DNA Replication and Repair

DNA replication and repair Daughter “ new “ strand Parental strand Hereditary information is encoded in DNA and reproduced in all cells of the body DNA Review Prokaryotic origins of replication • DNA Replication : Semiconservative model •• Eukaryotic BB replication forks replication “bubble” This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits → ion > Direction of replication . •• DNA is a polymer of nucleotides ,each consisting of a Start nitrogenous base, a sugar, and a phosphate group I DNA replication The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C) The copying of DNA is called DNA replication A The copying of DNA is remarkable in its speed and accuracy : : : ble” “bub icat repl ✓ ④ Synthesizing a New DNA Strand ✓ replication forks • c- j •!④• • • → → • → ← • • replication “bubble” v . Antiparellel The monomer - Nucleotide / Phosphate + Deoxyribose + nitrogenous Group Sugar base ON so • ( Three hydrogen bonds Bs . i. Two hydrogen bonds - In DNA replication, the parent molecule unwinds, and two new Boog " Since the two strands of DNA are complementary, each strand acts as a template ÷ two exact replicas of the "parental" molecule Parental molecule > Daughter “ new “ strand Parental strand 3 Antiparallel Elongation The lagging strand is synthesized as a series of segments called - ' scratch and adds RNA nucleotides one at a time using the parental DNA leading strand • • RNA primer for leading strand DNA polymerase . the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork - Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate Nucleotides added dATP, dTTP, dCTP and dGTP. DNA polymerase Parental DNA Sliding clamp Parental DNA Helicases -5 . BBB DNA ligase forms bonds between DNA fragments Proofreading and Repairing DNA Replicating the Ends of DNA Molecules 31 - There is no 3′end of a pre existing polynucleotide . Thus, repeated rounds of replication produce for DNA polymerase to add on to • - b- b- ⑧ - b- direction of 5’ to 3’ . ' ~ Parental strand Bazooka oh The primer will be removed and DNA polymerase won’t be able to replicate this part ' most of which have circular chromosomes Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules This is not a problem for prokaryotes, • Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres RNA primer -7 Gene 1 o Gene 1 p Gene 1 ' be missing from the gametes they produce r It has been proposed that the shortening of telomeres is connected to aging . An enzyme called telomerase catalyzes the ^ The shortening of telomeres might protect cells lengthening of telomeres in germ cells Replication from cancerous growth by limiting the number of x cell divisions × Gene 1 . With telomeres The DNA is saved and the erasing is in the telomeres Gene 1 If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually If there’s no telomeres Parts of the DNA will be ersed thus some geanes will be effected and some proteins won’t be created in the cell qgf ' Telomeres & Telomerase and cancer E shorter DNA molecules with uneven ends In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA - complete the 5′ ends of daughter DNA strands can also undergo spontaneous changes . of new DNA at a replication fork in the from going back together • • For linear DNA,the usual replication machinery cannot . DNA can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it ! 6 ' In mismatch repair of DNA, repair enzymes replace incorrectly paired nucleotides that have evaded the proofreading process . y DNA polymerases catalyze the synthesis each single-stranded DNA to prevent it DNA double helix straight Bow 0 DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides ' u Single-strand binding proteins bind to i god 5 . The DNA polymerase keeps elongate the leading strand n n J Topoisomerase keeps the 2 The lagging strand is now completed ( create ) an RNA primer helix at the replication forks origins of replication . primase can start Helicases untwist the double particular sites called . 6 start Replication begins at Single-strand binding proteins DNA polymerase . DNA polymerases DNA polymerase replaces the RNA primer lagging strand templet “ parent “ Deoxyguanosine triphosphate RNA primer - DNA polymerase 4 groups as a molecule of pyrophosphate The difference between dATP and ATP is that dATP has deoxyribose while ATP has ribose " Deoxycytidine triphosphate Okazaki fragment Okazaki fragment 2 Helicases viaa dehydration reaction, it loses two phosphate Deoxythymidine triphosphate origin of replication . RNA primer for fragment 2 DNA ligase As each monomer joins the DNA strand, . Deoxyadenosine triphosphate Okazaki fragment DNA polymerase detachs 3 lagging strand new “ daughter “ Bow pyrophosphate as a template ÷÷: A An enzyme called primase can start ( create ) an RNA primer chain from g RNA primer DNA polymerases Direction of replication • ° primase Batman • The primer is short (5–10 nucleotides long), and the 3 replication forks • 1 r The initial nucleotide strand is a short RNA primer DNA polymerase makes RNA primer for fragment 2 DNA polymerases can not initiate synthesis of a polynucleotide; can only add Parental strand nucleotides to the 3 prime origins of replication • . Parental strand Single-strand binding proteins Daughter “ new “ strand replication “bubble” origins of replication primase replication fork Helicases Enzymes called DNA polymerases catalyze the synthesis of . Daughter “ new “ strand in the direction away from the replication fork RNA primer leading strand new “ daughter “ RNA primer . Topoisomerase relieves the strain of twisting of the double helix by Most DNA polymerases require a primer and a DNA template strand Parental strand In the lagging strand, DNA polymerase must work leading strand templet “ parent “ DNA polymerase Helicases are enzymes that untwist the double helix at the replication forks Single-strand binding proteins bind to and stabilize single-stranded DNA I Parental strand B lagging strand Single-strand binding proteins • v Separated Parental strands Okazaki fragments, which are joined together by DNA ligase primase 2 daughter DNA molecules new DNA at a replication fork tf Separation Bo Topoisomerase breaking, swiveling, and rejoining DNA strands l DNA polymerases add nucleotides only to the free 3′ end of a growing strand; therefore, a new DNA strand can elongate only in the 5′ → 3′ direction papooses At the end of each replication bubble is a replication fork, a Y-shaped region where parental DNA strands are being unwound Each strand acts as a template for building new strand in replication 2 daughter DNA molecules y •• origin of replication , for building a new strand in replication 2 strands Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble” > • , daughter strands are built based on base-pairing rules ' There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist Replication telomerase : the enzyme that lengthen the telomeres # Gene 1 telomeres special nucleotide sequences at their ends called • Gene 1 BABB telomerase Gene 1 . Increase the length of telomeres