DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts when one double-stranded DNA molecule produces two identical copies of the molecule. The cell cycle (mitosis) also pertains to the DNA replication/reproduction process. The cell cycle includes interphase, prophase, metaphase, anaphase, and telophase. Each strand of the original double-stranded DNA molecule serves as template for the production of the complementary strand, a process referred to as semiconservative replication. Cellularproofreading and error toe-checking mechanisms ensure near perfect fidelity for DNA replication. In a cell, DNA replication begins at specific locations in the genome, called "origins". Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. In addition to DNA polymerase, the enzyme that synthesizes the new DNA by adding nucleotides matched to the template strand, a number of other proteins are associated with the fork and assist in the initiation and continuation of DNA synthesis. DNA replication can also be performed in vitro (artificially, outside a cell). DNA polymerases, isolated from cells, and artificial DNA primers are used to initiate DNA synthesis at known sequences in a template molecule. The polymerase chain reaction (PCR), a common laboratory technique, employs such artificial synthesis in a cyclic manner to amplify a specific target DNA fragment from a pool of DNA. -------------------------------------------------
DNA usually exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotideshaving the bases: adenine, cytosine, guanine, and thymine (commonly noted as A,C, G & T). A nucleotide is a mono-, di-, or triphosphate deoxyribonucleoside; that is, a deoxyribose sugar is attached to one, two, or three phosphates, and a base. Chemical interaction of these nucleotides forms phosphodiester linkages, creating the phosphate-deoxyribose backbone of the DNA double helix with the bases pointing inward. Nucleotides (bases) are matched between strands through hydrogen bonds to form base pairs. Adenine pairs with thymine (two hydrogen bonds), and cytosine pairs with guanine (three hydrogen bonds) because a purine must pair with a pyrimidine: a pyrimidine cannot pair with another pyrimidine because the strands would be very close to each other; in a purine pair, the strands would be too far apart and the structure would be unstable. If A-C paired, there would be one hydrogen not bound to anything, making the DNA unstable. DNA strands have a directionality, and the different ends of a single strand are called the "3' (three-prime) end" and the "5' (five-prime) end" with the direction of the naming going 5 prime to the 3 prime region. The strands of the helix are anti-parallel with one being 5 prime to 3 then the opposite strand 3 prime to 5. These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3' end of a DNA strand. The pairing of bases in DNA through hydrogen bonding means that the information contained within each strand is redundant. The nucleotides on a single strand can be used to reconstruct nucleotides on a newly synthesized partner strand. -------------------------------------------------
Main article: DNA polymerase
DNA polymerases adds nucleotides to the 3' end of a strand of DNA. If a mismatch is accidentally incorporated, the polymerase is inhibited from further extension. Proofreading removes the mismatched nucleotide and extension continues. DNA polymerases are a family of enzymes that carry out all forms of DNA...
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