compiled from other sources too

1. 2007-2008
DNA Replication

2. Objectives:
 Make a model of a DNA template to determine
the sequence of bases in the new DNA strand
 Explain the steps of DNA replication and the
enzymes involved in the process.

3. Activity (DNA makes DNA)

4. What is it?
A process of
copying and
duplicating a
DNA molecule

5. Essential?
 DNA replication needs to occur because
existing cells divide to produce new cells.
Each cell needs a full instruction manual to
operate properly so the DNA needs to be
copied before cell division so that each new
cell receives a full set of instructions.
 To ensure that each daughter cell gets a copy
of the genome, and therefore, successful
inheritance of genetic traits.

7. Anti-parallel strands
 Nucleotides in DNA
backbone are bonded from
phosphate to sugar between
3 & 5 carbons
 DNA molecule has “direction”
 complementary strand runs in
opposite direction
3
5
5
3

8. Bonding in DNA
….strong or weak bonds?
How do the bonds fit the mechanism for copying DNA?
3
5 3
5
covalent
phosphodiester
bonds
hydrogen
bonds

9. ENZYMES

10. How?
1. Opening of double helix
and separation of DNA
strand
2.Priming of template
strand
3. Assembly of new DNA
segment.

11. STEP1: REPLICATION FORK FORMATION
 An enzyme called helicase breaks the bond between nitrogenous bases. The two strands
of DNA split.
 The separation of the two single strands of DNA creates a ‘Y’ shape called a replication
‘fork’. The two separated strands will act as templates for making the new strands of
DNA.
 DNA is directional in both strands, signified by a 5' and 3' end. This notation signifies
which side group is attached the DNA backbone. The 5' end has a phosphate (P) group
attached, while the 3' end has a hydroxyl (OH) group attached.

13.  The leading strand is the simplest to replicate. Once the DNA strands
have been separated, a short piece of RNA called a primer binds to the
3' end of the strand. The primer always binds as the starting point for
replication. Primers are generated by the enzyme DNA primase.

STEP2: PRIMER BINDING

15. DNA
Polymerase III
STEP3: ELONGATION
During elongation, an enzyme called
DNA polymerase adds DNA
nucleotides to the 3′ end of the newly
synthesized polynucleotide strand. The
template strand specifies which of the
four DNA nucleotides (A, T, C, or G) is
added at each position along the new
chain.

16. Leading & Lagging strands
 Leading strand – synthesized continuously
since DNA polymerase is moving in the same
direction as the replication fork ( enzyme can
work only in 5’ to 3’ direction)
 Lagging strand – synthesized in small,
separate fragments, delayed because it must
wait for the leading strand to expose the
template strand. ( synthesis happens in the
direction of 3’ to 5’)

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

18.  Another enzyme called DNA ligase joins
Okazaki fragments together forming a single
unified strand.
 Once completed, the parent strand and its
complementary DNA strand coils into the
familiar double helix shape. In the end,
replication produces two DNA molecules, each
with one strand from the parent molecule and
one new strand.
STEP4: TERMINATION

20.  https://www.youtube.com/watch?v=TNKWgcFP
Hqw

21. assessment
1. Write the sequence of DNA bases that would
pair with this partial strand CAT TCA CTG.
2. The enzyme that unwinds the DNA to prepare
for replication is____.
3. The synthesis of DNA by DNA polymerase
occurs in the __ to ___ direction.
4. What enzymes are used to join bits of DNA?
5. DNA replication is semi-conservative. This
means that _____________.