Every cell in your body is produced by cell division. Before each cell divides, it must copy its genetic material in a process called D N A replication. Understanding of DNA replication comes largely from studies of E coli, bacteria that are found by the billions in your large intestine. Let's take a look at how D N A replication occurs in an E coli cell. As we zoom in, we see the D N A. At the origin of replication, the two strands of D N A separate, serving as templates for making new strands. The result is a replication bubble. The bubble grows in both directions, forming two replication forks. Let's zoom in on one of them. Many proteins work together at the replication fork. Only some are shown. Here, the D N A is unwound, and D N A polymerases, shown in orange, build new strands of D N A. Original parental D N A strands are shown in dark blue. Newly formed D N A strands are shown in light blue. Because strands in a D N A double helix run in opposite directions, the new strands must be made in different ways. One new strand, the leading strand, is built continuously. The other new strand, the lagging strand, is built in pieces. First, let's focus on the leading strand. D N A polymerase builds a new strand of D N A by adding D N A nucleotides one at a time. Each new nucleotide must pair up with its complementary nucleotide on the parental strand. Adding new nucleotides works the same way on both the leading and lagging strands. Each piece of the lagging strand begins with a short segment of R N A shown in red. A clamp surrounds the R N A and attaches to D N A polymerase, which builds the rest of the new piece as D N A. When the piece is finished, it is released from D N A polymerase. How are pieces of the lagging strand joined together? A different D N A polymerase removes R N A and replaces it with D N A. However, it cannot finish connecting the pieces. An enzyme called D N A ligase joins the pieces together. Growth of the leading and lagging strands continues on both sides of the replication bubble until there are two identical D N A molecules. Although bacteria are very different from humans, the process of D N A replication in bacteria is similar to what happens in your own cells.
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14. DNA Synthesis
Introduction to DNA Replication
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