DNA replication is the process by which a cell makes an identical copy of its DNA before cell division. It involves unwinding the DNA double helix at the replication fork, using DNA polymerases to add complementary nucleotides to each single strand, resulting in two identical DNA molecules each with one original and one new strand (semi-conservative replication). The leading strand is replicated continuously while the lagging strand replication occurs discontinuously in fragments joined by DNA ligase. Telomeres and the telomerase enzyme prevent DNA loss during replication.

1. Unit 5:
DNA & Protein Synthesis
Section 2: DNA Replication
Biology
Mrs. DeFord

3. DNA Replication
• Before a cell divides, the DNA must
make a second copy of the DNA.
This process of making a copy of
the DNA is called DNA replication.
• DNA replication is:
– Semi-conservative
– Semi-discontinuous
– bidirectional

4. DNA Replication
• DNA is maintained in a compressed,
supercoiled state. Before DNA can
be replicated, the DNA strands must
unwind and separate.

5. DNA Replication
• Helicase is the enzyme that
unwinds and unzips the DNA.
• As the helicase moves along the
DNA molecule, the weak hydrogen
bonds between the complimentary
nitrogen bases are broken.
• The DNA unzips, exposing the
nucleotides.
• The junction of the unwound
molecules is called a replication
fork.

6. DNA Replication
• Each strand of the double helix of
DNA is a template, or model, for
making the new strand.
• A new strand is formed by pairing
complementary bases with the old
strand.
• Two molecules are made. Each
has one new and one old DNA
strand. This is called semi-
conservative replication.

7. DNA Replication

8. DNA Replication

9. DNA Replication
• All known DNA polymerases work in a 5` to 3` direction,
so DNA replication is semi-discontinuous because one
strand is copied continuously and one strand is copied
discontinuously.
– The continuous strand is called the leading strand.
The 5’ carbon is located at the top of the leading
strand.
– The other strand, called the lagging strand, must
start replication in the replication fork and is made in
segments called Okazaki fragments. The 3’ carbon
is located at the lower section of the lagging strand.

10. • RNA primers are required for initiation of DNA synthesis
• Leading strand requires only one RNA primer
• Lagging strand requires one RNA primer for every
Okazaki fragment
• RNA primers are removed by specific enzymes and
replaced with DNA nucleotides
• Gaps are sealed with DNA ligases.
DNA Replication

11. • DNA polymerases
add on new
nucleotides to the
growing DNA
strand.
• Free floating
nucleotides attach
to their
complimentary
bases on the
template strand of
the DNA.
DNA Replication

12. The Telomere Problem
• When the DNA polymerase reaches the 5' end of the
leading template, synthesis of the leading daughter strand
is almost complete. When the RNA primer, which
occupied the daughter strand's 5' end is removed, the
overlapping single strand cannot be replicated. This
primer was located at the beginning of the leading strand
and at the end of the lagging strand. Thus, the 5' end of
each of these antiparallel daughter strands is one primer-
length shorter. With each succeeding replication, the
chromosome grows shorter and shorter.
DNA Replication

13. The Telomere Problem
• Fortunately, our chromosomes contains protective ends
called telomeres. Telomeres are highly repeated
sequences that cap both ends of our chromosomes like
the wax tips on shoelaces. Because telomeres are
nonsense DNA (i.e., they do not encode proteins), there is
no immediate consequences when telomeric sequence is
lost. Given enough replication events, telomere shortening
is correlated with triggering a natural cell death called
apoptosis. Cellular aging occurs when the cells are no
longer able to divide.
DNA Replication

14. The Telomere Problem
• Cells use a special enzyme called telomerase to fix this
problem. Telomerase makes it less likely that genes will be
damaged or lost during replication of rapidly dividing cells
by adding nonsense DNA to the ends of the lagging
strands.
• Telomerase is often switched off in normal adult cells. But
in cancer cells, telomerase may be switched on, resulting
in their ability to grow and divide rapidly.
DNA Replication