DNA replication is the process by which DNA copies itself in order to pass genetic information to daughter cells. It occurs through a semi-conservative process whereby the double helix unwinds and each strand acts as a template for new partner strands. DNA polymerase adds nucleotides to extend the new strands from primers in the 5' to 3' direction. On the leading strand synthesis is continuous but on the lagging strand it occurs discontinuously in fragments called Okazaki fragments that are later joined by DNA ligase. Telomerase adds repetitive sequences to the ends of lagging strands during replication to compensate for nucleotide losses and prevent shortening of chromosomes.

1. Gaziosmanpasha University
College of Medicine
Department of Medical Biology
DNA Replication
Instructed by:
Dr. Nihan Bozkurt
Prepared by:
Milat Hussein

2. DNA Definition
DNA  Deoxyribonucleic Acid
 DNA, is a biological macromolecule that carries hereditary
information in many organisms.
 are mostly present inside the nucleus.
 Some cytoplasmic organelles like the mitochondria also
contain DNA molecules.

3.  DNA molecules consist of two DNA strands.
 The double helix structure of DNA was first discovered in
1953 by James Watson, Francis Crick, and Rosalind Franklin.

4. Phosphate
group
Deoxyribose sugar
Nitrogenous bases
 Nucleotides consists of three parts:
1- deoxyribose sugar .
2- phosphate group.
3- nitrogenous bases .
 DNA Structure
 DNA molecules are polymers and are made up of many
smaller molecules, called nucleotides.

5.  Deoxyribose sugar :
 Deoxyribose is the five-carbon sugar molecule that helps form
the phosphate backbone of DNA molecules.
 Formula for deoxyribose is С₅Н₁₀О₄
 Phosphate group :
 chemical formula PO₄⁻³.
 It is a chemical compound made up of one phosphorus and
four oxygen atoms.
 When it is attached to a molecule containing carbon, it is
called a phosphate group.

6. There are four types of nitrogenous bases found in DNA
molecules.
 Adenine .
 Guanine .
 Cytosine .
 Thymine.

7.  Nucleotides are strongly linked together by phosphodiester
bonds.
 Together, the phosphate groups and sugars form the sugar-
phosphate backbone.
 The nitrogenous bases point inwards, like the rungs of a
ladder, and are joined together in base pairs.

8. nucleobase + Deoxyribose = nucleoside
nucleoside + phosphate = nucleotide
nucleotide chain = polynucleotide
2 polynucleotides = deoxyribonucleic acid (DNA)

9.  DNA Replication
 DNA replication is the basis for biological inheritance.
 Two identical DNA molecules will be produced from a single
double-stranded DNA molecule.
 DNA replication begins at specific locations in the genome,
called "origins".
 Unwinding of DNA at the origin, and synthesis of new strands,
forms a replication fork.
 Replication occurs in both prokaryote and eukaryote cells .

10.  DNA replication is a process by which DNA is copied
during the S or synthesis phase.

11.  General feature of DNA replication
• DNA replication is semi conservative.
• It is bidirectional process.
• It proceed from a specific point called origin.
• It proceed in 5’-3’ direction.
• It occur with high degree of fidelity.
• It is a multi-enzymatic process.

12.  Models of DNA replication
 Conservative : if both strands of the template DNA molecule
stayed bonded and intact while serving as a template for a
brand new and identical double-stranded DNA molecule.
 Semiconservative : postulates that the two original strands
of DNA separate, each serving as a template for a new strand –
creating DNA molecules that have one original strand and one
new strand.
 Dispersive :Each strand of both daughter molecules contain a
mixture of old and newly synthesized parts.

14.  Meselson and Stahl Experiment
 Proves that DNA replication is semiconservative .

15.  DNA replication occurs by three steps
1. Initiation: It involves recognition of the positions on a DNA
molecule where replication will begin.
2. Elongation:
- Leading strand synthesis
- Lagging strand synthesis
3. Termination :During this process, which is known as
replication termination, DNA synthesis is completed.

16.  Initiation of DNA replication:
 Initiator proteins recognize and bind to the replicator and
serve to initiate DNA replication.
 The first step of replication is taken by connecting these
proteins to their starting points.
 The activities that are provided by initiator proteins range
from recognition of the ori and recruitment of replication
factors, to melting of double-stranded DNA and replicative
DNA-helicase activity.

17.  Replication origin
 DNA replication origins in bacteria, archaea, plasmids,
bacteriophage, and viruses are specific sequences
essential for initiation of DNA replication, because they
contain binding sites for either their cognate helicase
loader or cognate DNA helicase
 At E.Coli, the replication begins with a single origin, called
OriC, which is 245 bp long.

18. DNA Replication Bubble
 Has the ability to grow in two directions.
 In each replication bubble, there are two replication forks.
 The functioning of the replication bubble occurs with the
enzyme DNA helicase .

20. DNA Replication Fork
 It was previously mentioned that each replication bubble
contains two replication forks.
 It first occurs at the origin point of synthesis and progresses as
the replication continues.
 which move in the opposite direction from origin.

21. Replication Bubble vs Replication Fork
Replication bubble is defined
as an opening that is present
within the DNA strand during
the initiation of replication.
Replication fork is defined as
structures that are present in
the replication bubble that
denotes the occurrence of
replication.

