Untitled Flashcards Set

  • Two DNA strands spilt forming two template (parent) strands

  • Each daughter molecule has one new strand and one pre-existing strand

  • Origin of replication is rich in A-T base pairs

    • A-T base pairs are double hydrogen bonded so its easier to break

  • Bidirectional replication→ Two sides to replication fork due to circular fashion of chromosome

  • Parent strand shortens (moves down) as daughter strand grows

    • Move outward forming a loop

  • Eukaryotic chromosomes have multiple origins of replication

  • DNA polymerases synthesise DNA

  • E.Coli has five types of DNA polymerase including:

    • DNA polymerase I

    • DNA polymerase III

  • DNA polymerase needs all four deoxynucleoside triphosphates (dATP, dCTP, dGTP and dTTP), a template strand (region of single-stranded DNA)

    • A primer is also needed, short piece of nucleic acid base-paired to template strand, acts as start point (must have 3’OH group)

    • Innermost oxygen molecule on 3’ binds to innermost phosphate on deoxytriphosphate

    • Binding splits phosphates releasing energy

    • Inorganic pyrophosphate split even further releasing energy for further replication

  • 5’—>3’ exonuclease degrades strands

  • DNA polymerase recognises free site and binds, start degrading strand in front of it and also start synthesising to avaoid gap

  • 3’—>5’ exonuclease activity is for proofreading

DNA replication in E. coli

  1. Partial unwinding by helicase

  2. Synthesis of primers by primase

  • Unwinding DNA molecule with fixed ends introduces positive supercoils

  • Positive supercoils are removed by DNA gyrase (type ii topoisomerase)

  1. DNA synthesis by DNA polymerase iii

  2. Removal of RNA primer by nick translation by DNA polymerase i

  • Gap created due to missing phosphodiester bond

  1. Sealing gaps between Okazaki fragments (short sections of DNA) by DNA ligase

  • Leading strand synthesised continuously, DNA polymerase adds nucleotides to deoxyribose ended strand in 5’ to 3’ direction

  • Lagging strand is synthesised in fragments

    • Nucleotides cannot be added to phosphate end