DNA replication is the process where a double-stranded DNA molecule is copied to produce two identical copies. It occurs during the S phase of the cell cycle. The DNA double helix unwinds and each strand serves as a template for new partner strands. DNA polymerase adds complementary nucleotides to each new strand, building them from the 5' to 3' direction. The leading strand is synthesized continuously while the lagging strand is synthesized discontinuously in fragments called Okazaki fragments that are later joined together. This semiconservative process results in two double-stranded DNA molecules that are identical to the original DNA.

1. DNA REPLICATION
SERWELAS, CLAUDINE M.
ESQUIVEL, CHERRY LYN
AAPD2-F
GENETICS
DR. LOS BANOS

2. What does DNA look like?
• DNA has..
• A sugar/phosphate
backbone
• 4 Chemical bases
Sugar

3. The bases
• It is composed of 4
chemical bases:
Adenosine (A), Thymine
(T), Cytosine (C) and
Guanine (G).
• A always pairs with T
• C always pairs with G
• The pairs are held
together by hydrogen
bonds (WNC’s).

4. What does DNA do?
• DNA is the genetic code.
• It determines our physical characteristics:
from our hair color to what we are allergic to.
• Our DNA codes for 20 amino acids which are
the building blocks of life.

5. What Is DNA Replication
• DNA Replication is the
process in which the DNA
within a cell makes an
exact copy of itself.
– Why does DNA replicate?
– During which phase of the
cell cycle does DNA
replicate?

6. DNA Replication models

7. The Three Possible DNA Replication
Models
• Conservative- would leave
the original strand intact
and copy it.
• Dispersive-would produce
two DNA molecule with
sections of both old and
new along each strand.
• Semiconservative –would
produce DNA molecule
with both one old strand
and one new strand.

8. DNA Replication
Replication occurs during Interphase
Replication Replication
fork fork
Replication
bubble
Hydrogen
bond
DNA replication is the process where an entire double-stranded
DNA is copied to produce a second, identical DNA double helix.

9. DNA Replication
DNA
helicase
• Helicase unwinds the double helix starting at a
replication bubble.
• The two strands separate as the hydrogen bonds
between base pairs are broken.
• Two replication forks form and the DNA is unwound in
opposite directions.

10. DNA Replication
•Helicase has completed unwinding the DNA strand.
•Single strand Binding Proteins (SSB) keep the two
strands from re-annealing (coming back together).

11. DNA Replication
Leading Strand
Primase RNA Primer
Lagging Strand
•Primase is an RNA polymerase that makes the RNA
primer.
•These primers “tell” the DNA polymerase where to start
copying the DNA.

12. DNA Replication
Leading Strand
3’ 5’
Direction of Replication
DNA Polymerase
5’ 3’
Direction of Replication Lagging Strand
• The DNA polymerase starts at the 3’ end of the RNA primer of
the leading stand CONTINUOUSLY.
• DNA is copied in 5’ to 3’ direction.
• DNA polymerase copies the lagging strand DIS- continuously.

13. DNA Replication
• The dis-continuous pieces of DNA copied on the lagging
strand are known as Okazaki fragments.

14. DNA Replication
Another DNA Polymerase removes the RNA primers and
replaces them with DNA.

15. DNA Replication
ligase
Finally the gaps in the sugar phosphate backbone are
sealed by DNA ligase
There are now 2 identical double helices of DNA.

16. REACTION:
o The DNA occurs simultaneously forming sister
chromatids.
o Nucleotides should always be in pair.
o Nucleotides are held together with loose
hydrogen bonds.
o Every cell in our body has the same copy of
DNA, and the DNA will copy itself trillion of
times in our lifetime.

17. o Every copy of the DNA contains half of
its original strand.
o DNA replication is semiconservative with each
existing strand serving as template for
synthesis of new strand.
o Replication begins at specific location called
REPLICATION.
o On one strand (leading strand) synthesis is
continuous
o On the other strand (lagging strand) synthesis
is discontinuous.

18. o The 2 strands is producing a series
Of okazaki fragments that must be
Ligased together

19. DNA Replication ANIMATION
• http://www.wiley.com/college/pratt/0471393878