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DNA Replication Lesson 2 • Label the ends of DNA strand as either 5’ or 3’. 5’ Leading Strand DNA polymerase III SSBPs 3’ 5’ primase gyrase helicase 5’ 3’ 3’ The leading strand is synthesized continuously during DNA replication 5’ Lagging Strand 5’ 3’ 3’ 5’ The laggin strand is synthesized discontinuously during DNAreplication 3’ Okazaki fragment 5’ Connecting Lagging Strands • DNA polymerase I – removes the RNA primer • DNA ligase – connects the sugarphosphate backbone of Okazaki fragments Mistakes in DNA Replication • less than 1 error in 107 (10 million) NTs • exonuclease – enzymes which can cut out sections of the DNA strand • DNA polymerase I and III and exonuclease work together to fix mistakes 1 A distorted section of DNA. 2 A nuclease enzyme cuts the damaged DNA strand at two points and the damaged section is removed. Nuclease DNA polymerase 3 Repair synthesis by a DNA polymerase fills in the missing nucleotides. DNA ligase 4 DNA ligase seals the Free end of the new DNA To the old DNA, making the strand complete. Replicating the Ends of DNA Molecules 5 End of parental DNA strands Leading strand Lagging strand 3 Last fragment Previous fragment RNA primer Lagging strand 5 3 Primer removed but cannot be replaced with DNA because no 3 end available for DNA polymerase Removal of primers and replacement with DNA where a 3 end is available 5 3 Second round of replication 5 New leading strand3 New lagging strand 5 3 Further rounds of replication Figure 16.18 Shorter and shorter daughter molecules • The ends of eukaryotic chromosomal DNA – Get shorter with each round of replication • Eukaryotic chromosomals – Have repeated short nucleotide sequences, called telomeres at their ends that do not code for amino acids. – postpone the erosion of genes near the ends of DNA molecules (e.g. in humans 100-1,000 TTAGGG) Figure 16.19 1 µm • Some cells (like cervical cancer cells) have an enzyme called telomerase – Lengthens the telomeres, thus restoring original length and compensating for shortening that occurs during DNA replication. – Contributes to the “indestructible” nature of cancer cells Explain the function of each of the following enzymes in DNA replication: DNA gryase DNA helicase DNA polymerase I DNA polymerase III DNA ligase RNA primase Relieves the tension produced by the unwinding DNA during replication Unwinds the DNA double helix by disrupting hydrogen bonds between base pairs Removes RNA primers and replaces them with the appropriate deoxyribonucleotides (DNA bases) during DNA replication Makes complementary strands of DNA (adds deoxyribonucleoside triphosphates to the 3’ end of the elongating strand Joins DNA fragments together by catalyzing the formation of a bond between the 3’ hydroxyl group and a 5’ phosphate group on the sugar-phosphate backbones Makes RNA primers • Review from previous lessons – Pg. 198 Section Review - #2, 3, 7 (info from previous lessons) • Material from today’s lesson – Pg. 204 Section Review - #2 – Pg. 205 Understanding Concepts - #8, 9, 10, 15, 18, 25, 33, 42