DNA replication

DNA replication is the process by which a cell makes exact copies of its DNA. This process occurs in all organisms and is essential to biological inheritance, cell division, and repair of damaged tissues. DNA replication ensures that each of the newly divided daughter cells receives its own copy of each DNA molecule. DNA most commonly occurs in double-stranded form, made up of two complementary strands held together by base pairing of the nucleotides comprising each strand. The two linear strands of a double-stranded DNA molecule typically twist together in the shape of a double helix. During replication, the two strands are separated, and each strand of the original DNA molecule then serves as a template for the production of a complementary counterpart strand, a process referred to as semiconservative replication. As a result, each replicated DNA molecule is composed of one original DNA strand as well as one newly synthesized strand. Cellular proofreading and error-checking mechanisms ensure near-perfect fidelity for DNA replication. DNA replication usually begins at specific locations known as origins of replication which are scattered across the genome. Unwinding of DNA at the origin is accommodated by enzymes known as helicases and results in replication forks growing bi-directionally from the origin. Numerous proteins are associated with the replication fork to help in the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new strands by incorporating nucleotides that complement the nucleotides of the template strand. DNA replication occurs during the S (synthesis) stage of interphase. DNA replication can also be performed in vitro (artificially, outside a cell). DNA polymerases isolated from cells and artificial DNA primers can be used to start DNA synthesis at known sequences in a template DNA molecule. Polymerase chain reaction (PCR), ligase chain reaction (LCR), and transcription-mediated amplification (TMA) are...