DNA Replication!
Do Now
• When do cells replicate?
• What enzymes are involved? How many
can you name?
• How do the cells replicate?Explain.
Objectives
• Explain what replication is and why it must
occur in our bodies.
• List the different enzymes involved in each
step of replication and what they do.
• Compare and contrast leading vs. lagging
strands.
Read the cartoon on
replication…
• How many daughter strands are formed?
• Where do the nucleotides come from in
the new strand of DNA?
• Which direction does DNA replication
occur?
• What type of molecules are used in DNA
replication?
Semiconservative Replication
 Parental strands of DNA separate
 Each strand serve as template
 DNA molecules produced have one strand
of parental DNA and one daughter strand
Enzymes involved
• Helicase- Unzips the DNA strand.
• Primase- RNA Primer (tells the next
enzyme where to begin!)
• DNA polymerase- adds nucleotides.
• DNA ligase- links together fragments.
Steps to DNA Replication
1. Unwinding
• DNA helicase (enzyme) unwinds and
unzips the DNA – breaking the hydrogen
bonds between the strands
• RNA primase adds a short segment of
RNA, called an RNA primer, on each DNA
strand.
Steps to DNA replication
DNA unwinds at different spots (Origins)
At the replication fork, an enzyme breaks the
hydrogen bonds between the bases.
Steps to DNA Replication
2. Base Pairing
• DNA polymerase (enzyme) adds
complementary nucleotides to parent DNA
strand
• Only adds to 3’ end of new DNA strand
2. Base Pairing (cont.)
• Two daughter strands of DNA are made
differently
– Leading Strand- Built continuously with out
breaks
– Lagging Strand- makes several small
fragments of DNA
• Built discontinuously
• Pieces are called Okazaki Fragments
• Fragments are connected by DNA Ligase
DNA Replication
Steps to DNA Replication
3. Joining
• DNA Polymerase removes RNA Primer
and fills it in place with DNA nucleotides
• DNA ligase links two sections
Steps to DNA Replication
(cont.)
• 4. There are 2 daughter strands- one
leading strand and one lagging strand
Leading and Lagging Strands
• http://www.fed.cuhk.edu.hk/~johnson/teac
hing/genetics/animations/dna_replication.h
tm
Mistakes in DNA
• DNA replication takes a lot of energy.
• During replication, mistakes are made.
(mutations)
• Enzymes “proof read” and repair the
mistakes.
Comparing DNA Replication in
Eukaryotes and Prokaryotes
• Eukaryotic DNA unwinds in multiple areas
as DNA is replicated.
• In prokaryotes, the circular DNA strand is
opened at one origin of replication.
Do Now
• Label the DNA Replication Diagram