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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD

DNA Replication

Prior to mitosis (the process by which non-reproductive or somatic cells divide) and meiosis (the process by which germ cell precursors divide to form sperm, egg, or other reproductive cells), each DNA double helix within a cell is duplicated. This process, called DNA replication, begins at precise locations encoded within DNA molecules. These sites are called origins of replication.

Enzymes called helicases facilitate unwinding of the DNA double helix. Once unwound, the DNA strands are replicated. DNA polymerase enzymes copy each strand of the DNA, relying on the complementarity of DNA base pairing to replicate each strand faithfully. Because of the precise process by which each strand of DNA is used as the template to produce another strand, DNA replication is said to be semi-conservative. That is, each new DNA double helix is made up of one old strand and one new strand of DNA.

On the leading strand, DNA is replicated in long 5' to 3' segments. However, on the lagging strand, DNA synthesis must proceed 5' to 3' in short fragments called Okazaki fragments. Okazaki fragments, believed to be ~1,000 to 2,000 bases in length, are joined together by enzymes called ligases to complete replication of the lagging strand.

DNA polymerase has a built-in proofreading capability. Because of this, DNA replication is a remarkably reliable process. In humans, the error rate is believed to be less than one base pair per billion.