DNA replication

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The text under the title „DNA replication “written by Brooker analyses the process in which existing DNA strands are used to make new ones. In this text author explains how the structures of DNA are involved in this process by overviewing bacterial DNA replication and some interesting features of eukaryotic DNA replication.

First chapter presents the basic characteristics of DNA replication. Process begins when two complementary strands unwind and each parental strand is used as template to synthesize new daughter strands acccording to the AT/GC rule. Those strands are identical so they contain the same information. According to the author, some scientists in the late 1950s thought of three models of DNA replication: conservative when parental strands stay together, semiconservative when the new DNA strand consists of one newly made strand and one old strand and dispersive model when DNA contains segments of both parental and newly made strands. Meselson and Stahl proved that DNA replication is semiconservative with an experiment whhere they labeled DNA with heavy and light isotopes of nitrogen and used centrifugation.

Second chapter examines how DNA replication occurs within bacteria. Synthesis begins at a single origin of replication and continues bidirectionally until two replication forks meet each other. Di

istinction is made between different types of proteins that are involved in E. coli DNA replication. To initiate the replication, DnaA proteins bind to five DnaA box sequences at the origin and unwind the AT-rich region. Following that, two DNA helicases bind to single-stranded DNA and move in a 5’ to 3’ direction to maintain the replication fork moving. In front of DNA helicase travels DNA gyrase which alleviates positive supercoiling. The author points out that in order to keep the DNA strands separated replication requires single-strand binding proteins. At the next step of replication primase synthesizes RNA primers, which have the leading strand and the lagging strand, and DNA polymerase synthesizes complementary strands of DNA. As there are five types of DNNA polymerase, the author notes that polymerase III is responsible for the most of the DNA replication. Several features are enumerated. Firstly, DNA polymerase needs an RNA primer to start the synthesis. Secondly, it can attach nucleotides only in the 5’ to 3’ direction. Because of these two features, there are some differences between the synthesis of leading strand and lagging strand. The leading strand is made continuously in the direction of the replication fork while the lagging strand consists of short Ok
kazaki fragments and is made in the direction away from the fork. To continue the replication, the RNA primers have to be removed by DNA polymerase I, which later synthesizes DNA to fill the empty area. Then DNA ligase completes the replication by catalyzing a covalent bond between fragments. Later the text presents the main facts about enzymatic features of DNA polymerase. It is a processive enzyme that uses deoxynucleoside triphosphates to synthesize new DNA strands. When replication comes to an end it is terminated at the pair of termination sequences, called ter s. . .

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