BIOL 1441 1st Edition Lecture 29 Outline of Last Lecture I DNA II Transformation III DNA replication model Outline of Current Lecture I DNA replication model II DNA replication III Repairing DNA Current Lecture I II DNA replication model a Semi conservative model i 2 strands of the parental molecule separate and each functions as a template for synthesis of a new complementary strand ii CORRECT MODEL b Competing models i Conservative model 2 parental strands reassociate after acting as templates for new strands restoring the parental double helix ii Dispersive model each strand of both daughter molecules contains a mix of old and newly synthesized DNA DNA Replication a 6 billion base pairs in each cell takes a few hrs to copy b Very few errors 1 in 10 billion nucleotides c Copying of DNA is remarkable in its speed and accuracy d More than a dozen enzymes and other proteins participate in DNA replication e Different in prokaryotes than in eukaryotes f Begins at origins of replication 2 DNA strands are separated opening up a replication bubble i Prokaryotes single origin of replication These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute g h i j k l m n o p ii Eukaryotes may have hundreds of origins of replication Proceeds in both directions from each origin until the entire molecule is copied At the end of each replication bubble is a replication fork a Y shaped region where new DNA strands are elongating Elongating a New DNA Strand i DNA polymerase enzyme that catalyzes elongation of new DNA at a replication fork ii Rate of elongation 500 nucleotides per second in bacteria 50 per second in human cells iii Bacteria DNA polymerase I III iv Humans at least 11 different DNA polymerases v Actually adding nucleotide triphosphate vi ATP adnosine triphosphate except sugar in ATP is ribose not deoxyribose Triphosphate Nucleotides i Highly reactive triphosphate tails have unstable cluster of negative charge ii As each nucleotide is added to growing DNA chain it loses 2 phosphate groups forming pyrophosphate iii Subsequent hydrolysis of pyrophosphate into 2 molecules of inorganic phosphate exergonic rxn drives polymerization rxn of growing DNA chain Antiparallel Elongation i 2 strands oriented in opposite directions affects replication ii DNA polymerase can only add nucleotides to the free 3 end of a growing strand 1 Can only move 1 way down strand A new DNA strand can only elongate in the 5 to 3 direction Only orientation DNA polymerase can add bases How does the 2nd strand 3 5 replicate i Replication is discontinuous ii DNA polymerase jumps ahead elongates in segments Lagging strand replication i Leading strand DNA polymerase adds DNA continuously moving toward the replication fork ii Lagging strand DNA polymerase must work in direction away from replication fork 1 Lagging strand synthesized as a series of segments Okazaki fragments 2 Joined together by DNA ligase glues them Priming DNA Synthesis i DNA polymerase cannot initiate synthesis of a polynucleotide III 1 Can only add nucleotides to 3 end ii Need an RNA primer short segment of RNA iii Primase start an RNA chain from scratch 1 Binds primer and begins replication iv Leading strand one primer v Lagging strand each Okazaki fragment must be primed separately q Other Proteins That Assist DNA Replication i Helicase untwists double helix separates the template DNA strands at the replication fork ii Single strand binding protein binds stabilizes single stranded DNA until used as template iii Topoisomerase corrects overwinding ahead of replication forks by breaking swiveling rejoining DNA strands iv Primase synthesizes an RNA primer at the 5 ends of the leading strand and the Okazaki fragments v DNA pol III continuously synthesizes the leading strand and elongates Okazaki fragments vi DNA pol I removes primer from the 5 ends of the leading strand and Okazaki fragments replacing primer with DNA and adding to adjacent 3 ends vii DNA ligase joins the 3 end of the DNA that replaces the primer to the rest of the leading strand joins lagging strand fragments Repairing DNA a DNA polymerase proofread newly made DNA replacing any incorrect nucleotides b DNA damage oxygen radicals radioactivity X rays UV light spontaneous changes under normal conditions c Cells monitor repair DNA continuously i 130 repair enzymes in humans d Mismatch Repair i Enzymes correct errors in base pairing e Thymine dimer i Covalent bonding of two adjacent thymine residues within a DNA molecule often catalyzed by UV radiation or chemical mutagenic agents f Nucleotide excision repair i Enzymes but out and replace damages stretches of DNA g Replicating the Ends of DNA Molecules i Eukaryotic DNA telomeres at ends of nucleotide sequences repetitive short nucleotide sequence ii Telomeres do not prevent the shortening of DNA molecules they postpone erosion of genes near the ends of DNA molecules h Errors in replication i Polymerase adds wrong nucleotide if mistake gets through can cause mutation ii Cancer 1 Normal shortening of telomeres protects body from cancer limits number of cell divisions somatic cells can undergo 2 More cell divisions more DNA replication more errors 3 Cancer cells have telomerase activity lengthen telomeres keep dividing immortal a Also keep acquiring more genetic mutations