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GENETICS 120:352 CERVANTES SPRING 208 PRACTICE PROBLEMS SERIES 6 CHAPTER 10: DNA REPLICATION AND SYNTHESIS (SECTIONS 10.3-10.8) 1. The discovery of Okazaki fragments lead to conclude that DNA synthesis is: a. Discontinuous b. Continuous c. Semiconservative d. All of the above HINT: Okazaki fragments are DNA fragments found on the lagging strand during DNA replication. a. Correct. DNA must be synthesized on both strands of the double helix at the same time. However, one strand runs in a 5’ to 3’ direction and the other runs in the 3’ to 5’ direction. In order for replication to occur on both strands in the same direction simultaneously, one strand must be made in a discontinuous fashion (in short pieces called Okazaki fragments) and reannealed later. b. Incorrect. DNA must be synthesized in a 5’ to 3’ direction, and both strands must be synthesized simultaneously. One strand can be synthesized in a continuous fashion, but the other strand cannot. c. Incorrect. DNA is replicated in a semiconservative manner; however, this was not suggested by the discovery of Okazaki fragments. d. Incorrect. In order for replication to occur on both strands in the same direction simultaneously, one strand must be made in short pieces, which are called Okazaki fragments. This discovery explained how this event is possible. 2. Which one of the following proteins and enzymes do not function at the origin of replication in Escherichia coli ? a. DNA ligase b. SSBPs c. Helicase d. DnaA, DnaB and DnaC proteins HINT: Consider the events that occur at the origin—recognition, binding, unwinding, stabilization and primer synthesis. a. Correct. DNA ligase catalyzes the last reaction in DNA replication; it seals the nick and joins together the Okazaki fragments. This reaction does not occur at the origin of replication. b. Incorrect. Single-stranded binding proteins are important to keep the unwound DNA from spontaneously coiling into a helix at the origin. c. Incorrect. Helicases unwind the DNA at the origin so that a replication fork can be established (and subsequently maintained). Incorrect. These proteins are essential for recognition, initial binding, and initial unwinding of the DNA helix at the origin of replication. 3. Which DNA polymerase (with a 5’ to 3’ exonuclease activity) is responsible for the removal of the RNA primers at the origin and at the 5’ ends of the Okazaki fragments in E. coli ? a. Primase b. DNA polymerase I c. DNA polymerase II d. RNase H HINT: The enzyme removes the RNA as it simultaneously synthesizes the replacement DNA. a. Incorrect. The enzyme primase does not degrade, but rather synthesizes, the RNA primer. b. Correct. The 5’ to 3’ exonuclease activity of DNA polymerase I is responsible for RNA primer removal. c. Incorrect. DNA polymerase II is probably involved in repair, not the removal and replacement, of RNA primers.
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