semikonservativen

Semiconservative replication, known in German-language texts as semikonservativen Replikation, is the standard model for DNA replication in cells, in which each daughter DNA molecule consists of one parental strand and one newly synthesized strand. The term reflects that each new molecule inherits half of the original molecule as a template.

The model was experimentally demonstrated in 1958 by Meselson and Stahl using Escherichia coli and isotopic labeling of DNA. After growth in heavy nitrogen (N-15) and subsequent replication in light nitrogen (N-14), density-gradient centrifugation showed that after one generation all molecules were hybrid, and after a second generation there was a mix of hybrid and light molecules. These results supported semiconservative replication over conservative or dispersive alternatives.

Mechanistically, replication begins at origins with helicase unwinding the double helix, producing replication forks. Each parental strand serves as a template for synthesis of a new strand by DNA polymerases, which add nucleotides in the 5' to 3' direction. The leading strand is synthesized continuously, while the lagging strand is made in short Okazaki fragments that are joined by ligase. Primase lays RNA primers; proofreading by DNA polymerases ensures fidelity; topoisomerases relieve torsional stress, and sliding clamps enhance processivity.

Semiconservative replication occurs in both prokaryotes and eukaryotes, though regulatory differences exist. In eukaryotes, multiple origins and chromatin structure influence initiation and timing, while bacteria typically have a single circular chromosome with faster replication. The model underpins DNA repair and genetic inheritance and provides a framework for understanding mutations and genome maintenance.

Historically, semiconservativity contrasted with earlier, now-disfavored models of fully conservative or dispersive replication. It remains a foundational concept in molecular biology and genetics education.