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Modern Genetic Analysis Glossary
L
lagging strand In DNA replication, the strand that is synthesized apparently in the 3 to 5 direction, by the ligation of short fragments synthesized individually in the 5 to 3 direction.
l (lambda) phage One kind ("species") of temperate bacteriophage.
l dgal A l phage carrying a gal bacterial gene and defective (d) for some phage function.
lampbrush chromosomes Large chromosomes found in amphibian eggs, with lateral DNA loops that produce a brushlike appearance under the microscope.
lateral inhibition The signal produced by one cell that prevents adjacent cells from acquiring its fate.
lawn A continuous layer of bacteria on the surface of an agar medium.
leader An untranslated segment at the 5 end of mRNA between the transcriptional and translational start sites.
leader sequence The sequence at the 5 end of an mRNA that is not translated into protein.
leading strand In DNA replication, the strand that is made in the 5 to 3 direction by continuous polymerization at the 3 growing tip.
leaky mutant A mutant (typically, an auxotroph) that results from a partial rather than a complete inactivation of the wild-type function.
leaky mutation A mutation that confers a mutant phenotype but still retains a low but detectable level of wild-type function.
lesion A damaged area in a gene (a mutant site), a chromosome, or a protein.
lethal gene A gene whose expression results in the death of the individual expressing it.
Leu Leucine (an amino acid).
library A collection of DNA clones obtained from one DNA donor.
ligand See ligandreceptor interaction.
ligandreceptor interaction The interaction between a molecule (usually of extracellular origin) and a protein on or within a target cell. One type of ligandreceptor interaction can be between steroid hormones and their cytoplasmic or nuclear receptors. Another can be between secreted polypeptide ligands and transmembrane receptors.
ligase An enzyme that can rejoin a broken phosphodiester bond in a nucleic acid.
line A group of identical pure-breeding diploid or polyploid organisms, distinguished from other individuals of the same species by some unique phenotype and genotype.
LINE (long interspersed element) A type of large repetitive DNA segment found throughout the genome.
linear tetrad A tetrad that results from the occurrence of the meiotic and postmeiotic nuclear divisions in such a way that sister products remain adjacent to one another (with no passing of nuclei).
linkage The association of genes on the same chromosome.
linkage group A group of genes known to be linked; a chromosome.
linkage map A chromosome map; an abstract map of chromosomal loci, based on recombinant frequencies.
locu (loci) See gene locus.
Lys Lysine (an amino acid).
lysis The rupture and death of a bacterial cell on the release of phage progeny.
lysogen See lysogenic bacterium.
lysogenic bacterium A bacterial cell capable of spontaneous lysis due, for example, to the uncoupling of a prophage from the bacterial chromosome.

© 1999 by W. H. Freeman and Company. All rights reserved.