How Dna Replication Occurs

DNA replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission. During DNA replication, the two nucleotide strands of the original double helix separate along the strands. Because the two strands are complementary, each strand serves as a template to make a new complementary strand. After replication, the two identical double-stranded DNA molecules separate and move to the new cells formed during cell division, as shown in Figure 10-9.

Steps of DNA Replication

The process of DNA replication is shown in Figure 10-10. In step Q, enzymes called helicases separate the DNA strands. Helicases move along the DNA molecule, breaking the hydrogen bonds between the complementary nitrogenous bases. This action allows the two DNA strands of the double helix to separate from each other. The Y-shaped region that results when the two strands separate is called a replication fork.

During step ©, enzymes called DNA polymerases add complementary nucleotides, found floating freely inside the nucleus, to each of the original strands. As the nucleotides on the newly forming strand are added, covalent bonds form between the adjacent nucleotides. Covalent bonds form between the deoxyribose sugar of one nucleotide and the phosphate group of the next nucleotide on the growing strand. Hydrogen bonds form between the complementary nitrogenous bases on the original and new strands.

By step G, DNA polymerases finish replicating the DNA and fall off. The result is two separate and identical DNA molecules that are ready to move to new cells in cell division.

In each new DNA double helix, one strand is from the original molecule, and one strand is new. This type of replication is called semi-conservative replication because each of the new DNA molecules has kept (or conserved) one of the two (or semi) original DNA strands.

Helicase separates the DNA strands.

DNA polymerases add nucleotides DNA polymerases are released.


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