Biol 1411 1st Edition Lecture 21 Outline of Last Lecture I DNA Structure a Transformation b Genetic Material c Structure Outline of Current Lecture I DNA Structure II DNA Replication Current Lecture Antiparallel strands o polarity of strand is determined by the sugar phosphate bonds o phosphate groups connect to the 3 C of one sugar and the 5 C of the next sugar o the two chain ends differ one is free 5 phosphate group the 5 end the other is a free 3 hydroxyl group the 3 end o by convention DNA sequence is written 5 to 3 as a single stand DNA is a double helix o Nitrogenous bases exposed in the major and minor grooves of DNA helix and accessible to proteins o Proteins interact with exposed base pairs o Distinct chemical signatures along the helix determined by base composition in major or minor groove 4 key features of DNA structure o it is a double stranded helix of uniform diameter o it is right handed o strands in antiparallel orientation based on 5 and 3 carbons of deoxyribos sugar o outer edges of nitrogenous bases are exposed in the major and minor grooves FUNCTION OF DNA o DNA has four important functions double helical structure is essential Genetic material stores genetic information millions of nucleotides base sequence encodes huge amounts of information Genetic material is susceptible to mutation a change in informationpossibly a simple alteration to a sequence 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 Genetic material is precisely replicated in cell division by complementary base pairing Genetic material is expressed as the phenotype nucleotide sequence determines sequence of amino acids in proteins 3 models of DNA replication o Semiconservative parent serves as a template and new molecules have one old and one new strand o Conservative original helix only serves as a template for synthesis of new helix o Dispersive Parent fragments serve as templates assembling old and new parts into molecules How is DNA replicated o Meselson and stahl showed the semiconservative replication was the correct model o They used density labeling to distinguish parent DNA strands from new DNA strands o DNA was labeled with 15N making it denser then DNA containing 14N Evidence for semiconservative model o Results of messelson stahl experiment explained by the semiconservative model o If conservative the first generation of individuals would have been both high and low density but not intermediate o If dispersive density in the first generation would be intermediate but this density would not be present in subsequent generation How is DNA replicated o The replication complex interacts with the template strands o All chromosomes have at least one region called origin of replication ori o Proteins in the replication complex bind to a DNA sequence in ori o New nucleotides are added to the new strand at the 3 end Sequence of added nucleotides determined by complementary base pairing o The enzyme DNA polymerase creates a phospodiester bond between internal phosphate at 5 carbon and hydroxyl group at 3 carbon o The replication fork is the site where DNA unwinds to expose bases o One new strand the leading strand is oriented to grow at its 3 end as the fork opens o The lagging stand is oriented so that its exposed 3 end gets farther from the fork o DNA begins with a short primer a starter strand complementary to the DNA template o Primase an enzyme synthesizes RNA one nucleotide at a time o DNA polymerase adds nucleotides to the 3 end of the RNA primer Polymerases are processive meaning they catalyze many polymerizations each time they bind to DNA o Synthesis of the lagging strand occurs in small discontinuous stretches okazaki fragments o Each okazaki fragment requires its own primer synthesized by the primase o DNA polymerase III adds nucleotides to the 3 end until reaching the primer of the previous fragment o DNA polymerase I completes synthesis of lagging strand by removing RNA primer and replacing with DNA o DNA ligase catalyzes the phosphodiester bond between adjacent nucleotides