meiosis and end up in different gametes (sperm and
ova) and are passed on to offspring.
law of independent assortment Gregor Mendel’s
second law, which says that after gametes form, the
separation of alleles for one gene is independent of the
separation of alleles for other genes; genes located on
nonhomologous chromosomes are independent from
one another. This is not true for genes that are linked,
where genes are located close to one another on the
same chromosome.
lead discovery The process of identifying active new
chemical entities that, by subsequent modification, can
be transformed into a clinically useful DRUG.
lead generation The term applied to strategies devel-
oped to identify compounds that possess a desired but
nonoptimized biological activity.
leading strand One of two new DNA strands that is
being replicated continuously, unlike the lagging strand.
The strand is made in the 5’ to 3’ direction, placing an
–OH group at the 3’ end for continuous polymerization
(adding nucleotides) at the 3’ growing tip by DNA poly-
merase that moves forwardwhen the template strands
unwind at the replication fork. It is the DNA polymerase
that plays the pivotal role in the process of life, since it is
responsible for duplicating the genetic information.
See alsoLAGGING STRAND.
lead optimization The synthetic modification of a
biologically active compound to fulfill all stereoelec-
tronic, physicochemical, pharmacokinetic, and toxico-
logic requirements for clinical usefulness.
leaf primordia Young leaves at the tip of a shoot
formed by the shoot apical meristem.
leaf veins Anetlike network called reticulate vena-
tion in vascular tissue in dicots. The veins are in paral-
lel venation to each other in monocots.
learned behavior Behavior that is not fixed but
acquired by trial and error or by observing others.leghemoglobin A monomeric HEMOGLOBINsynthe-
sized in the root nodules of leguminous plants that are
host to nitrogen-fixing bacteria. Has a high affinity
for dioxygen and serves as an oxygen supply for the
bacteria.
See alsoNITROGEN FIXATION.legume A pod-bearing plant that is a member of the
Leguminosae (Fabaceae) or pea or bean family. These
plants form symbiotic relationships with certain nitro-
gen-fixing bacteria (rizobia) in their root nodules to
acquire nitrogen for growth. Examples include beans,
peas, lentils, alfalfa, clover, and wildflowers like wild
blue lupine (Lupinus perennis).lek A place where males display in groups and
females choose for the purpose of fertilization; a special
kind of polygynous mating system.lemur Primates (prosimians) of the superfamily
Lemuroidea that live in trees; found only on
the island of Madagascar and the adjacent tiny
Comoro Islands. Related to monkeys, they have large
eyes, foxlike faces, and long furry tails (not prehen-
sile). There are only about 30 to 50 living species,
17 of which are endangered. They are small,
ranging from about an ounce up to about the size of
a house cat.lenticle A pore where gases are exchanged in the
stems of woody plants.Leonardo da Vinci (1452–1519) ItalianScientist,
Artist Leonardo da Vinci, one of the greatest minds of
all time, was born on April 15, 1452, near the town of
Anchiano near Vinci. He was an illegitimate child of a
notary, Piero da Vinci, and a peasant woman named
Caterina. In his teenage years he became an apprentice
in one of the best art studios in Italy in 1469, that of198 law of independent assortment