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Reminder: where we started CAMPBELL : exploring life, chapter 1.

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Presentation on theme: "Reminder: where we started CAMPBELL : exploring life, chapter 1."— Presentation transcript:

1 Reminder: where we started CAMPBELL : exploring life, chapter 1

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5 The cell cycle

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7 Development of an organism

8 DNA replication why we are talking about replication E. coli bacterium : single chromosome : 4.6 million nucleotide pair Human cell : 46 DNA molecules: 6 billion base pairs

9 Important questions for considering replication of double helix Two strands of DNA : high affinity >> created by various interactions present – Therefore…………. – A mechanism is required to open up the strands and get access to the bases

10 Unwind and over winding

11 Must be highly accurate Proof reading mechanism is required

12 Must be very rapid: Enzyme action

13 Components of replication

14 Replication at many sites: origin of replication at many sites in Eukaroytes New strand of DNA elongates

15 Three replication bubbles by Transmission electron microscopy

16 Catalysis of elongation DNA polymerases The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells

17 Elongation of new DNA strand

18 Antiparallel elongation Double helix Strands are antiparallel 5’---3’, 3’---5’ This antiparallel nature also should be reproduced in the newly formed double helix This is achieved by adding nucleotides only to the 3’ free end of a growing DNA strand, never to the 5’ end by DNA polymerase. Thus a new strand only elongate from 5’ to 3’direction. LEADING strand…

19 Incorporation of a nucleotide into a DNA strand : elongation step

20 What happened to the other template strand? Formation of LAGGING strand? 5’—3’ direction is mandatory for DNA POLIII to work DNA polIII works opposite to the replication fork Small fragments of DNA are added, OKAZAKI fragments (1000-2000 nucleotide long in E.coli), 100-200 in eukaryotes DNA ligase joins the sugar-phosphate bond of these fragments

21 DNA replication: leading and lagging strand Leading strand (DNA pol III) Lagging strands

22 Priming DNA synthesis: synthesis of lagging strand

23 Summary of DNA replication

24 Proofreading and repairing

25 Replicating the ends of DNA molecule


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