Living systems store, retrieve, transmit, and respond to information essential to life processes.ģ.A Heritable information provides for continuity of life.ģ.A.1 DNA, and in some cases RNA, is the primary source of heritable information.ġ.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.ģ.3 The student is able to describe representations and models that illustrate how genetic information is copied for transmission between generations. 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. 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. 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. 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, or 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. As was stated previously, DNA replication is more complex than simply unzipping the double helix and making new complementary strands.
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