CHAPTER 6 DNA REPLICATION REPAIR AND RECOMBINATION 2009 Garland Science Publishing DNA Replication 6 1 The process of DNA replication requires that each of the parental DNA strands be used as a to produce a duplicate of the opposing strand a catalyst b competitor c template d copy 6 2 DNA replication is considered semiconservative because a after many rounds of DNA replication the original DNA double helix is still intact b each daughter DNA molecule consists of two new strands copied from the parent DNA molecule c each daughter DNA molecule consists of one strand from the parent DNA molecule and one new strand d new DNA strands must be copied from a DNA template 6 3 The classic experiments conducted by Meselson and Stahl demonstrated that DNA replication is accomplished by employing a mechanism a continuous b semiconservative c dispersive d conservative 6 4 If the genome of the bacterium E coli requires about 20 minutes to replicate itself how can the genome of the fruit fly Drosophila be replicated in only 3 minutes a The Drosophila genome is smaller than the E coli genome b Eucaryotic DNA polymerase synthesizes DNA at a much faster rate than procaryotic DNA polymerase c The nuclear membrane keeps the Drosophila DNA concentrated in one place in the cell which increases the rate of polymerization d Drosophila DNA contains more origins of replication than E coli DNA 6 5 Meselson and Stahl grew cells in media that contained different isotopes of nitrogen 15N and 14N so that the DNA molecules produced from these different isotopes could be distinguished by mass A B C Explain how light DNA was separated from heavy DNA in the Meselson and Stahl experiments Describe the three existing models for DNA replication when these studies were begun and explain how one of them was ruled out definitively by the experiment you described for part A What experimental result eliminated the dispersive model of DNA replication 6 6 Indicate whether the following statements are true or false If a statement is false explain why it is false A When DNA is being replicated inside a cell local heating occurs allowing the two strands to separate B DNA replication orgins are typically rich in G C base pairs C Meselson and Stahl ruled out the dispersive model for DNA replication D DNA replication is a bidirectional process that is initiated at multiple locations along chromosomes in eucaryotic cells 6 7 Answer the following questions about DNA replication A On a DNA strand that is being synthesized which end is growing the 3 end the 5 end or both ends Explain your answer B On a DNA strand that is being used as a template where is the copying occurring relative to the replication origin 3 of the origin 5 or both 6 8 How does the total number of replication origins in bacterial cells compare with the number of origins in human cells a 1 versus 100 b 5 versus 500 c 10 versus 1000 d 1 versus 10 000 6 9 The chromatin structure in eucaryotic cells is much more complicated than that observed in procaryotic cells This is thought to be the reason that DNA replication occurs much faster in procaryotes How much faster is it a 2 b 5 c 10 d 100 6 10 DNA polymerase catalyzes the joining of a nucleotide to a growing DNA strand What prevents this enzyme from catalyzing the reverse reaction a hydrolysis of PPi to Pi Pi b release of PPi from the nucleotide c hybridization of the new strand to the template d loss of ATP as an energy source 6 11 Figure Q6 11 shows a replication bubble Figure Q6 11 A B C D E F On the figure indicate where the origin of replication was located use O Label the leading strand template and the lagging strand template of the righthand fork R as X and Y respectively Indicate by arrows the direction in which the newly made DNA strands indicated by dark lines were synthesized Number the Okazaki fragments on each strand 1 2 and 3 in the order in which they were synthesized Indicate where the most recent DNA synthesis has occurred use S Indicate the direction of movement of the replication forks with arrows 6 12 Which of the following statements about the newly synthesized strand of a human chromosome is true a It was synthesized from a single origin solely by continuous DNA synthesis b It was synthesized from a single origin by a mixture of continuous and discontinuous DNA synthesis c It was synthesized from multiple origins solely by discontinuous DNA synthesis d It was synthesized from multiple origins by a mixture of continuous and discontinuous DNA synthesis 6 13 You have discovered an Exo mutant form of DNA polymerase in which the 3 to 5 exonuclease function has been destroyed but the ability to join nucleotides together is unchanged Which of the following properties do you expect the mutant polymerase to have a It will polymerize in both the 5 to 3 direction and the 3 to 5 direction b It will polymerize more slowly than the normal Exo polymerase c It will fall off the template more frequently than the normal Exo polymerase d It will be more likely to generate mismatched base pairs 6 14 A molecule of bacterial DNA introduced into a yeast cell is imported into the nucleus but fails to replicate the yeast DNA Where do you think the block to replication arises Choose the protein or protein complex below that is most probably responsible for the failure to replicate bacterial DNA Give an explanation for your answer a primase b helicase c DNA polymerase d initiator proteins 6 15 Most cells in the body of an adult human lack the telomerase enzyme because its gene is turned off and is therefore not expressed An important step in the conversion of a normal cell into a cancer cell which circumvents normal growth control is the resumption of telomerase expression Explain why telomerase might be necessary for the ability of cancer cells to divide over and over again 6 16 Which diagram accurately represents the directionality of DNA strands at one side of a replication fork 6 17 Indicate whether the following statements are true or false If a statement is false explain why it is false A Primase is needed to initiate DNA replication on both the leading strand and the lagging strand B The sliding clamp is loaded once on each DNA strand where it remains associated until replication is complete C Telomerase is a DNA polymerase that carries its own RNA molecule to use as a primer at the end of the lagging strand D Primase requires a proofreading function that ensures there are no errors in the RNA primers used for