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  Sequential Process of Replication

Sequential Process of Replication:

Initiation of DNA replication:
Initiation of DNA synthesis takes place at a site known as origin of replication.

  • In prokaryotes, just only one origin.
  • In eukaryotes, there are more than one origins.
  • The origin contains a short sequence of A = T base pairs.


Replication bubbles:

The two complementary strands of DNA divide at a site of replication to make a bubble. In eukaryotes several replication bubbles take place.

Unwinding of parental DNA:

Later than the initiation point is located the unwinding of the DNA occurs once in every 10 nucleotide pairs. This permits the “strand separation”.

In prokaryotes the unwinding of the structure takes place along with the help of an enzyme called helicase that needs energy of hydrolysis of 2 ATP molecules per base pair broken. Other protein single strand binding protein that is abbreviated as SSB, binds to the unwound DNA to prevent (rewinding) rejoining.

When the DNA polymerase cannot initiate replication a primer is a required and the primer is the small nucleotide of RNA, synthesised via enzymes primase.

1735_initiation of replication.jpg


                                                                   Diagram: Initiation of Replication

Under the affect of DNA polymerase, in the existence of Mg2+, the double strands of the DNA working like a template (or primer) separate through cleaving the hydrogen bonds among the complementary bases. In the cellular sap the deoxyribo nucleoside triphosphates are attracted from solution to create hydrogen bonds along with their complementary bases on the separated strands of the (primer RNA) template dictates the sequence where the monomers are assembled.
Polymerisation

Throughout this reaction, every incoming nucleotide loses a pyrophosphate group and creates an ester linkage along with the 3' hydroxyl group of the deoxyribose on the previously exists last nucleotide. This linkage is known as “phosphodiester linkage”.

 

1689_replication fork.jpg

 
                                                              Diagram: Replication Fork

The parental strands run in the direction of antiparallel. Synthesis takes place concurrently on both strands, but at dissimilar rates. No enzymes can synthesize 3' → 5' direction and a single enzyme cannot synthesize both of the strands. The single enzyme replicates one strand that is known as leading strand in a continuous way in 5' to 3' direction (forward), it replicates the another strand, lagging strand in a discontinuous way and polymerising just only few (250) nucleotides again run in 5' to 3' at backward direction. This is known as semi continuous DNA synthesis. The recently synthesized DNA is made like discontinuous small fragments known as Okazaki fragments and joined through the enzyme known as ligase.
So the 2 daughter double helices are formed each contains an old strand of the primer DNA and a complementary new strand. The last composition and nucleotide sequence of every strand is similar with the subsequent strand in the template (parent) DNA. This procedure has been termed as replication.

 

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