DNA Replication PDF

DNA Replication AP Biology 2007-2008 AP Biology  DNA Replication  Purpose: cells need to make a copy of DNA before dividing so each daughter cell has a complete copy of genetic information  3 proposed Models of Replication AP Biology  Mese...

DNA Replication AP Biology 2007-2008 AP Biology  DNA Replication  Purpose: cells need to make a copy of DNA before dividing so each daughter cell has a complete copy of genetic information  3 proposed Models of Replication AP Biology  Meselson and Stahl Experiment AP Biology  Semi-Conservative Model  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  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 AP do the bonds fit the mechanism for copying DNA? Biology  DNA Replication  Large team of enzymes coordinates replication AP Biology 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  RNA Primase  Adds small section of RNA (RNA primer) to the 3’ end of template DNA  Why must this be done? DNA polymerase 3 (enzyme that builds new DNA strand) can only add nucleotides to existing strands of DNA AP Biology  Replication: 3rd step  Build daughter DNA strand  add new complementary bases  With the help of the enzyme DNA polymerase III DNA Polymerase III AP Biology  Replication: 4th step  Replacement of RNA primer by DNA  Done by DNA polymerase I 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 AP Biology  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  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 AP Biology