replicon is a unit of replication

1. replicon Concept and enzymes used in
replication
Mrs. Praveen Garg
VITS College, Satna

2.  A replicon is DNA molecule containing an origin of
replication region essential for initiate to replication.
 Replicon is a segment of chromosome that can replicate
with its own initiator and termination sequence, located
in chromosome.
 One replicon has one origin of replication and
termination sequences.
 Jacob, Brenner and Cuzin developed a replicon model as
the unit in which the cell controls or regulate individual
acts of replication.
 It is also known as unit of replication.
INTRODUCTION

3.  The number of replicon per chromosome varries and
depend on the size of the chromosome:
Single copy replication control:
 A control system in which there is only one copy of a
replicon present in it.
 Bacterial chromosome and plasmid have single replicon
per chromosome.
Multiple copy replication control:
 A control system in which there are multiple replicon
present in it.
 Eukaryotic chromosome have larger and multiple
replicon per chromosome.

4. • Replicon concept is given by Jacob et al., in 1963.
• The replicon initiates and completes synthesis once per cell
cycle.
• For this method, an initiator protein interacted with a DNA
sequence, called a replicator, to start replication.
• The replicator can be identified genetically as a DNA
sequence required for replication, whereas the origin is
defined by physical or biochemical methods as the DNA
sequence at which replication begins.
• Initiator proteins have now been identified for some
replicons, such as the DnaA protein in E. coli and the Origin
Recognition Complex in the yeast Saccharomyces cerevisiae.
CONCEPT

5. • Replicons can be linear or circular.
• Replicated region appear as eye within non replicated
DNA.
• Replication fork is initiated at the origin and then moves
sequentially along DNA.
• Replicon is unidirectional when a single replication fork
is created at an origin.
• Replicon is bidirectional when an origin creates two
replication forks that move in opposite directions.

7. Prokaryotic Replicon
• A Bacterial genome is usually have a single circular
replicon that replicate bidirectionally from a single
origin.
• The origin of E.Coli is oriC, is 245bp in length.
• Two replication fork met around the circle but there are
ter site that cause termination.

9. • Eukaryotic genome have a multiple replicon (many oriC)
region, each with its own origin.
• The progression into S phase is tightly controlled.
• Individual replicon are activated at perticular time in S
phase.
• Eukaryotic replicon are 40 to 100kb in length.
• Initiation of DNA synthesis in a replicon is highly
regulated.
• Replication in higher eukaryotes depend upon position of
nucleosome, type of histone modification, DNA
methylation etc.
Eukaryotic Replicon

11. ENZYMES USED IN REPLICATION
• Replication is a process in which each DNA strand
synthesized its own complementary DNA.
• There are various enzymes used in the process of replication.
• The function of all enzymes is mostly similar to prokaryotes
and eukaryotes.
• But the name of all enzymes is different in both.

12. Prokaryotic enzymes Eukaryotic enzymes
Dna A Helicase
Dna B Single standed binding protein
Dna C DNA polymerase III
Dna G DNA polymerase I
Gyrase Topoisomerase
Ligase Sliding clamp
Clamp loader

13. Dna A
• It bind to 9 mer sequence and unwind to 13 mer
sequences present at oriC site and forms an open
complex during initiation of replication.
• It is ATP requiring process, cause double stranded DNA
to melt forming ssDNA.
• It is first protein that bind to DNA to initiate replication.

14. • It is primosome constituents and consist of six subunits.
• It unwinds DNA during replication.
• It is responsible for the extension of open complex
during replication.
• It is essential for binding of Dna B protein to DNA
strands.
• Its help to the formation of two bidirectional replication
fork.
• After start to replication, it remove from the binding site.
Dna B
Dna C

15. • Helicase enzyme often used to separate double stranded
DNA molecule.
• This enzymes break the hydrogen bond between two strand
using the energy by ATP hydrolysis.
• These are protein involved in unwinding of DNA molecule.
• It is hexamer protein and move with the replication fork.
• There are four type of helicase: Dna A, Dna B, Rep A
protein, and Helicase II.
• The role of helicase is to unwind the duplex DNA and
provide single stranded DNA for replication.
DNA Helicases

16. • It is called as DNA unwinding protein.
• Single stranded binding protein (SSB) bind to single
stranded DNA to prevent annealing and to protect the
DNA from being digested by nucleases.
• It is found in both prokaryotes and eukaryotes.
• It is tetramer structure, contain four sub unit.
• The major function of SSB is to prevent recoiling of DNA
strand after its unwinding by helicases.
• Its stabilized the single stranded DNA.
• Its play major role in replication.
Single Stranded Binding Protein

18. • Primase enzyme involved in replication.
• DNA primase is a type of RNA polymerase which creates
an RNA primer.
• DNA polymerase use this primer to replicate template
ssDNA.
• Primase catalyzes the synthesis of short RNA segment
called a primer complementary to ssDNA molecule.
• Primase play important role in replication because no
known DNA polymerase can initiate the synthesis of
DNA strand without an initial RNA or DNA primer.
• After completing replication, RNA primers is removed by
5’- 3’exonucleases.
DNA Primase (Dna G)

19. • Gyrase is an enzyme that relieves strain while double
stranded DNA is unwound by helicase.
• This cause negative supercoiling of DNA.
• Negative supercoiling of bacterial DNA by gyrase
influences all the metabolic processes essential for
replication.
• The ability of gyrase to relax positive supercoiling during
DNA replication.
• Its work similar to topoisomerase II found in eukaryotes.
DNA Gyrase

21. • DNA ligase are responsible
for connecting DNA
segment during replication.
• It catalyze the
phosphodiester bond
between two DNA segment.
• It required to free OH
group at 3’end of one DNA
and Phosphate group at 5’
end of other DNA for
creating phosphodiester
bond.
• It is energy dependent
process.
DNA Ligase

22. • It is responsible for removing supercoils in the helix.
• There are two types of topoisomerase:
DNA Topoisomerase I:
 Reversibly cut a single stranded of double helix.
 They have both nuclease (strand cutting) and ligase
(sealing) activity.
 They do not require ATP.
DNA Topoisomerase II:
 It tightly bind to DNA double helix and break in both
strand.
 It pass through the break and reseal the break.
 It introduce negative supercoils using energy as ATP.
Topoisomerase

24. • DNA is replicated by enzyme DNA polymerases.
• These enzyme utilize ssDNA as template on which
complementary strand is synthesized using dNTPs.
• DNA polymerase require a free 3’OH group as a primer for
synthesizing a new DNA strand.
• DNA polymerase synthesize DNA in 5’-3’ direction.
• It is categorized into three types in prokaryotes.
• DNA Polymerase I: Remove RNA primer or proof reading.
• DNA Polymerase II: Help in DNA repair.
• DNA Polymerase III: Add nucleotide in 5’- 3’ direction.
DNA Polymerase

25. It is classify in to six types in eukaryotes.
DNA polymerase α: Initiate replication and
synthesize primers
DNA polymerase β: replication with low fidelity
DNA polymerase γ: polymerization in mitochondria
DNA polymerase δ: elongation
DNA polymerase ε: proofreading and filling gap

26. • Slinding clamp hold DNA polymerase and prevent this
enzyme from dissociate from the template DNA strand.
• Slinding clamp bind with DNA by the help of clamp
loader.
• This is energy dependent process.
Slinding Clamp

27. THANK YOU