Biol 1411 1st Edition Lecture 21Outline of Last Lecture I. DNA Structurea. Transformationb. Genetic Materialc. Structure Outline of Current Lecture I. DNA StructureII. DNA ReplicationCurrent Lecture- Antiparallel strands: o polarity of strand is determined by the sugar-phosphate bondso phosphate groups connect to the 3’ C of one sugar, and the 5’C of the next sugaro the two chain ends differ: one is free 5’ phosphate group- the 5’ end; the other isa free 3’ hydroxyl group – the 3’ endo by convention, DNA sequence is written 5’ to 3’ as a single stand- DNA is a double helixo 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 structureo it is a double-stranded helix of uniform diametero it is right-handedo strands in antiparallel orientation based on 5’ and 3’ carbons of deoxyribos sugaro outer edges of nitrogenous bases are exposed in the major and minor grooves - FUNCTION OF DNAo DNA has four important functions- double-helical structure is essential: Genetic material stores genetic information- millions of nucleotides; basesequence encodes huge amounts of information Genetic material is susceptible to mutation- a change in information-possibly 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 complementarybase pairing Genetic material is expressed as the phenotype- nucleotide sequence determines sequence of amino acids in proteins - 3 models of DNA replicationo Semiconservative: parent serves as a template and new molecules have one old and one new strando Conservative: original helix only serves as a template for synthesis of new helixo 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 modelo They used density labeling to distinguish parent DNA strands from new DNA strandso DNA was labeled with 15N, making it denser then DNA containing 14N. - Evidence for semiconservative modelo 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 intermediateo 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 strandso 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 baseso One new strand, the leading strand, is oriented to grow at its 3’ end as the fork openso 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 templateo Primase- an enzyme- synthesizes RNA one nucleotide at a timeo 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 DNAo Synthesis of the lagging strand occurs in small, discontinuous stretches –okazaki fragments o Each okazaki fragment requires its own primer, synthesized by the primaseo 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