U3L3 - DNA Replication - Specifics copy DNA Replication DNA helicase replication fork Step 1: As DNA

  • View
    4

  • Download
    0

Embed Size (px)

Text of U3L3 - DNA Replication - Specifics copy DNA Replication DNA helicase replication fork Step 1: As DNA

  • DNA Replication

    The Specifics

  • - Before DNA replication can occur, highly coiled and condensed DNA molecule must be “straightened” into linear sequence of nucleotidesEnzymes serve this function.- Enzymes serve this function

    DNA Replication The Specifics

  • DNA Replication

    DNA helicase

    replication fork

    Step 1: As DNA uncoils and unwinds, a class of enzymes called topoisomerases relieve tension on molecule

    Step 2: DNA helicase breaks hydrogen bonds that hold two strands together - produces a “replication fork”

    Topoisomerase

  • DNA Replication

    Step 4: RNA primase attaches RNA nucleotides (primers) to 3´ end of each strand

    RNA primers

    3´ 5´

    Single stranded binding proteins (SSBP) attach to each strand to prevent hydrogen bonds from reforming

    Step 3:

    SSBP

  • DNA Replication Step 5:

    3´ 5´

    DNA polymerase III5´

    Starting at primers DNA polymerase III adds complimentary DNA nucleotides to each strand

    DNA polymerase III constructs new strand in 5

    ´-3´ direction only

  • DNA Replication 3´

    3´ 5´

    NOTICE: One strand (leading) is built continuously towards replication fork

    Cont inuou

    s

    Other strand (lagging) is built discontinuously away from replication fork

    Discontinuous

    3´ 5´

    Okazaki fragments Lagging strand is made up

    of short pieces called Okazaki fragments

  • DNA polymerase I replaces RNA primers with DNADNA ligase joins the Okazaki fragments.

    Okazaki fragments

    DNA Replication 3´

    3´ 5´

    Step 6:

    DNA ligase

    DNA ligase joins Okazaki fragments DNA polymerase I

    Error Correction: Most occurs during Step 5 when DNA polymerase III is adding complimentary DNA nucleotides

    Step 7:

  • T C

    DNA Replication

    T

    AT CGAG GATG GCCCT

    GACTC

    3´5´ 5´3´ ACCGGGA

    New strand is made from 5’ to 3’

    1000 nucleotides added per second in bacteria

    50 nucleotides added per second in Eukaryotes

    When this happens, enzyme usually backs up and corrects error, before continuing on

    Step 7: Error Correction: Most occurs during Step 5 when DNA polymerase III is adding complimentary DNA nucleotides Occasionally, DNA polymerase III will make a mismatch error

  • DNA Replication

    C T

    AT CGAG GTG GCCCT

    GACTC

    3´5´ 5´3´ ACCGGGA

    A

    G

    Uncorrected Mismatch Error

    Step 7: Error Correction: Most occurs during Step 5 when DNA polymerase III is adding complimentary DNA nucleotides Occasionally, DNA polymerase III will make a mismatch error

    About one error in a million base pairs is not corrected by DNA polymerase III

    When this happens, enzyme usually backs up and corrects error, before continuing on

  • DNA Replication

    C CAC

    T

    AT CGAG GTG GCCCT

    GACT

    3´5´ 5´3´ CGGGA

    G

    A

    Special DNA repair complexes made up of proteins and enzymes, including DNA polymerases I and DNA polymerase II, remove a small section of nucleotides around error

    Step 7:

  • C

    DNA Replication

    CAC T

    AT CGAG GTG GCCCT

    GACT

    3´5´ 5´3´ CGGGA

    A

    T

    Special DNA repair complexes made up of proteins and enzymes, including DNA polymerases I and DNA polymerase II, remove a small section of nucleotides around error

    Step 7:

    Missing section is replaced by DNA polymerase IIId by DNA ligase.

  • C

    DNA Replication

    CAC T

    AT CGAG GTG GCCCT

    GACT

    3´5´ 5´3´ CGGGA

    A

    T

    This phosphodiester bond is missing

    Missing phosphodiester bond of last nucleotide is joined by DNA ligase

    Special DNA repair complexes made up of proteins and enzymes, including DNA polymerases I and DNA polymerase II, remove a small section of nucleotides around error

    Step 7:

    Missing section is replaced by DNA polymerase IIId by DNA ligase.