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Published byMildred Tucker Modified over 9 years ago
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Reminder: where we started CAMPBELL : exploring life, chapter 1
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The cell cycle
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Development of an organism
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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
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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
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Unwind and over winding
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Must be highly accurate Proof reading mechanism is required
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Must be very rapid: Enzyme action
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Components of replication
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Replication at many sites: origin of replication at many sites in Eukaroytes New strand of DNA elongates
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Three replication bubbles by Transmission electron microscopy
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Catalysis of elongation DNA polymerases The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells
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Elongation of new DNA strand
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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…
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Incorporation of a nucleotide into a DNA strand : elongation step
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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
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DNA replication: leading and lagging strand Leading strand (DNA pol III) Lagging strands
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Priming DNA synthesis: synthesis of lagging strand
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Summary of DNA replication
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Proofreading and repairing
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Replicating the ends of DNA molecule
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