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Dna replication

  1. Presented by: Aparna Gupta, Priya Maurya, Shiva Pardesi & Avishek Majumder M. Sc Biotechnology (2nd sem) DNA Replication
  2. 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.
  3. 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.
  4. Modes of DNA Replication
  5. 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.
  6. Modes of DNA Replication
  7. 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.
  8. Modes of DNA Replication
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. 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’.
  14. 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.
  15. 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
  16. 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.