DNA Replication Lecture 2

DNA Replication Lecture 2 Éilliking H bond phosph odiester bond 5 final strand Ippolymerase th TY III II 00 sing strand 8807 88 n bonds Jl'd Helicase FWIW.NO w33 b.wj4 H bondage int d wigwag Singh strand binding protein RIA Midas's free ends We Primaraxd 3 5 m DNA N i PolyPte...

DNA Replication Lecture 2 Éilliking H bond phosph odiester bond 5 final strand Ippolymerase th TY III II 00 sing strand 8807 88 n bonds Jl'd Helicase FWIW.NO w33 b.wj4 H bondage int d wigwag Singh strand binding protein RIA Midas's free ends We Primaraxd 3 5 m DNA N i PolyPtertase E leading Strand 1 Helicase 2 Replication fork 3 Primase RNA primers AT T N Primer 4 DNA Polymerase Ta f f leading strand Okazaki fragments Primer if primase 1121 Helicases imbN Primers signs 5 3 DNA polymerase 99 by exonuclease DNA ligase At the end of this lecture, you need to understand: • The process of DNA replication. • The role of different enzymes used in DNA replication. • Replication of eukaryotic telomeres, the role in aging and cancer • Semi -conservative = each one of the parent DNA strands is passed to the daughter DNA + one new strand for each DNA replication starts from middle Intime'S'm In cell cycle, DNA replication takes place in S phase, in the nucleus of eukaryotic cells . E O Origins of Replication • sites along the DNA molecule where enzymes start the DNA replication - then proceeds in both directions to form “bubbles” DNA Replication Bubbles : DNA duplicates in both directions eventually all meet to form whole replicated strand 00 EM of DNA replication man A DNA Helicase • The enzyme unwinds the chain and breaking the H - bonds between the complementary base pairs (A - T, G - C). o single strand binding proteins and topoisomerases • SSB’s Stabilize the DNA strands as they are being replicated, prevent rejoining of DNA strands. • Topoisomerases relieves the stress of strands unwinding. I E o Primase Produces RNA nucleotides into a primer D RNA Primer • Nucleotides for the starting point for DNA replication • Short strands of RNA, temporary Ios Replication Forks Y -shaped regions of replicating DNA molecules where new strands are growing. o DNA Polymerases • DNA Polymerase III • Elongates the strand by adding DNA nucleotides on leading strand in the direction of 5` to 3` • Also proofreads and corrects the DNA strand • DNA Polymerase I • Cuts off RNA primers and fills in with DNA (between Okazaki fragments) – lagging strand • Can proofread 1St bond H bond between the bases Polymerase A E S Primer sunt I 4 long short made from and move Okazaki segments with helicase in the opposite direction of DNA helicase Anti - parallel strand builds in the opposite direction (but always in 5’ to 3’ direction) o Leading Strand Lagging Strand • Template strand of DNA • Continuous addition of nitrogenous bases • in 5’ to 3’ direction • Other DNA strand • Forms short strands of Okazaki fragments (that will be joined later) • in the 5’ to 3’ direction • OKAZAKI FRAGMENTS • The short strands of newly made DNA fragments on the lagging strand are called Okazaki fragments after the Japanese Biochemist Reiji Okazaki. • Enzyme: DNAEnzyme: DNA LigaseLigase a linking enzyme joins the strands Example: joining two Okazaki Example: joining two Okazaki fragments together.fragments together. Lagging Strand Okazaki Fragment 2Okazaki Fragment 2 DNA ligaseDNA ligase Okazaki Fragment 1Okazaki Fragment 1 5 ’ 5 ’ 3 ’ 3 ’ DNA LIGASE is the enzyme that joins the Okazaki fragments (sugar - phosphate backbone) with covalent bonds My Ka How Fast? • Prokaryotic DNA polymerase can work at about 1000 bases per second . • Eukaryotic DNA polymerase can work at about 50 bases per second . Telomeres • At the ends of each chromosome is a protective cap called a telomere. • In differentiated cells, each time a cell divides, the telomeres are snipped shorter É Telomerase • enzyme which adds DNA sequence repeats ("TTAGGG" in all vertebrates) to the 3' end of DNA strands (an overhang ). x