DNA REPLICATION IN PROKARYOTES

1. Prokaryotic DNA
replication
Presented by;
Tahmina Mazumder
17/BBT/69

2. Contents
• Introduction
• Types of replication
• History
• Enzymes and proteins involved in the process
• Initiation
• Elongation
• Termination
• Conclusion
• Reference

3. Introduction
• The process of replication involves the formation of two
double helixes which are exactly identical to that of original
double helix.
• The process of replication occurs during interphase
between the two mitotic cycles.
• It especially occurs during the S phase of interphase.

4. Types of dna replication
• Warburg suggested that Watson crick model of DNA can be replicated
by three methods;
1. Conservative replication method: it means that the double
stranded molecule is conserved as such and a new copy is
synthesised from the old molecule.
2. Dispersive replication: in this type of replication the old dna
molecules are disintegrated and two new molecules are
synthesised.
3. Semiconservative replication: in this type the two strands of
dna separates and maintains their integrity, each will
synthesised from the pool of nucleotides its complementary
strand.The result would be a newly synthesised molecule that
would carry or conserve one of the two strands from the
parent molecule and the other strand will be entirely new.

6. History
• In 1958 the scientist named meselson and stahl performed an experiment
that provides a strong evidence to prove that DNA replication was
semiconservative in nature.
• They grew E.coli bacteria in a medium containing heavy nitrogen isotope
N15 , I.e, they used radiotracer technique.
• The DNA labelled with N15 is allowed to be replicated in a medium
containing N14. As the replication was semi conservative the generations of
DNA obtained should contain one of the dna strand labelled with N15 and
the other strand labelled with N14.
• The resulting molecule will have the density which is the intermediate
between N15 DNA and N14 DNA.
• E.coli cells are allowed to grow on N15 culture medium for about 14- cell
generations so that almost all the nitrogen (N14) in the DNA is replaced by
N15 .

8. Some important enzymes
and proteins involved in the
process.• DNA A: onset of initiation
• DNA B (helicase) : unwinds the dna molecule
• DNA C (helicase loader) : loads the dna helicase
• DNA G (dna primase): synthesises RNA primer
• DNA polymerase III
• DNA polymerase I
• clamp loaders
• Sliding clamp
• Ssb proteins: prevents the unwinded dna from rewinding
• RNA primer

9. Process of replication
• The process is carried out in 3 steps;
• Initiation
• Elongation
• Termination

11. Initiation of replication
• For the initiation of replication from Ori-C, dna A forms complex with the atp
molecule.
• This complex now combines at Ori- C region in order to promote the
unwinding of DNA in a region of 13 by sequences.
• The opening occurs from the ori c region to form a replication fork.
• During the unwinding, the excessive super coiling of DNA also occurs ahead
of the replication fork.
• In case of circular DNA molecule the problem of supercoiling is overcomes by
topoisomerase enzyme.
• Unwinding is followed by the synthesis of rna primers by dna primase or dna g
through its interaction with DNA’s thus further elongating by the DNA
polymerase III HOLOENZYME.

13. Elongation
• The elongation phase of replication includes two distinct but related operation; leading
strand synthesis and lagging strand synthesis.
• Synthesis of DNA by DNA polymerase occurs in 5’ —-> 3’ direction only.
• The parent dna is first unwind by dna helicase.
• Each separated strand is stabilised by ssb proteins.
• Leading strand synthesis is more straight forwRd of the two and begins with the
synthesis of primase at the replication origin.
• Deoxyribonucleotides are added to the primer by DNA polymerase III
• Lagging strand synthesis is accomplished in short Okazaki fragments.the synthesis of
each Okazaki fragment is quite complex in its coordination with the leading strand.
• The gaps of both the strands are then filled by the DNA polymerase I holoenzyme and
the nicks are closed by the enzyme ligase thus forming the leading and lagging strands.

15. Termination
• The two replication fork of the circular E. coli chromosome meet at the
terminus region containing multiple copies of a 20bp sequence called
ter(terminus) sequence.
• The ter sequence are arranged on the chromosome to create a sort of trap
that a replication fork can enter but cannot leave. The ter sequence function
as binding sites for a protein called tus(terminus utilisation substance)
• The tus-ter complex functions per replication cycle.
• When either replication fork encounters a functional tus ter complex, it halts.
The other fork halts when it meets the first arrested fork
• Separation of the cage aged circles in E. coli require topoisomerase type 2.
• The separated chromosome then segregated into two daughter strands.

17. Conclusion
• Dna replication in prokaryotes employs a large number of
proteins and enzymes, each of which plays a critical role
during the process. One of the key enzyme is the enzyme
DNA polymerase which adds nucleotides one by one to
the growing dna chain that are complementary to the
template strand.

19. Reference
• Www.courses//lumenlearning.com
• www.biotechnika.org

20. Thank you