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  • 1.DNA Replication
    • BHOORO MAL KISHNANI
  • BSMT 3 RDYEAR
  • SDLS LNH

2. Introduction

  • It is the process that can duplicate the DNA of a cell
  • 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.
  • DNA Replication

3.

  • In the eukaryotes DNA is formed in two strands, each composed of units called Nucleotides.
  • The two strands look like two chains that form theDNA Double Helix .
  • TheDNA Replication Processis capable of opening the Double Helix and separating the two strands.
  • Then the two strands are copied

4.

  • Result two new DNA molecules are created.
  • The next step is the cell division.
  • After that a daughter cell is created.
  • In its nucleus lies a copy of the parental DNA.

5. 6. DNA Replication models

  • There are three possible models that describe the accurate creation of the daughter chains:
  • Semiconservative Replication
  • Conservative Replication
  • Dispersive Replication

7. Semiconservative Replication

  • DNA Replication would create two molecules.
  • Each of them would be a complex of an old (parental and a daughter strand).

8. Conservative Replication

  • The DNA Replication process would create a brand new DNA double helix made of two daughter strands while the parental chains would stay together.

9. Dispersive Replication

  • According to this model the Replication Process would create two DNA double-chains, each of them with parts of both parent and daughter molecules.

10.

  • The correct model isSemiconservative DNA Replicationwas proved by the experiment ofMeselson - Stahl .

11. Steps of DNA ReplicationStep 1

  • The first major step for theDNA Replicationto take place is the breaking of hydrogen bonds between bases of the two antiparallel strands.
  • The unwounding of the two strands is the starting point.
  • The splitting happens in places of the chains which are rich in A-T.

12.

  • That is because there are only two bonds between Adenine and Thymine (there are three hydrogen bonds between Cytosine and Guanine).
  • Helicaseis the enzyme that splits the two strands.
  • The initiation point where the splitting starts is called "origin of replication".
  • The structure that is created is known as " Replication Fork ".

13. 14. Step 2

  • One of the most importantsteps of DNA Replicationis the binding ofRNA Primasein the the initiation point of the 3'-5' parent chain.
  • RNA Primasecan attract RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due to the hydrogen bonds between the bases.
  • RNA nucleotides are the primers (starters) for the binding of DNA nucleotides

15. 16. Step 3

  • Theelongationprocess is different for the 5'-3' and 3'-5' template.
  • a) 5'-3' Template : The 3'-5' proceeding daughter strand -that uses a5'-3' template - is calledleading strandbecauseDNA Polymerase can "read" the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc).

17. 18.

  • b) 3'-5'Template : The3'-5' templatecannot be "read" by DNA Polymerase . The replication of this template is complicated and the new strand is calledlagging strand .
  • In the lagging strand the RNA Primase adds more RNA Primers.DNA polymerase reads the template and lengthens the bursts.

19.

  • The gap between two RNA primers is called " Okazaki Fragments ".
  • The RNA Primers are necessary for DNA Polymerase to bind Nucleotides to the 3' end of them.
  • The daughter strand is elongated with the binding of more DNA nucleotides.

20. 21. Step 4

  • In the lagging strand theDNA Pol I- exonuclease -reads the fragments and removes the RNA Primers.
  • 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).

22.

  • Each new double helix is consisted of one old and one new chain.
  • we call
  • semiconservative replication .

23. 24. Step 5

  • The laststep of DNA Replicationis theTermination .
  • This process happens when the DNA Polymerase reaches to an end of the strands.
  • When the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap (because there is no primer

25.

  • So, the end of the parental strand where the last primer binds isn't replicated.
  • These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are calledtelomeres .
  • As a result, a part of the telomere is removed in every cycle of DNA Replication.

26. Step 6

  • The DNA Replication is not completed before amechanism of repairfixes possible errors caused during the replication.
  • Enzymes likenucleasesremove the wrong nucleotides and the DNA Polymerase fills the gaps.

27. 28. Speed of DNA Replication

  • The Genome of complex eukaryotes is huge and the process of DNA Replication should be incredibly fast.
  • It is amazing that a Chromosome of 250 million pair of bases can be replicated in several hours.
  • The speed of DNA replicationfor the humans is about 50 nucleotides per second per replication fork (low speed comparing to the speed of the bacterial DNA Replication).

29.

  • 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).
  • The speed of DNA replicationin 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.

30. Enzymes of DNA Replication

  • Helicase : Unwounds a portion of the DNA Double Helix RNA Primase : Attaches RNA primers to the replicating strands. DNA Polymerase delta () : Binds to the 5' - 3' strand in order to bring nucleotides and create the daughter leading strand. DNA Polymerase epsilon () : Binds to the 3' - 5' strand in order to create discontinuous segments starting from different RNA primers.

31.

  • Exonuclease (DNA Polymerase I) : Finds and removes the RNA PrimersDNA Ligase : Adds phosphate in the remaining gaps of the phosphate - sugar backbone Nucleases : Remove wrong nucleotides from the daughter strand.

32.