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Labeling The incorporation of a radioactive nucleotide into a nucleic acid molecule.

Lac selection A means of identifying recombinant bacteria containing vectors that carry the lac Z’ gene. The bacteria are plated on a medium that contains an analogue of lactose that gives a blue colour in the presence of β- galactosidase activity.

Lactose intolerance  Inability to digest lactose because of lack of intestinal lactase. This leads to accumulation of large amounts of gas and fluid in large intestine, which causes pain and diarrhea.

Lagging strand  Newly synthesized DNA strand formed at the growing fork as short, discontinuous segments, called Okazaki fragments, which are later joined by DNA ligase. Although overall lagging strand synthesis occurs in the 3’ to 5’ direction, each Okazaki fragment is synthesized in the 5’ to 3’ direction.

Lambda (λ) phage  A bacteriophage that infects E.coli; derivatives of which are extensively used as cloning vectors.

Laminar flow The flow of a fluid that closely follows the shape of a streamlined surface without turbulence. Used in connection with laminar airflow cabinets to imply a stable flow of air over the work area such as to minimize turbulence.

Laminar flow cabinet or hood A work station with filtered air flowing in a laminar nonturbulent flow parallel to (horizontal laminar flow) the work surface, to maintain the sterility of the work.

Langerhans’ cells  Antigen-presenting cells of the skin which emigrate to local lymph nodes to become dendritic cells; they are very active in presenting antigen to T cells.

L-Dopa  Dopamine precursor administered to pateints with Parkinson’s disease.

Leading strand  Newly synthesized DNA strand formed by continuous synthesis in the 5’ to 3’ direction at the growing fork. The direction of leading-strand synthesis is the same as movement of the growing fork.

Lectin pathway  A pathway of complement activation, initiated by mannan-binding lectin (MBL), which intersects the classical pathway.

Leukemia Malignant disease of the hemopoietic system, evident as circulating blast cells

Leukocyte  White blood cell.

Leukotrenes  A collection of metabolites of arachidonic acid which have powerful pharmacological effects.

Ligand  Any molecule, other than an enzyme substrate, that binds tightly and specifically to a macromolecule, usually a protein, forming a macromolecule-ligand complex, which usually signals some cellular response.

Ligase (DNA ligase) An enzyme that repairs single-stranded discontinuities in double-stranded DNA molecules in the cell. Purified DNA ligase is used in gene cloning to join DNA molecules together.

Linkage  The condition where two genes are both present in close proximity on a single chromosome and are usually inherited together.

Linkage analysis A technique for mapping the chromosomal position of a gene by comparing its inheritance pattern with that of genes and other loci whose map positions are already known.

Linker A synthetic, double-stranded oligonucleotide used to attach sticky ends to a blunt-ended molecule.

Lipofection Transfection of DNA by fusion with lipid encapsulated DNA.

Lipopolysaccharide (LPS)  A product of some Gram-negative bacterial cell walls which can act as a B-cell mitogen.

Lipsome  Spherical phospholipid bilayer structure with an aqueous interior that forms in vitro from phospholipids and may contain protein.

Locus coeruleus  Brainstem nucleus that projects to many brain parts and is implicated in directed attention.

Log phase  ref. Growth curve

Low-density lipoprotein (LDL)  protein-lipid aggregate that is major carrier of plasma cholesterol to cells.

LPS  = Lipopolysaccharide.

Lung compliance  Change in lung volume caused by given change in transpulmonary pressure. The greater the lung compliance, the more stretchable the lung wall.

Lymphokines  A generic term for molecules other than antibodies which are involved in signaling between cells of the immune system and are produced by lymphocytes (cf. interleukins)

Lymphoma A solid tumor of lymphoid cells.

Lysogen A bacterium that harbors a prophage.

Lysogenic cycle  Series of events in which a bacterial virus of a certain type enters a host cell and its DNA is incorporated into the host-cell genome in such a way that the virus (the prophage) lays dormant. The association of a prophage with the host-cell genome is called lysogeny. By various mechanisms, the prophage can be activated so that it replicates, produces new viral particles, and lyses the bacterium.

Lysosome  Small membrane-bounded organelle having an internal pH of 4-5 and containing hydrolytic enzymes, which aid in the digestion of material ingested by phagocytosis and endocytosis.

Lysozyme An enzyme that weakens the cell walls of certain types of bacteria.

Lytic infection cycle The pattern of infection displayed by a phage that replicates and lyses the host cell immediately after the initial infection. Integration of the phage DNA molecule into the bacterial chromosome does not occur.

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