Every cell in your body is produced by cell division. Before each cell divides it must copy its genetic material in a process called DNA 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 DNA replication occurs in an E. coli cell. As we zoom in, we see the DNA. At the origin of replication, the two strands of DNA 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 DNA is unwound, and DNA polymerases [polymerases = pol-IM-er-aces], shown in orange, build new strands of DNA. Original, parental DNA strands are shown in dark blue. Newly formed DNA strands are shown in light blue. Because strands in a DNA 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. DNA polymerase builds a new strand of DNA by adding DNA nucleotides [nucleotides = NEW-clee-oh-tides] 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 RNA, shown in red. A clamp surrounds the RNA and attaches to DNA polymerase, which builds the rest of the new piece as DNA. When the piece is finished, it is released from DNA polymerase. How are pieces of the lagging strand joined together? A different DNA polymerase removes RNA and replaces it with DNA. However, it cannot finish connecting the pieces. An enzyme called DNA ligase [ligase = LIE-gace] joins the pieces together. Growth of the leading and lagging strands continues on both sides of the replication bubble until there are two identical DNA molecules. Although bacteria are very different from humans, the process of DNA replication in bacteria is similar to what happens in your own cells. Copyright 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
