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DNA Replication Notes Purpose: To make an identical copy of DNA for cell division Process: Background: DNA must always be protected inside the nucleus except during cell division (mitosis); it will get destroyed by enzymes in the cytoplasm. enzymes and freenucleotides are available for replication in the nucleus. Part 1 Initiation: an enzyme called helicase attaches to DNA and breaks the Hydrogen bonds to unwind and separate the complementary strands. This creates a replication fork Part 2 Elongation: Another enzyme called DNA polymerase attaches to one strand. It reads the DNA code, and attaches complementary nucleotides to the original exposed strand. After it attaches each complementary nucleotide, it proofreads for mistakes. Replication is Quick: More Helicase and DNA Polymerase enzymes can attach at other sites on the strands to speed up the process. This creates the appearance of bubbles along the strand called replication bubbles. DNA molecule is antiparallel: Complementary strands run in opposite directions. Scientists label the ends 3’ and 5’. Leading and Lagging Strands: DNA polymerase can only attach new nucleotides to the 3’ end of the new DNA strand. This means that it must constantly back track to copy parts of the strand newly exposed by helicase. Since this process takes longer than the other strand, it is called the lagging strand. The Strand that does not need to do this is called the leading strand. The fragments that are created in the lagging strand are called Okazaki fragments. A third enzyme, Ligase, fuses the sugar-phosphate backbone of the Okazaki fragments together. Part 3 Termination: When complete, the result should be two perfect copies of the original. Though 3.3 billion base pairs of the human genome can be copied in a matter of hours, few mistakes (mutations) occur. This process is semiconservative: Each new molecule has one original strand and one new strand. ** Note: Make sure you understand the roles of the three enzymes, what semi-conservative means, and where in the cell this occurs and why.