Bio 118 1st Edition Lecture 14 Outline of Last Lecture I Photosynthesis a Two Linked Sets of Reactions b Nature of Light Energy c Photosynthetic Pigments Absorb Light d Role of Carotenoids and Other Accessory Pigments e Resonance f Reaction Center g Two Types of Reaction Centers h Electrons Participate in Redox Reactions i Chemiosmosis and Photophosphorylation j Oxygenic Photosynthesis k How Does Photosystem 1 Work l Summary of Photosystems m The Z Scheme n The Calvin Cycle and Carbon Fixation o The Calvin Cycle p The Importance of Rubisco q Connection to Global Warming 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 Outline of Current Lecture I DNA s Primary Structure II DNA Has Directionality III DNA s Secondary Structure IV DNA Strands Are Templates for DNA Synthesis V How Do the New DNA Strands Form VI A Comprehensive Model for DNA Synthesis VII Characteristics of DNA Polymerases VIII How Does Replication Get Started IX How is the Helix Opened Stabilized X How is the Leading Strand Synthesized XI Primase XII The Lagging Strand XIII How is the Lagging Strand Synthesized XIV The Discontinuous Replication Hypothesis XV The Discovery of Okazaki Fragments XVI DNA Synthesis Enzymes Are Well Organized XVII Replicating the Ends of Linear Chromosomes XVIII Repairing Mistakes DNA Damage XIX If DNA is Damaged How is It Detected XX What if DNA Polymerase Misses an Error XXI Repairing Damaged DNA XXII Nucleoside Excision Repair System Current Lecture DNA s Primary Structure Has 2 major components o Backbone Made of the sugar and phosphate groups of deoxyribonucleotides o Series of nitrogen containing bases that project from the backbone DNA Has Directionality One end has an exposed hydroxyl group on the 3 carbon of deoxyribose Other end has an exposed phosphate group on the 5 carbon DNA s Secondary Structure Watson Crick proposed 2 DNA strands line up in the opposite direction to each other antiparallel fashion Antiparallel strands twist to form a double helix Secondary structure is stabilized by complementary base pairing o A denine T hymine o G uanine C ytosine DNA Strands are Templates for DNA Synthesis Watson Crick suggested the existing strands of DNA served as a template for the production of new strands Bases were added to new strands according to complementary base pairing How Do New DNA Strands Form Three Possibilities 3 alternative hypotheses for how the old and new DNA strands interact during replication o Semiconservative replication Parental DNA strands separate each used as template for synthesis of new strand Daughter molecules each consist of one old and one new strand o Conservative replication Parental molecule serves as template for synthesis of entirely new molecule o Dispersive replication The parent molecule cut into sections Daughter molecules contain old DNA interspersed with newly synthesized DNA Comprehensive Model for DNA Synthesis Meselson Stahl s Key Experiment o Showed that each parental DNA strand is copied in its entirety semi conservative replication o Did not illustrate a mechanism for this process o DNA polymerase is the enzyme that catalyzes DNA synthesis o Discovery of DNA polymerase cleared the way for understanding DNA replication reactions Characteristics of DNA Polymerases Critical characteristic of DNA polymerases can only work in one direction DNA polymerases can add deoxyribonucleotides to only the 3 end of a growing DNA chain DNA synthesis always proceeds in the 5 3 direction DNA polymerization is exergonic because monomers that act as substrates in reaction deoxyribonucleoside triphosphates have high potential energy because of their 3 closely packed phosphate groups How Does Replication Get Started A replication bubble forms in a chromosome that is actively being replicated Grows as DNA proceeds Synthesis is bidirectional replication process begins at a single location in bacterial chromosomes origin of replication Eukaryotes also have bidirectional replication but synthesis does not start at end of chromosomes but has multiple origins of replications replication ubbles Replication fork is the T shaped region where the DNA is split into 2 separate strands How Is The Helix Opened and Stabilized Several proteins are responsible for opening and stabilizing the double helix o Enzyme helicase catalyzes the breaking of hydrogen bonds between the 2 DNA strands to separate them o Single strand DNA binding proteins SSBPs attach to separated strands to prevent from closing o Enzyme topoisomerase cuts rejoins the DNA downstream of the replication fork relieving tension in the helix due to the unwinding How is the Leading Strand Synthesized DNA polymerase requires a free 3 hydroxyl OH group to commence there isn t one available therefore it requires a primer to start replication A few nucleotides possessing free 3 hydroxyl groups bond to the template strand is the primer made of RNA o Provides a free 3 hydroxyl group that can combine with incoming dNTP to form phosphodiester bond later RNA segment removed Primase A type of RNA polymerase that synthesizes a short RNA segment that serves as primer DNA Polymerase 3 then adds bases to the 3 end of the primer creates the leading strand Then leads into the replication fork Synthesized continuously in the 5 3 direction The Lagging Strand The other DNA strand Synthesized discontinuously in the direction away from the replication fork because DNA synthesis must proceed in 5 3 direction How is the Lagging Strand Synthesized Synthesis starts when primase synthesizes a short stretch of RNA again acting as a primer DNA polymerase 3 then adds bases to the 3 end of the primer DNA polymerase moves away from replication fork as helicase continues to open the replication fork exposing DNA Discontinuation Replication Hypothesis Once primase synthesizes an RNA primer on the lagging strand DNA polymerase might synthesize short fragments of DNA along lagging strand These fragments would later be linked together to form a continuous whole strand Tested by Okazaki colleagues Discovery of Okazaki Fragments Lagging strand is synthesized as short discontinuous fragments called Okazaki fragments DNA polymerase 1 then removes the RNA primer at the beginning of each Okazaki fragment fills in the gap Enzyme DNA ligase joins the Okazaki fragments to form a continuous strand Because fragments are synthesized independently joined
View Full Document
Unlocking...