Pierce Benjamin A Chapter 9 Genetics Essentials Concepts and Connections 2nd ed New York W H Freeman 2013 N pag Print CHAPTER 9 9 2 FIG 9 1 p 235 Semiconservative replication each of the original strains remains intact despite no long being combined in the same molecule the original DNA molecule is semi conserved o Each resulting DNA molecule is interspersed with fragments of old and new DNA o One round of replication produces 2 hybrid molecules each with half original and o After second round of replication the molecules are hybrid and consist half new DNA entirely of new DNA o Additional rounds of DNA would result in fewer and fewer hybrid molecules In conservative replication the entire double stranded DNA molecule serves as a template for replication The original DNA molecule is fully conserved o After 1st round of replication 50 of molecules are identical to the original o After 2nd round 25 of the molecules have entirely original DNA and 75 have 50 consist of entirely new DNA entirely new DNA o After each round the number of new DNA molecules increases while the number of original DNA molecules remains constant In dispersive replication both nucleotide strands break down into fragments which serve as templates for the new DNA molecules o Always produces hybrid molecules containing some original some new DNA o Proportion of new DNA increases within the molecules after each replication event Meselson and Stahl s Experiment o Distinguished old DNA from new DNA within E coli cells to determine which o Used N14 common form infused bacteria crossed by N15 rare heavy form mode of replication infused bacteria centrifugation o Distinguished between the two with an equilibrium density gradient o After several days of spinning two layers will form o DNA from N15 bacteria transferred to the medium with N14 and allowed 1 round of replication produced a single band at a position intermediate between that expected for N14 and N15 agreeing with the dispersive and semi conservative models o After a 2nd round of replication in N14 two bands of equal intensity appeared one at the intermediate mark and one at the N14 mark so semi conservative replication o The band representing N14 DNA became progressively stronger Replication is semi conservative o Backed up by Meselson and Stahl No evidence ever found for conservative or dispersive replication Semi conservative replication can occur several different ways depending on the nature of the template o Linear vs circular Replicons individual units of replication each contains a replication origin where replication starts and continues until the entire replicon has been replicated bacterial chromosomes have 1 replication origin o eukaryotic chromosomes contain many Theta replication common in circular DNA generates a structure resembling the Greek letter theta synthesized o Double stranded DNA begins to unwind at the replication origin o Single stranded nucleotide strands serve as templates on which new DNA can be o The unwinding of the double helix generates a loop replication bubble o Unwinding may be at one or both ends of the bubble o Replication fork the point of unwinding where the 2 single nucleotide strands separate from the double stranded helix There may be 2 replication forks one at each end bidirectional replication eventually meet Simultaneous unwinding and replicating the DNA until they If there is 1 replication fork it proceeds around the circle to produce 2 complete circular DNA molecule each having 1 old and 1 new nucleotide strand o replication has 1 replication origin Eukaryotic cells have way too much DNA to have a single replication origin They have thousands Typical eukaryotic replicons are 20 000 300 000 bp o At each replication origin DNA unwinds and produces a replication bubble with 2 replication forks o When replication forks of adjacent replicons run into each other fuse to form long stretches of new DNA o The replication fusion of all the replicons 2 identical DNA molecules The original nucleotide strand does not break in either theta or linear eukaryotic replication 3 major groups of replication processes o 1 a template consisting of single stranded DNA o 2 raw materials substrates to be assembled into a new nucleotide strand o 3 enzymes and other proteins that read the template assemble the substrates together into a DNA molecule The DNA molecule unwinds exposing bases which act as a template for assembling new polynucleotide strands which will be complementary and antiparallel to the template Raw materials for new DNA o dNTPs deoxyribonucleoside triphosphates each with a deoxyribose sugar and a base a nucleoside attached to 3 phosphate groups nucleotides are added to the 3 OH group of the growing nucleotide strand the 3 OH group of the last nucleotide strand attacks the 5 phosphate group of the incoming dNTP o a phosphodiester bond is created between the 2 nucleotides FIG 9 6 p 239 DNA polymerases enzymes that synthesize DNA synthesis requires many enzymes and proteins for proper function o can add nucleotides to the 3 end of the growing strand o so new DNA strands always elongate in the 5 3 direction As DNA unwinds due to its antiparallel nature one template is exposed in the 5 3 direction and the other in the 3 5 direction The template strand exposed at the 3 5 direction allows the new strand to be synthesized continuously in the 5 3 direction the leading strand undergoes continuous replication The 2nd template strand is exposed in the 5 3 direction and requires Okazaki fragments o After a little has been unwound synthesis proceeds in a short distance in the 5 3 direction until the template runs out Okazaki fragment Then after more has been exposed by unwinding another Okazaki fragment beings FIG 9 8 p 240 o 1 On the lower template strand synthesis proceeds continuously in 5 3 direction the same as that of unwinding o 2 On the upper template strand synthesis begins at the fork proceeds in the direction opposite that of unwinding so it soon runs out of template o 3 Synthesis starts again on the upper strand at the fork each time proceeding away from the fork o 4 Synthesis on the upper strand is discontinuous Short fragments of DNA produced by discontinuous synthesis Okazaki fragments Reiji Okazaki discovered them o In each bacterial cell range from 1000 2000 nucleotides o In eukaryotic cells range from 100 200 nucleotides in length 9 3 Bacterial replication requires a large of enzymes proteins Replication has 4
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