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The function of DNA is information transfer and storage

The function of DNA is information transfer and storage. 1. DNA is copied to more DNA in DNA replication 2. Gene expression i.e. Transcription- synthesis of RNA from only one strand of a double stranded DNA helix

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The function of DNA is information transfer and storage

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  1. The function of DNA is information transfer and storage 1. DNA is copied to more DNA in DNA replication 2. Gene expression i.e. Transcription- synthesis of RNA from only one strand of a double stranded DNA helix Translation- ribosome mediated synthesis of a polypeptide from a messenger RNA molecule

  2. DNA replication A polymerisation reaction with the typical 3 phases • initiation, • elongation, • termination

  3. Initiation occurs at replication origins

  4. Initiation • occurs at Ori C (origin of chrm replication) • a 245 bp region with 2 conserved sequences • 3 tandem 13bp repeats • 4 copies of a 9 bp sequence

  5. Dna A protein molecules with bound ATP bind to 9 bp repeats • Facilitated by HU protein • The 3 x 13 bp repeats are sequencially denatured to give the open complex • A complex formed by Dna B and Dna C bind to the melted region, Dna C is released • In the presence of SSB protein and Dna gyrase the Dna B helicase unwinds the DNA in preparation for priming and DNA synthesis

  6. Replication occurs at origins

  7. Elongation • Leading and lagging strand synthesis • Parental DNA is unwound (helicases) and topological stress removed (gyrases/topoisomerases) • Separated strand stabilised by SSB • Leading strand • Primase (DnaG protein) synthesises a short RNA primer at origin • DNA pol III builds complimentary strand from ds primer • continuous process • Lagging • Primase synthesizes many RNA primers along lagging strand • Each primer is extended by DNA pol III • Synthesis proceeds in 5' to 3' direction i.e. the direction opposite to the fork movement • Synthesis is discontinuous in the form of multiple Okazaki fragments

  8. lagging continued.......... • Synthesis continues until fragment extends as far as the primer of the previously added Okazaki sequence • Note both strands are synthesised by a single asymetric dimer of DNA pol III that moves in the direction of the replication fork • This is achieved in the case of the lagging strand by the DNA looping around the part of the dimer DNA pol III

  9. DnaB (helicase) and Dna G (primase) together form the primosome • 1000 nucleotides of new DNA added per second to each strand • RNA primers are removed (exonuclease activity) and replaced with DNA (polymerase) by DNA pol I and the remaining nick is sealled by a ligase using NAD as a cofactor

  10. Termination • Eventually the 2 relication forks of E.coli meet at a terminus region containing many copies of a 20 bp sequence called Ter • These sequences are binding sites for a protein called Terminal utilization substance (Tus) • Tus-Ter complex arrests the replication fork in one direction

  11. The other replication fork halts when they meet. • the few hundred base pairs in between the protein complexes are replicated by an as yet unknown mechanism resulting in 2 interlinked circular chromosomes • separation requires Topoisomerase IV • separate chrm are then segregated at cell division

  12. Proteins at the E coli replication Fork SSB binds to ssDNA and stabilizes it DnaB protein (helicase) DNA unwinding; primosome constituent Dna G protein (Primase) RNA primer synthesis; primosome constituent DNA pol III New strand elongation DNA pol I Excision of primers and filling of gaps DNA ligase Ligation DNA gyrase supercoiling (DNA topoisomerase II)

  13. DNA polymerases DNA pol I DNA pol II DNA pol III Mol.wt. (Daltons) 103,000 88,000 900,000 polymerizatn rate 16-20 7 250-1000 (nucleotides/second) 3' to 5' exonuclease activity yes yes yes 5' to 3‘ exonuclease activity yes no no Functions proof reading and repair replicatn

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