22.  Some of the major proteins in DNA replication
include the following:
 Helicase:
- An enzyme that opens the double helix by breaking the
hydrogen bonds between complimentary base pairs.
- separates the double strands of DNA molecules.
- DNA Helicase moves in the direction of 5‘3' or 3‘5
along single chain DNA.
 Single-strand DNA- binding proteins (SSBPs) :
- These proteins stabilize the individual strands of DNA to
prevent them from reconnecting.
 Topoisomerase:
- Because unwinding of the DNA by helicase creates
tension further down the strand.

23. • Primase:
- An enzyme that adds a primer (which is a short segment of
ribonucleic acid, known as RNA) where DNA polymerase III
will attach.
• DNA polymerase III: which add DNA bases .
• DNA polymerase I: An enzyme that replaces the RNA primer
with DNA.
• DNA ligase :fills in the gaps between Okazaki fragments.
• Sliding clamp: A protein that holds DNA polymerase III in place

26. Replicating the Leading Strand
 Replication is continuous.
 There are no fragments.
 DNA polymerase iii adds nucleotides in the direction of 5’ to 3’ .
 DNA polymerase only works in the direction of 5’ to 3’ .

27. Replicating the Lagging Strand
Synthesizing in the opposite direction of the replication fork. This
is the first Okazaki fragment.
Once DNA polymerase III reaches the first Okazaki fragment
primer.
DNA ligase connects the segments of DNA by closing the sugar-
phosphate backbone.

28.  DNA Polymerase.
 DNA polymerase :
- is an important enzyme group involved in
DNA synthesis, repair, and replication, these enzymes are
found in all living organisms.
- It is a complex enzyme, It is an enzyme that carries out
polymerization of DNA, as it is clear from its name DNA
polymerase.
 It is mainly of three types in prokaryotes ;
- pol-I
- pol-II
- pol-III
 In eukaryotes, it is of five kinds ;
- pol-α
- pol-β
- pol-Ƴ
- Pol- δ
- pol-Ɛ.

29.  Prokaryotic DNA Polymerase Types and Function
• DNA Polymerase I: is coded by polA gene. It is a single
polypeptide and has a role in recombination and repair. It has
both 5’→3’ and 3’→5’ exonuclease activity. DNA polymerase
Ⅰ removes the RNA primer from lagging strand by 5’→3’
exonuclease activity and also fills the gap.
• DNA Polymerase II : is coded by polB gene. It is made up of 7
subunits. Its main role is in repair and also a backup of DNA
polymerase III. It has 3’→5’ exonuclease activity.
• DNA Polymerase III : is the main enzyme for replication
in E.coli. It is coded by polC gene. The polymerization and
processivity rate is maximum in DNA polymerase III. It also has
proofreading 3’→5’ exonuclease activity.

30.  Differences between Eukaryotic DNA Polymerase vs
Prokaryotic DNA Polymerase:
Prokaryotic Eukaryotic
DNA pol III is involved in in vivo
replication of DNA.
DNA pol α and delta are
involved in in vivo replication
of nuclear DNA, whereas DNA
pol gamma for mitochondrial
DNA.
DNA pol I is the repair enzyme. DNA pol beta and epsilon
function as DNA repair
enzymes.
Primase synthesizes primer in
EColi
DNA pol alpha synthesizes
primers for both leading and
lagging strands

31.  Telomerase Definition
Telomerase is an enzyme found inside our cells, which may be
related to the aging process.
 It adds short, repetitive “caps” to our DNA strands.
 Each time our cells divide, they must replicate their DNA so
that each daughter cell gets a full set of operating
instructions.
 This finding suggests that use of artificial telomerase may
prevent cancer from occurring.

32.  Telomere replication on the lagging strand is as follows:
1. Telomerase attaches to the very end of the lagging strand,
overhanging the un replicated portion of DNA.
2. Using its own RNA template, telomerase synthesizes the
extending telomere, adding additional bases to the 3’ end of
the lagging strand.
3. Primase adds the primer on the telomere.
4. DNA polymerase III binds to the primer and moves opposite of
telomerase to complete the synthesis of the lagging strand.

33. Prokaryotes DNA replication Eukaryotes DNA replication
1. It occurs inside the cytoplasm. It occurs inside the nucleus.
2. There is single origin of replication. Origin of replications are numerous.
3. DNA polymerase Ill carries out both
initiation and elongation.
Initiation is carried out by DNA
polymerase α while elongation by
DNA polymerase δ andε .
4. DNA repair and gap filling are done
by DNA polymerase I.
The same are performed by DNA
Polymerase β.
5. RNA primer is removed by DNA
polymerase I .
RNA primer is removed by DNA
Polymerase β.
6. Okazaki fragments are large, 1000-
2000 nucleotides long.
Okazaki fragments are short, 100-
200 nucleotides long.
7. Replication is very rapid, some
2000
base pairs per second.
Replication is slow, some 100
nucleotides per second.
8. DNA gyrase is needed. DNA gyrase is not needed.
9. The DNA is circular and double-
stranded.
The DNA is linear and double-
stranded.

34. Thank you