1. Ch. 12.2: Replication of DNA
Section objective:
Summarize DNA replication

2. 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.

3. 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
Nucleotides
Parental molecule of DNA
Both parental strands serve as templates
Two identical daughter molecules of DNA

4. 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

5. Copying DNA
Matching bases allows DNA to be easily copied

6. DNA replication
DNA Helicase: enzyme responsible for uncoiling the double helix and unzipping the weak hydrogen bonds between the base pairs

7. DNA replication Enzyme DNA polymerase adds new bases
DNA bases in nucleus
Enzyme
DNA polymerase
adds new bases
DNA
polymerase

8. Copying DNA Build daughter DNA strand
use original parent strand as “template”
add new matching bases
synthesis enzyme = DNA polymerase
DNA
Polymerase

9. Replication Adding bases5 3
energy
DNA
Polymerase III
Adding bases
can only add nucleotides to 3 end of a growing DNA strand
need a “starter” nucleotide to bond to
strand only grows 53
energy
DNA
Polymerase III
DNA
Polymerase III
energy
DNA
Polymerase III
The energy rules the process.
energy 3 5

10. 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

11. Leading & Lagging strands
Okazaki
Limits of DNA polymerase III
can only build onto 3 end of an existing DNA strand  5
Okazaki fragments 5 5 3 5 3 5 3
ligase
Lagging strand 3
growing
replication fork 3 5
Leading strand  3 5
Lagging strand
Okazaki fragments
joined by ligase
“spot welder” enzyme 3
DNA polymerase III
Leading strand
continuous synthesis

12. New copies of DNA Get 2 exact copies of DNA to split between new cells
polymerase
DNA
polymerase