![]() An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. The origin of replication is recognized by certain proteins that bind to this site. coli, which has a single origin of replication on its one chromosome (as do most prokaryotes), it is approximately 245 base pairs long and is rich in AT sequences. How does the replication machinery know where to begin? It turns out that there are specific nucleotide sequences called origins of replication where replication begins. It is now known that DNA pol III is the enzyme required for DNA synthesis DNA pol I and DNA pol II are primarily required for repair. In prokaryotes, three main types of polymerases are known: DNA pol I, DNA pol II, and DNA pol III. When the bond between the phosphates is broken, the energy released is used to form the phosphodiester bond between the incoming nucleotide and the growing chain. The addition of nucleotides requires energy this energy is obtained from the nucleotides that have three phosphates attached to them, similar to ATP which has three phosphate groups attached. One of the key players is the enzyme DNA polymerase, also known as DNA pol, which adds nucleotides one by one to the growing DNA chain that are complementary to the template strand. The process is quite rapid and occurs without many mistakes.ĭNA replication employs a large number of proteins and enzymes, each of which plays a critical role during the process. This means that approximately 1000 nucleotides are added per second. coli has 4.6 million base pairs in a single circular chromosome and all of it gets replicated in approximately 42 minutes, starting from a single origin of replication and proceeding around the circle in both directions. These questions address the following standards:ĭNA replication has been extremely well studied in prokaryotes primarily because of the small size of the genome and the mutants that are available. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. Use the ten steps summarizing the process of DNA replication listed just prior to Table 14.1 in the text as an outline for discussing DNA replication in prokaryotes. A Learning Objective merges required content with one or more of the seven Science Practices. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The replicated DNA molecules now consist of one original template strand and one newly synthesized strand. (Think of the activities on the lagging strand as analogous to trying to walk on a moving sidewalk that is moving in the opposite direction.) The RNA primers are replaced by DNA nucleotides, and ligase seals the DNA, creating phosphodiester bonds between the 3'-OH of one end and the 5'-phosphate of the other strand. However, replication of the other strand-called the lagging strand-occurs in a direction away from the replication fork, in short stretches of DNA known as Okazaki fragments. One strand-called the leading strand-is synthesized continuously in the direction of the replication fork (the direction in which helicase is separating the two strands), with polymerase adding new nucleotides one-by-one. DNA polymerase recognizes the 3'-OH end as its landing site thus, polymerase “reads” the template strand in the 3' to 5' direction and builds the complementary DNA polymer in the 5' to 3' direction. The enzyme primase synthesizes RNA primers to initiate DNA synthesis by DNA polymerase, which can add nucleotides only in the 5' to 3' direction. Single-strand binding proteins keep the fork open. The enzyme helicase opens up the DNA at the point where hydrogen bonds connect the strands, resulting in the formation of a Y-shaped replication fork. Replication in prokaryotes starts from a sequence of nucleotides on the chromosome called the origin of replication-the point at which the DNA opens up or unzips. ![]() What is the role of DNA polymerase and other enzymes and proteins in supporting replication?Īs was stated previously, DNA replication is more complex than simply unzipping the double helix and making new complementary strands.How is DNA replicated in prokaryotes, and what are the roles of the leading and lagging strands and Okazaki fragments in the process?.In this section, you will explore the following questions:
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