
helicase, primase, DNA polymerase and ligase.
24-2
DNA Replication
• Found only in the nucleus
• Origin of replication
• Multiple origins on a
chromosome
• Occurs during S phase of the
cell cycle
• Semiconservative  each
new DNA strand has 1 new/1
old strand
24-3
DNA Replication Enzymes
• DNA helicase unzips DNA 
unwinds it
• At “replication fork” 
• “unzipped” bases form H bonds with
“free” nucleotides
• Primase assembles RNA Primer
• “seats” DNA Polymerase
• DNA polymerase bonds sugar
to phosphate  makes new
strand of DNA
• (DNA Ligase (see Okazaki
Fragments))
24-4
Accuracy
• DNA Polymerase also “proofreads”
• Very accurate (only 1 error every billion
nucleotides!)
• 20 repair enzymes “edit”
goofs  mutagens (heat,
chemicals, radiation, etc.)
• EXACT SEQUENCE is VERY IMPORTANT
24-5
Okazaki fragments
• DNA is “anti-parallel”
• Sugar/phosphate groups “point” in diff.
direction
• 5’3’ “leading strand”
• 3’5’ “lagging strand”
• DNA Polymerase only assembles new strand
in 5’3’ direction
• Leading strand is OK
• Lagging strand forms in segments 
Okazaki fragments
• DNA Ligase ties sugar/phosphates together
24-6