label Chemical group, radioactive atom or fluorescent dye added to a molecule
in order to follow its progress through a biochemical reaction or to locate it
spatially. Also, as a verb, to add such a group or atom to a cell or molecule.
lagging strand One of the two newly synthesized strands of DNA found at a
replication fork. The lagging strand is made in discontinuous lengths that are
later joined covalently.
lambda bacteriophage (g bacteriophage) Virus that infects E. coli. Widely
used as a DNA cloning vector
lamellipodium (lamellipodia) Flattened, sheetlike protrusion supported by a
meshwork of actin filaments, which is extended at the leading edge of a
crawling animal cell.
laminin Extracellular matrix protein found in basal laminae, where it forms a
sheetlike network.
lamin see nuclear lamin
lampbrush chromosome Paired chromosome in meiosis in immature
amphibian eggs, in which the chromatin forms large stiff loops extending out
from the linear axis of the chromosome.
leading strand One of the two newly synthesized strands of DNA found at a
replication fork. The leading strand is made by continuous synthesis in the 5 -
to-3 direction.
lectin Protein that binds tightly to a specific sugar. Abundant lectins from plant
seeds are often used as affinity reagents to purify glycoproteins or to detect
them on the surface of cells.
leptotene The first phase of division I of meiosis, in which the paired
duplicated homologous chromosomes condense and become visible in the light
microscope.
lethal mutation A mutation that causes the death of the cell or the organism
that contains it.
leucine zipper Structural motif seen in many DNA-binding proteins in which
two a helices from separate proteins are joined together in a coiled-coil (rather
like a zipper), forming a protein dimer.
leucine-rich repeat protein (LRR protein) Common type of receptor serine/
threonine kinase in plants. Characterized by a tandem array of leucine-rich
repeat sequences in the extracellular portion.
leucocyte see white blood cell
leukemia Cancer of white blood cells.
ligand Any molecule that binds to a specific site on a protein or other
molecule. (From Latin ligare, to bind.)
ligase Enzyme that joins together (ligates) two molecules in an energydependent
process. DNA ligase, for example, joins two DNA molecules
together end to end through phosphodiester bonds.
light chain One of the smaller polypeptides of a multisubunit protein such as
myosin or immunoglobulin. Abbreviated as L chain in immunoglobulins.
lineage analysis Tracing the ancestry of individual cells in a developing
embryo.
linkage (1) Mutual effect of the binding of one ligand on the binding of
another that is a central feature of the behavior of all allosteric proteins. (2) Coinheritance
of two genetic loci that lie near each other on the same
chromosome. The closer together the two loci, that is, the greater the linkage,
the lower the frequency of recombination between them.
lipase Enzyme that catalyzes the cleavage of fatty acids from the glycerol
moiety of a triglyceride.
lipid Organic molecule that is insoluble in water but tends to dissolve in
nonpolar organic solvents. A special class, the phospholipids, forms the
structural basis of biological membranes.
lipid bilayer Thin bimolecular sheet of mainly phospholipid molecules that
forms the core structure of all cell membranes. The two layers of lipid
molecules are packed with their hydrophobic tails pointing inward and their
hydrophilic heads outward, exposed to water.
lipid raft Small region of the plasma membrane enriched in sphingolipids and
cholesterol.
lipophilic see hydrophobic
liposome Artificial phospholipid bilayer vesicle formed from an aqueous
suspension of phospholipid molecules.
local mediator Secreted signal molecule that acts at short range on adjacent
cells.
locus In genetics, the position of a gene on a chromosome. Different alleles of
the same gene all occupy the same locus.
long-term potentiation Long-lasting increase (days to weeks) in the
sensitivity of certain synapses in the hippocampus. Induced by a short burst of
4..4754 (50

repetitive firing in the presynaptic neurons.
low-density lipoprotein (LDL) Large complex composed of a single protein
molecule and many esterified cholesterol molecules, together with other lipids.
The form in which cholesterol is transported in the blood and taken up into
cells.
LTP see long-term potentiation
lumen Cavity enclosed by an epithelial sheet (in a tissue) or by a membrane
(in a cell).
lymph Colorless fluid derived from blood by filtration through capillary walls.
Carries lymphocytes in a special system of ducts and vessels the lymphatic
vessels.
lymphocyte Type of white blood cell responsible for the specificity of
adaptive immune responses. There are two main types: B cells, which produce
antibody, and T cells, which interact directly with other effector cells of the
immune system and with infected cells. T cells develop in the thymus and are
responsible for cell-mediated immunity. B cells develop in the bone marrow in
mammals and are responsible for the production of circulating antibodies.
lymphoid organ Organs involved in the production or function of
lymphocytes, such as thymus, spleen, lymph nodes, and tonsils.
lysis Rupture of a cell's plasma membrane, leading to the release of cytoplasm
and the death of the cell.
lysogeny State of a bacterium in which it carries the DNA of an inactive virus
integrated into its genome. The virus can subsequently be activated to replicate
and lyse the cell.
lysosome Membrane-bounded organelle in eucaryotic cells containing
digestive enzymes, which are typically most active at the acid pH found in the
lumen of lysosomes.
lysozyme Enzyme that catalyzes the cutting of polysaccharide chains in the
cell walls of bacteria.