DNA Replication PDF

DNA Replication Learning Goals: 1.To understand the importance of DNA replication to living organisms 2.To understand the roles that various enzymes play in DNA replication. 3.To understand the process of DNA replication that results in the leadin...

DNA Replication Learning Goals: 1.To understand the importance of DNA replication to living organisms 2.To understand the roles that various enzymes play in DNA replication. 3.To understand the process of DNA replication that results in the leading and lagging strands. AP Biology 2007-2008  DNA Replication ▪ Purpose: cells need to make a copy of DNA before dividing so each daughter cell has a complete copy of genetic information ▪ Happens during S phase of interphase ▪ Must be done correctly for cell growth, repair, and normal cell functioning. AP Biology  3 proposed models of replication AP Biology  Meselson and Stahl Experiment ▪ https://www.youtube.com/watch?v=JcUQ_TZCG0w 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  Enzymes involved in Replication ▪ Helicase (the unzipper) ▪ Topoisomerase (the unwinder) ▪ Primase (the initializer) ▪ DNA Polymerase III (the builder) ▪ DNA Polymerase I (the replacer) ▪ Ligase (the gluer) AP Biology  Step 1: DNA unwinds ▪ Unwind DNA ◆ Helicase enzyme ▪ unwinds part of DNA helix (it unzips by disrupting the H bonds) ▪ stabilized by single-stranded binding proteins ⬥ PREVENTS DNA MOLECULE FROM CLOSING! ▪ DNA topoisomerase ⬥ Enzyme that prevents tangling upstream from the replication fork helicase topoisomerase single-stranded binding proteins AP Biology replication fork  Step 2: Create a Primer ◆ RNA Primase ▪ Adds small section of RNA (RNA primer) to the 3’ end of template DNA on both parent strands ⬥ Why must this be done? DNA polymerase 3 (enzyme that builds new DNA strand) can only add nucleotides to existing strands of DNA in the 5’ to 3’ direction AP Biology  Step 3: Build the new Strand ▪ Build daughter DNA strand ◆ add new complementary bases starting from end of RNA primer ◆ With the help of the enzyme DNA DNA polymerase III Polymerase III ◆ Proofreads and makes AP Biology any corrections  Building the new strand ▪ When DNA polymerase III builds the new strand, it adds DNA nucleotides in the form of nucleoside triphosphates ▪ Cleaving the last 2 phosphate groups releases energy that drives the formation of the phosphodiester linkage between C3 of the previous nucleotide and phosphate AP Biology AP Biology  Step 4: Replace primer with DNA ▪ Replacement of RNA primer by DNA nucleotides ▪ Done by DNA polymerase I, and works to replace the primers on both strands ▪ Ligase works to form new phosphodiester linkages, linking the new “pieces” of DNA together AP Biology  Okazak i Leading & Lagging strands Limits of DNA polymerase III ◆ can only build onto 3′ end of an existing DNA strand 5′ ts 3′ Okaza 5′ ki f r 3′ ag m 5′ en 3′ 5′ 5′ 3′ Lagging strand ligase growing 3′ 5′ replication fork Leading strand Lagging strand ✔ 3′ 5′ 3′ DNA polymerase III ◆ Okazaki fragments ◆ joined by ligase Leading strand ▪ “spot AP Biology welder” enzyme ◆ continuous synthesis  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 replication 3′ primase 5′ fork DNA polymerase III 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′ 5′ replication fork 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 Comparing Leading to Lagging Strand ▪ Lagging strand ◆ Built discontinuously (in many pieces called Okazaki fragments) ◆ Built away from the fork ◆ Several RNA primers ▪ Leading Strand ◆ Built continuously (in one piece) ◆ Built towards the fork ◆ One RNA primer AP Biology  Mind Map: DNA Replication AP Biology