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Prokaryotic DNA Replication

Prokaryotic DNA Replication. By Rajani Yadav M.Sc.(Prev.). http://powerpointpresentationon.blogspot.com. DNA structure

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Prokaryotic DNA Replication

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  1. Prokaryotic DNA Replication By Rajani Yadav M.Sc.(Prev.) http://powerpointpresentationon.blogspot.com

  2. DNA structure DNA is a double helical structure. The two strands in the DNA backbone run anti-parallel to each other. One of the DNA strands is built in the 5' → 3' direction, while the complementary strand is built in the 3' → 5' direction.Nucleotide Forms backbone of DNA. When a nucleotide base forms hydrogen bonds with a complementary base on the other strand, they form a base pair: Adenine pairs with thymine and cytosine pairs with guanine. These pairings are often expressed with C:::G and A::T where the dots indicate hydrogen bonds.

  3. Introduction The process by which a parent molecule synthesizes two daughter DNA molecules is called as “Replication”. This process is important in all known life forms and the general mechanisms of DNA replication are not the same in prokaryotic and eukaryotic organisms. Theoretically replication of double stranded DNA can be conservative, dispersive or semi-conservative.

  4. Conservative mode Of Replication Here a parent DNA molecule synthesizes two DNA molecules out of them, one is the conserved copy of parent DNA and the other is completely new. Dispersive Mode of Replication Here both the strands of parent DNA break into several fragments. Each broken fragment synthesizes its own new complementary copies. Later on all the fragments unite together and result in the formation of two daughter DNA molecule. Semi-conservative mode of Replication Here both the strands of DNA start separating from each other. Both the separated strands synthesizing its complementary copy. Finally two daughter DNA molecules are formed and each of them contain one old and one new strand.

  5. Mechanism Of DNA Replication DNA replication in E. coli is originates at a single origin of replication (OriC). Initiation of replication in double stranded DNA Molecule: The initiation of replication is mediated by DnaA, a protein that binds to a region of the origin known as the DnaA box In E. coli, there are 4 DnaA boxes, each of which contains a highly conserved 9bp sequence 5' - TTATCCACA - 3'.

  6. Initiation Of Replication Binding of DnaA to this region causes it to become negatively supercoiled. The enzyme that control degree of supercoiling is Topoisomerase. Dna A protein separates the strand at 13 mers region by the help of HU and IHF proteins. Due to this open complex forms at 13 mers.

  7. Diagram showing SSB proteins

  8. In order for DNA replication to continue, SSB is needed to prevent the single strands of DNA from forming any secondary structures and to prevent them from reannealing. Unwinding is followed by the synthesis of RNA primer by Dna G Primase. Primer during DNA Replication in both Prokaryotes and Eukaryotes are short RNA molecules. The RNA primer synthesis is complementary to both strands of the duplex DNA.The main purpose of synthesis of RNA primase is to provide free 3’OH group to DNA polymerase III. The enzyme is considered as main replication enzyme.

  9. B) Elongation : Leading Strand: The strand which synthesizes continuously in 5’ to 3’ direction is called leading strand. Leading strand always moves in same direction at the movement of replication fork. Chain elongation takes place by an enzyme DNA Polymerase III, This enzyme attracts the complementary bases & forms bonds between nucleotides. In this way chain elongation takes place step by step on leading strand. Lagging Strand: Synthesis of lagging strand is more complicated because DNA polymerase add bases only to the 3’ end of DNA strand. Since synthesis of DNA strand occurs from 3’ to 5’ direction due to which small okazaki fragments are formed. These fragments contains a short RNA primer at 5’ end.

  10. Digestion of RNA Primer: To complete the synthesis of okazaki fragments within the lagging strand three additional events occur. Removal of RNA primer by enzyme DNA polymerase I Synthesis of DNA by polymerase I in the area where primer have been removed. Covalent attachment by DNA ligase. C) Termination : Termination of DNA replication in E. coli is completed through the use of termination sequences and the Tus protein or TBP (Termination Binding Protein). These sequences allow the two replication forks to pass through in only one direction, but not the other.

  11. THANKS

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