Presented by: Aparna Gupta, Priya
Maurya, Shiva Pardesi & Avishek
Majumder
M. Sc Biotechnology (2nd sem)
DNA Replication

DNA Replication
 Prior to cell division, the DNA material in the original
cell must be duplicated so that after cell division, each
new cell contains the full amount of DNA material.
 The process of DNA duplication is usually
called replication.
 The replication is termed semiconservative since
each new cell contains one strand of original DNA and
one newly synthesized strand of DNA.
 Occurs during Interphase (S phase) of the Cell
Cycle.

Modes of DNA Replication
1. Semiconservative:
 In this pattern the parental strand separate into
individual strand. Each of this strand act template for
the synthesis of new strand.
 In the semi-conservative model, the two parental
strands separate and each makes a copy of itself.
After one round of replication, the two daughter
molecules each comprises one old and one new
strand. Note that after two rounds, two of
the DNA molecules consist only of new material, while
the other two contain one old and one new strand.

Modes of DNA Replication

Modes of DNA Replication
2. Conservative:
 Conservative: When parental strand of DNA form a
daughter molecule without separation of strand.
 In the conservative model, the parental molecule
directs synthesis of an entirely new double-stranded
molecule, such that after one round of replication, one
molecule is conserved as two old strands. This is
repeated in the second round.

Modes of DNA Replication

Modes of DNA Replication
3. Dispersive:
 Parental DNA breaks up into small fragments which
then synthesize complementary strands that is
randomly.
 In the dispersive model, material in the two parental
strands is distributed more or less randomly between
two daughter molecules. In the model shown here, old
material is distributed symmetrically between the two
daughters molecules. Other distributions are possible.

Modes of DNA Replication

Mechanism
1. Initiation:
 At the time of replication the two strand of parental
DNA strands separate like the uncoiling of a cotton
thread forming “Y” shaped replicating fork.
 The origin of replication is called Ori C.
 In the absence of Ori, replication will not occur.
 Various enzymes are involved in the separation, like,
Helicases which most important as they cut hydrogen
bonds between two N2 bases.

Mechanism
 DnaA is a protein which will bind to the Ori C at the
9nt region. To this protein complex, another complex
protein DnaB will bind. They together form a
prepriming complex.
 The uncoiling occurs due to a helicase, DnaB.
 But when the coiling stops there occurs a turgidity or
tensional stress which is reduced to Topoisomerase.
Topoisomerase is a type of endonuclease that also
induces a cut to one strand of DNA.
 DNA gyrase a type of topoisomerase prevents
supercoiling of DNA.

Mechanism
 DNA replication always occurs in the 5’-3’ direction.
 DNA Polymerase III creates links between the
nucleotide, it creates a strand that is complementary
to the original strand.

Mechanism
2. Elongation:
 The replicating fork now has a lagging strand 3’-5’
and a leading strand 5’-3’.
 DNA Polymerase III is the main replicating enzyme,
having 3’-5’ and 5’-3’ exonuclease activity.
 RNA Polymerase enzymes synthesize a RNA Primer.
This primer is attached to the lagging strand in the 3’-
5’ strand.
 After the primer gets attached, the DNA Polymerase
enzymes gets attached on the free 3’ end.
 The Okazaki fragments formation takes place in the
lagging strand.

Mechanism
 After RNA Primer is in place DNA Nucleotides are
added by DNA Polymerase III
 Eventually the segment of DNA will run into another
RNA Primer. The DNA segments are Okazaki
Fragments.
 The lagging strand is formed of small segments, the
okazaki fragments because replication occurs only on
5’-3’ direction.
 Once the fragments are formed DNA Polymerase I
replaces the RNA Primer with DNA nucleotide.
 Each fragment starts with a RNA Primer and the
fragments are joined by DNA ligase.
 The complementary strand formation takes place in
the leading strand 3’-5’.

Mechanism
 In the lagging strand DNA replication is discontinous
 DNA is replicated in segments that become joined
together.
 In the leading strand replication is continous.
 Helicase unwinds and unzips the double helix.

Mechanism
5’
5’
3’
3’
5’
3’
Parental DNA
5’
3’
5’
Okazaki fragments
Primer
Gap filled byDNA Ligase
Elongation by DNA Polymerase IIIPrimer is removed by DNA Polymerase I

Mechanism
3. Termination:
 For termination, first a termination point is appointed.
This is done by TUS protein.
 Termination happens when the DNA Polymerase
reaches to an end of the strands. The ends of linear
chromosomal DNA consists of non-coding DNA that
contains repeat sequences and are called telomeres.
As a result, a part of the telomere is removed in every
cycle of DNA replication.