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  1. 1. DNA Replication <ul><li>BHOORO MAL KISHNANI </li></ul><ul><li>BSMT 3 RD YEAR </li></ul><ul><li>SDLS LNH </li></ul>
  2. 2. Introduction <ul><li>It is the process that can duplicate the DNA of a cell </li></ul><ul><li>Every cell (of eukaryotes or prokaryotes) has one or more DNA (or RNA) polymer molecules that need to duplicate in order the cell duplication to take place. </li></ul><ul><li>DNA Replication </li></ul>
  3. 3. <ul><li>In the eukaryotes DNA is formed in two strands, each composed of units called Nucleotides. </li></ul><ul><li>The two strands look like two chains that form the DNA Double Helix . </li></ul><ul><li>The DNA Replication Process is capable of opening the Double Helix and separating the two strands. </li></ul><ul><li>Then the two strands are copied </li></ul>
  4. 4. <ul><li>Result two new DNA molecules are created. </li></ul><ul><li>The next step is the cell division. </li></ul><ul><li>After that a daughter cell is created. </li></ul><ul><li>In its nucleus lies a copy of the parental DNA. </li></ul>
  5. 6. DNA Replication models <ul><li>There are three possible models that describe the accurate creation of the daughter chains: </li></ul><ul><li>Semiconservative Replication </li></ul><ul><li>Conservative Replication </li></ul><ul><li>Dispersive Replication </li></ul>
  6. 7. Semiconservative Replication <ul><li>DNA Replication would create two molecules. </li></ul><ul><li>Each of them would be a complex of an old (parental and a daughter strand). </li></ul>
  7. 8. Conservative Replication <ul><li>The DNA Replication process would create a brand new DNA double helix made of two daughter strands while the parental chains would stay together. </li></ul>
  8. 9. Dispersive Replication <ul><li>According to this model the Replication Process would create two DNA double-chains, each of them with parts of both parent and daughter molecules. </li></ul>
  9. 10. <ul><li>The correct model is Semiconservative DNA Replication was proved by the experiment of Meselson - Stahl . </li></ul>
  10. 11. Steps of DNA Replication Step 1 <ul><li>The first major step for the DNA Replication to take place is the breaking of hydrogen bonds between bases of the two antiparallel strands. </li></ul><ul><li>The unwounding of the two strands is the starting point. </li></ul><ul><li>The splitting happens in places of the chains which are rich in A-T. </li></ul>
  11. 12. <ul><li>That is because there are only two bonds between Adenine and Thymine (there are three hydrogen bonds between Cytosine and Guanine). </li></ul><ul><li>Helicase is the enzyme that splits the two strands. </li></ul><ul><li>The initiation point where the splitting starts is called &quot;origin of replication&quot;. </li></ul><ul><li>The structure that is created is known as &quot; Replication Fork &quot;. </li></ul>
  12. 14. Step 2 <ul><li>One of the most important steps of DNA Replication is the binding of RNA Primase in the the initiation point of the 3'-5' parent chain. </li></ul><ul><li>RNA Primase can attract RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due to the hydrogen bonds between the bases. </li></ul><ul><li>RNA nucleotides are the primers (starters) for the binding of DNA nucleotides </li></ul>
  13. 16. Step 3 <ul><li>The elongation process is different for the 5'-3' and 3'-5' template. </li></ul><ul><li>a) 5'-3' Template : The 3'-5' proceeding daughter strand -that uses a 5'-3' template - is called leading strand because DNA Polymerase ä can &quot;read&quot; the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc). </li></ul>
  14. 18. <ul><li>b) 3'-5'Template : The 3'-5' template cannot be &quot;read&quot; by DNA Polymerase ä. The replication of this template is complicated and the new strand is called lagging strand . </li></ul><ul><li>In the lagging strand the RNA Primase adds more RNA Primers. DNA polymerase å reads the template and lengthens the bursts. </li></ul>
  15. 19. <ul><li>The gap between two RNA primers is called &quot; Okazaki Fragments &quot;. </li></ul><ul><li>The RNA Primers are necessary for DNA Polymerase å to bind Nucleotides to the 3' end of them. </li></ul><ul><li>The daughter strand is elongated with the binding of more DNA nucleotides. </li></ul>
  16. 21. Step 4 <ul><li>In the lagging strand the DNA Pol I - exonuclease -reads the fragments and removes the RNA Primers. </li></ul><ul><li>The gaps are closed with the action of DNA Polymerase (adds complementary nucleotides to the gaps) and DNA Ligase (adds phosphate in the remaining gaps of the phosphate - sugar backbone). </li></ul>
  17. 22. <ul><li>Each new double helix is consisted of one old and one new chain. </li></ul><ul><li>we call </li></ul><ul><li>semiconservative replication . </li></ul>
  18. 24. Step 5 <ul><li>The last step of DNA Replication is the Termination . </li></ul><ul><li>This process happens when the DNA Polymerase reaches to an end of the strands. </li></ul><ul><li>When the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap (because there is no primer </li></ul>
  19. 25. <ul><li>So, the end of the parental strand where the last primer binds isn't replicated. </li></ul><ul><li>These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are called telomeres . </li></ul><ul><li>As a result, a part of the telomere is removed in every cycle of DNA Replication. </li></ul>
  20. 26. Step 6 <ul><li>The DNA Replication is not completed before a mechanism of repair fixes possible errors caused during the replication. </li></ul><ul><li>Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps. </li></ul>
  21. 28. Speed of DNA Replication <ul><li>The Genome of complex eukaryotes is huge and the process of DNA Replication should be incredibly fast. </li></ul><ul><li>It is amazing that a Chromosome of 250 million pair of bases can be replicated in several hours. </li></ul><ul><li>The speed of DNA replication for the humans is about 50 nucleotides per second per replication fork (low speed comparing to the speed of the bacterial DNA Replication). </li></ul>
  22. 29. <ul><li>But the human Genome can be copied only in a few hours because many replication forks take place at the some time (multiple initiation sites). </li></ul><ul><li>The speed of DNA replication in bacteria is much longer (about 1000 nucleotides per second) and that is a reason why during the process of bacterial replication the rate of errors is much higher. </li></ul>
  23. 30. Enzymes of DNA Replication <ul><li>Helicase : Unwounds a portion of the DNA Double Helix RNA Primase : Attaches RNA primers to the replicating strands. DNA Polymerase delta (ä) : Binds to the 5' - 3' strand in order to bring nucleotides and create the daughter leading strand. DNA Polymerase epsilon (å) : Binds to the 3' - 5' strand in order to create discontinuous segments starting from different RNA primers. </li></ul>
  24. 31. <ul><li>Exonuclease (DNA Polymerase I) : Finds and removes the RNA Primers DNA Ligase : Adds phosphate in the remaining gaps of the phosphate - sugar backbone Nucleases : Remove wrong nucleotides from the daughter strand. </li></ul>