Cellular Metabolism and Reproduction: Mitosis and Meiosis
The DNA molecule must be duplicated before cell di-
vision. The molecule separates where the hydrogen bonds
hold the two chains of nucleotides together and a new copy
of the DNA chain is constructed (Figure 4-9). The first step
is the unwinding of the molecule. This is accomplished by
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helicase enzymes that separate the hydrogen bonds be-
tween the base pairs and stabilize the nucleotide chains of
the double helix. Then new nucleotides are added to the
separated chains by DNA polymerase, another enzyme. In
this way, a copy of the DNA molecule is constructed.Guanine Cytosine
Adenine Thymine(^) Uracil
Original DNA molecule
DNA molecule unwinds
Old strand
New strand
Nucleotide
Old strand
New strand
New DNA molecule
New DNA molecule^
Figure 4- 9 DNA replication. The two chains of nucleotides separate at the base pairs and are
duplicated.
(^) ®
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