DNA Replication

Document Sample
DNA Replication Powered By Docstoc
					             DNA Replication




AP Biology                     2007-2008
     Double helix structure of DNA




“It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic
  AP Biology
material.”                                                Watson & Crick
   Directionality of DNA
     You need to      PO4                       nucleotide
        number the
        carbons!
            it matters!                           N base

                              5 CH2
               This will be                  O
              IMPORTANT!!
                              4         ribose        1


                                   3             2
AP Biology
                                        OH
                                                          5
   The DNA backbone                         PO4
     Putting the DNA
        backbone together                                   base
                                           5 CH2

            refer to the 3 and 5                  O
                                           4                   1
             ends of the DNA                     C
                                                3         2
               the last trailing carbon       O
                                           –O P O


             Sounds trivial, but…
                                               O                base
                 this will be                5 CH2
                IMPORTANT!!                                O
                                                4                   1

                                                     3         2
                                                      OH
AP Biology                                                3
   Anti-parallel strands
    Nucleotides in DNA
       backbone are bonded from
       phosphate to sugar
       between 3 & 5 carbons        5   3
         DNA molecule has
          “direction”
         complementary strand runs
          in opposite direction
        THIS WILL CAUSE A
          PROBLEM FOR
          REPLICATION
AP Biology                            3   5
  Bonding in DNA
                              hydrogen
                               bonds
                    5                          3


      covalent
   phosphodiester
       bonds

                    3
                                                5


….strong or weak bonds?
How do
AP Biology the bonds fit the mechanism for copying DNA?
   Copying DNA
    Replication of DNA
         base pairing allows
          each strand to serve
          as a template for a
          new strand
         new strand is 1/2
          parent template &
          1/2 new DNA



AP Biology
                                      Let’s meet
                                      the team…
   DNA Replication
      Large team of enzymes coordinates replication




AP Biology
   Replication: 1st step
  Unwind DNA
         helicase enzyme
           unwinds part of DNA helix
           stabilized by single-stranded binding proteins
               PREVENTS DNA MOLECULE FROM CLOSING!
           DNA gyrase
               Enzyme that prevents tangling upstream from the replication
                fork
                                                helicase             gyrase




    single-stranded binding proteins
AP Biology                                        replication fork
   Replication: 2nd step
   Add RNA primer
         DNA BY RNA Primase
              Why must this be done?
                   DNA can’t be added to an existing strand of
                    nucleotides




AP Biology
                  Replication: 3rd step
                             Build daughter DNA
                              strand
                               add new
                                complementary bases
                               With the help of the

                                enzyme DNA
                                polymerase III
                                       But…
                                    Where’s the
                                   We’re missing
                                      ENERGY
                 DNA                 something!
                                  for the bonding!
             Polymerase III            What?

AP Biology
   Energy of Replication
   Where does energy for bonding usually come from?
                                 We come
                               with our own
                                 energy!
    You                                                  energy
 remember                                              energy
   ATP!
 Are there
 other ways
to get energy
  out of it?
      Are there
     other energy
     nucleotides?                         And we
       You bet!                       leave behind a
                      ATP
                      GTP
                      CTP
                      TTP               nucleotide!    GMP
                                                       AMP
                                                       ADP
                                                       TMP
                                                       CMP
AP Biology      modified nucleotide
   Energy of Replication
      The nucleotides arrive as nucleosides
            DNA bases with P–P–P
               P-P-P = energy for bonding
            DNA bases arrive with their own energy source
             for bonding: by breaking off two phosphate
             groups
            bonded by enzyme: DNA polymerase III




     ATP                GTP              TTP       CTP

AP Biology
   4th step
     Replacement of RNA primer by DNA
     Done by DNA polymerase I




AP Biology
   Before we solve the Problem. Lets
   review DNA replication




AP Biology
   Limits of DNA polymerase III
      DNA polyermase III can only add
       nucleotides to an existing strand




      DNA polymerase III can only add nucleotides
       to 3 end of a DNA strand
        WHY IS THAT A PROBLEM?




AP Biology
                                                                    Okazaki


         Leading & Lagging strands
Limits of DNA polymerase III
    can only build onto 3 end of
                                                                                      5
     an existing DNA strand


    3                               5
                                            3
                                                 5
                                                               3     5        5
                                                                                     3

                                                                           Lagging strand
                                                      ligase
                  growing       3
               replication fork
    5
                                                                           Leading strand



Lagging strand
                                                                                3
                                                                                     5


                                                                                      3
                                          DNA polymerase III
    Okazaki fragments
    joined by ligase                            Leading strand
AP Biology
     “spot   welder” enzyme                             continuous synthesis
AP Biology
    DNA replication on the lagging strand
RNA primer is added
 built by primase
 serves as starter sequence for DNA polymerase III

HOWEVER short segments called Okazaki fragments
  are made because it can only go in a 5 3 direction

                                                                       5


                                                       3     5       3
                                               5
                                          3
    3                              5

                 growing       3                                   primase
              replication fork           DNA polymerase III
    5

                                                                   RNA 5


 AP Biology                                                            3
         Replacing RNA primers with DNA
NEXT DNA polymerase I
    removes sections of RNA       DNA polymerase I
     primer and replaces with                               5

     DNA nucleotides                                        3


    3
                              5            ligase
                growing       3
             replication fork
    5

                                                      RNA   5


                                                            3

STRANDS ARE GLUED
TOGETHER BY DNA LIGASE
AP Biology
   Lagging strand DNA replication review




AP Biology
                                             Houston, we
                                            have a problem!
         Chromosome erosion
All DNA polymerases can
only add to 3 end of an           DNA polymerase I
existing DNA strand                                                 5

                                                                    3


    3
                              5
                growing       3
             replication fork      DNA polymerase III
    5

                                                              RNA   5


Loss of bases at 5 ends                                            3

in every replication
  chromosomes get shorter with each replication
 Biology
AP limit to number of cell divisions?
         Telomeres
Repeating, non-coding sequences at the end
of chromosomes = protective cap
                                                                      5
    limit to ~50 cell divisions
                                                                      3


    3
                                   5
                     growing       3                           telomerase
                  replication fork
    5

                                                                      5
Telomerase
                                                      TTAAGGG TTAAGGG 3
    enzyme extends telomeres
    can add DNA bases at 5 end
    different level of activity in different cells
AP Biology
          high in stem cells & cancers -- Why?
   Replication fork
                      DNA
                  polymerase III       lagging strand
     DNA
 polymerase I
                                                                    3’
                           Okazaki                  primase
                          fragments                                      5’
  5’           ligase
    3’                                             5’    SSB

                                                        3’     helicase

                                                DNA
                                            polymerase III
 5’          leading strand
  3’
                        direction of replication
AP Biology
                                   SSB = single-stranded binding proteins
    DNA polymerases
     DNA polymerase III
          1000 bases/second!                 Roger Kornberg
                                                   2006
          main DNA builder

      DNA polymerase I
          20 bases/second
          editing, repair & primer removal

DNA polymerase III                            Arthur Kornberg
     enzyme                                        1959




 AP Biology

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:24
posted:2/26/2012
language:English
pages:25