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.