L

Labeling: The incorporation of a radioactive nucleotide into a nucleic acid molecule.
Lagging strand: The strand of newly replicated DNA that is synthesised discontinuously in the opposite direction from the replication fork. DNA synthesis proceeds in the 5' to 3' direction.
Lambda: A temperate phage that infects E. coli. Derivatives of phage lambda are widely used as cloning vectors.
Lambdoid: A group of phage that are related to the E. coli phage Lambda. Lambdoid phage share a similar gene organization and regulate the lysis-lysogeny decision in a similar way. Examples of lambdoid phage are the E. coli phage 434 and the Salmonella phage P22.
Latent period: The time between the initial infection of a cell with a virus and the production and release of new virions via lysis of the bacterial cell (contrast with eclipse period).
Leading strand: The strand of newly replicated DNA that is synthesised continuously in the same direction as the replication fork. DNA synthesis proceeds in the 5' to 3' direction.
Leaky mutation: A nucleotide substitution that changes the amino acid sequence of a protein that results in partial loss of its activity.
Ligase (DNA ligase): An enzyme that repairs single-stranded discontinuities in double-stranded DNA molecules in the cell. Ligase joins a 3'-OH residue of a deoxyribonucleotide to the 5'-phosphate residue of an adjacent deoxyribonucleotide. Purified DNA ligase is used in gene cloning to join DNA molecules together.
Ligation: The formation of a phosphodiester bond between two adjacent bases separated by a single-strand break. Catalyzed by DNA ligase.
Linkage: The tendency of genes located close together on the same DNA molecule to be coinherited. Typically expressed as percent coinheritance of two genetic markers.
Linkage disruption: A genetic rearrangement or insertion that alters the coinheritance of otherwise linked genes.
Linkage map: A genetic map assembled from recombination data that shows the order of mutant sites and genes along a nucleic acid molecule.
Linked: Two genes located close together on the same DNA molecule that are coinherited at detectable frequency during recombination. See Linkage.
Linker: A synthetic, double-stranded oligonucleotide used to attach sticky ends to a blunt-ended molecule.
Locus (loci): The position on a chromosome where a particular genetic trait resides. Sometimes used to describe multiple genes that affect the same function.
LPS: Lipopolysaccharide. A major componant of the outer layer of the outer membrane of Gram-negative bacteria.
Lysate: A solution containing progeny phage resulting from the lysis of a population of bacterial cells by infecting phage.
Lysis: Disruption of cells with release of the contents.
Lysogen: A bacterial cell carrying a phage genome as a repressed prophage.
Lysogenic cycle: The pattern of phage infection that involves integration of the phage DNA into the host chromosome.
Lysogenic conversion: Expression of particular genes by a prophage that confer a novel phenotype on the host (e.g. expression of a phage encoded toxin).
Lysogeny: The ability of a temperate bacteriophage to maintain itself as a quiescent prophage until induced into the lytic cycle.
Lysozyme: An enzyme that hydrolyzes the peptidoglycan within the cell walls of bacteria.
Lytic cycle: The development of a bacteriophage, either after infection of a host bacterium or after induction of a prophage, resulting in production and release of free progeny phage particles, and lysis of the host cell.
Lytic growth or lytic cycle: A phage infection where the phage replicates, matures, and ultimately lyses the bacterial cell, releasing free phage.
Lytic phage: A phage that can only enter the lytic cycle when it infects a sensitive bacterial cell.