Deoxyribonucleic Acid (DNA)

DNA replication, the process by which the double-stranded DNA molecule reproduces itself, is a complicated process, even in the simplest organisms. DNA synthesis—making new DNA from old—is complex because it requires the interaction of a number of cellular components and is rigidly controlled to ensure the accuracy of the copy, upon which the very life of the organism depends. This adds several verification steps to the procedure. Though the details vary from organism to organism, DNA replication follows certain rules that are universal to all.

DNA replication (duplication, or copying) is always semi-conservative. During DNA replication the two strands of the parent molecule unwind and each becomes a template for the synthesis of the complementary strand of the daughter molecule. As a result both daughter molecules contain one new strand and one old strand (from the parent molecule), hence the term semi-conservative. The replication of DNA always requires a template, an intact strand from the parent molecule. This strand determines the sequence of nucleotides on the new strand. Wherever the nucleotide on the template strand contains the base A, then the nucleotide to be added to the daughter strand at that location must contain the base T. Conversely, every T must find an A to pair with. In the same way, Gs and Cs will pair with each other and with no other bases.

Replication begins at a specific site called the replication origin when the enzyme DNA helicase binds to a portion of the double stranded helix and "melts" the bonds between base pairs. This unwinds the helix to form a replication fork consisting of two separated strands, each serving as a template. Specific proteins then bind to these single strands to prevent them from repairing. Another enzyme, DNA polymerase, proceeds to assemble the daughter strands using a pool of free nucleotide units which are present in the cell in an "activated" form.

High fidelity in the copying of DNA is vital to the organism and, incredibly, only about one error per one trillion replications ever occurs. This high fidelity results largely because DNA polymerase is a "self-editing" enzyme. If a nucleotide added to the end of the chain mismatches the complementary nucleotide on the template, pairing does not occur. DNA polymerase then clips off the unpaired nucleotide and replaces it with the correct one.

Occasionally errors are made during DNA replication and passed along to daughter cells. Such errors are called mutations. They have serious consequences because they can cause the insertion of the wrong amino acid into a protein. For example, the substitution of a T for an A in the gene encoding hemoglobin causes an amino acid substitution which results in sickle cell anemia. To understand the significance of such mutations requires knowledge of the genetic code.