DNA Replication Lecture 2
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RCSI Bahrain
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Summary
This document is a lecture on DNA replication, focusing on the various enzymes involved and the intricacies of the process. It describes the role of enzymes like Helicase, Primase, and DNA Polymerases. The lecture also covers eukaryotic telomeres, their role in cell division and aging.
Full Transcript
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