DNA Replication Making New DNA Molecules Chapter 12 Section 12-2 Pg. 297 - 299

Overview of DNA Replication DNA replication is the process by which the cell copies its DNA. Happens in the S phase (synthesis) of cell cycle. The double helix separates, and each strand serves as a template on which a new strand is built. Each new piece of DNA has one old strand and one new strand.

Step 1: Separation of the Strands In order for the DNA to replicate, the two strands need to be separated. This happens easily because the hydrogen bonds holding them together are weak. An enzyme called helicase breaks the hydrogen bonds and separates the strands. This separation creates a replication fork. Helicase will unwind the DNA in several different locations. This helps to speed up the process.

Step 2: Building New DNA A second enzyme called DNA polymerase attaches to the DNA strands to assemble the new strands. As DNA polymerase moves along the existing strands, new complementary strands are assembled using the base-pairing rules. The DNA polymerase joins the nucleotides by covalent bonds (a dehydration synthesis reaction) in the backbone and hydrogen bonds between the 2 strands.

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Step 3: Proofreading DNA replication is very accurate, with only about one error in 10,000 paired nucleotides. Mutations are mistakes or changes to the DNA. Some of them can be fixed, and some cannot. Some can have serious effects, and some do not affect the cell at all. Enzymes proofread and repair many of the mutations that can occur. This reduces the error rate to about 1 in every 1 billion base pairs.

End Result of Replication Replication results in the creation of two identical molecules of DNA. The process is described as being semi-conservative. “semi” = half “conserve” = to save Replication is semi-conservative, because each new molecule is made of one old strand of DNA and one newly created strand.