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Chapter 12

BIOL 1000 Chapter 12 - DNA Structure, Replication, and Oganization.docx

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BIOL 1000
Jennifer Steeves

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Chapter 12 – DNA Structure, Replication, and Organization 12.2 DNA Structure • X-Ray Diffusion – When X-rays are focused through isolated macromolecules or crystals of purified molecule, the x-rays deflected are in patterns called diffraction patterns  Provides info about organization of the molecular components  Data indicated that DNA was a highly ordered, multiple stranded structure with a repeating substructure spaced every 3.4 A° (1 angstrom = 1 x m) • Watson and Crick discovered that a DNA molecule consists of 2 polynucleotide chins twisted around each other into a right-handed double helix - held together by Hydrogen bonds • Each nucleotide consists of deoxyribose (a carbon sugar); a phosphate group; a cyclic nitrogen containing a compound called a base • Adenine bonds with Thymine through 2 Hydrogen bonds • Guanine bonds with Cytosine through 3 Hydrogen bonds • The deoxyribose sugars are linked by phosphate groups to form an alternating sugar- phosphate backbone – Connected by a phosphodiester bond • The two strands are complementary  Makes DNA suited to store & transmit genetic info • The sugar-phosphate backbones of the 2 complementary strands are Antiparallel • Stacked (base-pairs) 34° apart, with 10 base pairs/turn (360°) • Chargaff’s Rule – Concentration of Purines (T & C) = Concentration of Pyrimidines (A & G)  T & A as well as G & C were present in DNA with fixed interrelationship 12.3 DNA Replication • Replication entails the transmission of genetic info to the progeny • Human DNA takes only few hours to copy (46 DNA Molecules; billions of base pairs) • Bacterial DNA replicates in less than an hour • Synthesized from deoxynucleoside 5-triphosphate precursors (dNTPs) • Semi-Conservative Replication  The two strands of a parental DNA molecule unwind and each serves as a template for the synthesis of a complimentary copy  Base pair specificity provides basis for duplication • 3 Types of Replication: 1. Semiconservative 2. Conservative – parental strand remains intact 3. Dispersive – parental strand & progeny become intertwined through fragmentation, synthesis, and rejoining process Before Replication 1. Primer Function (Primase) – adds RNA primer on template strand to initiate replication  Used as a starting point for nucleotide assembly by DNA Polymerases 2. Template – provides ssDNA (single-stranded) that will direct the addition of each complimentary deoxynucleotide Replicating Apparatus • DNA Replication is complex and is carried out by a multi enzyme complex  Replicating Apparatus/replisome • Occurs in the 5’ – 3’ direction of the new strand (3’ – 5’ on the old strand) • Replication Fork – The junction between the newly separated strands and the non- replicated double stranded DNA • Leading Strand – Continuous strand synthesized toward the Replication Fork • Lagging Strand – Discontinuous strand synthesized away from the Replication Fork • Okazaki Fragments – A short fragment of DNA formed on the lagging strand during replication  100-1000 base pairs in length (only in eukaryotes) • Helicase – Enzyme that separates and unwinds the DNA strands • DNA Gyrase (Topoisomerase) – Relieves strain while double-helix DNA is being unwound by Helicase  Cutting the DNA, turning the DNA on one side in the direction opposite to that of the twisting force, and rejoining the two strands. • DNA Single Stranded Binding Proteins – Prevents parent DNA strands from annealing to each other once they have been separated by Helicase; stabilizes ssDNA.  Replication is 100x faster with these attached • Primase (RNA polymerase) –which makes short RNA primers in the 5’ – 3’ direction using ssDNA as a template • DNA Polymerase III – The synthesis of DNA is catalysed by DNA polymerase; can only add dNTPs to the 3’ end and forms polynucleotides (Main replication enzyme of E. Coli)  dATP, dTTP, dGTP, and dCTP • Sliding DNA Clamps – Increase the degree of productivity of DNA Polymerase  Surround the DNA and binds to the DNA Polymerase tethering it to the Template • DNA Polymerase I – To complete the DNA replication RNA primers must be removed  Using its 5’ – 3’ exonuclease activity removes the RNA Primer on the lagging strand and fills in the necessary nucleotides between the Okazaki Fragments in 5’ – 3’ direction
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