replication

(also reduplication, autoreproduction, and autosynthesis), the process of the self-reproduction, or self-copying, of nucleic acids, genes, and chromosomes that occurs in all living cells. The mechanism of replication is based on the enzymatic synthesis of deoxyribonucleic acid (DNA) or ribo-nucleic acid (RNA), which occurs in accordance with the matrix principle.

The model of the structure of DNA—the double helix—was proposed in 1953 by J. Watson and F. Crick. This model both explained how genetic information is inscribed in a DNA molecule and made it possible to understand and experimentally study the chemical mechanisms of the duplication of genetic material. The strict specificity of the pairing of nitrogen bases in a DNA molecule is responsible for the complementarity of the sequences of bases in two chains and guarantees the precision of replication. The guanine-cytosine pair is stabilized by three hydrogen bonds, and the adenine-thymine pair is stabilized by two hydrogen bonds, thereby sharply diminishing the probability of the incorrect pairing of bases.

According to Watson and Crick, the process of DNA replication provides for the rupture of hydrogen bonds and the unwinding of the strands of a double helix; it also provides for the synthesis of complementary chains on solitary strands. As a result of DNA replication, two similar molecules arise from a single double-chain DNA molecule. In each daughter molecule there is one polynucleotide chain that is parental and one that is resynthesized (the semiconservative mechanism of replication).

DNA replication is a multistage genetically controlled process in which many enzymes participate, including DNA poly-merase, DNA ligase, and DNA methylase. The replication of DNA having a double chain has a fixed beginning and end and an oriented direction. The noncontinuous synthesis of new chains starts where the complementary chains of the parental DNA unwind (Figure 1).

At the beginning of each synthesized segment of a DNA chain there is a priming strand of RNA that consists of 50–100 nucleotides. The fragments of the DNA chains, each containing about 1,000 nucleotides, gradually form on the RNA with the participation of DNA polymerase. The primer RNA is then destroyed by ribonuclease, while the synthesized DNA fragments are combined to form long chains by the enzyme polynu-cleotideligase.

Replication is unique in viruses and bacteriophages that have DNA composed of a single strand. DNA composed of a single chain [the (+)-chain] is introduced into the host cell, after which a complementary chain [the (–)-chain] is synthesized on the (+)-chain serving as a matrix. New single-chain (+)-chains, which form new viral particles, are synthesized on the double-helix molecule that has formed (the replicative form). Viruses and bacteriophages that contain RNA replicate by the same principle. Therefore, in all known cases of replication, DNA and RNA pass through a stage during which molecules have a double chain.

Eucaryotes are higher organisms whose cells contain a formed nucleus. The main genetic function in eucaryotes is performed by complex structures known as chromosomes, which consist of DNA, RNA, proteins, and other substances. The replication of DNA and other chromosomal components occurs during interphase, which precedes cell division. The doubled chromosomes subsequently disjoin and become evenly distributed between daughter cells. Thus, all hereditary information is transmitted relatively unchanged from cell to cell from one generation to the next.