Biotechnology

part of a biological test or testing pro-
cess (QPCR-Quantitative PCR, RT-
PCR-Reverse Transcriptase PCR, etc.).
See also 
AUTORADIOGRAPHY
,
CELL
,
DEOXYRIBO-
NUCLEIC ACID
 (
DNA
),
GENE EXPRESSION ANALY-
S I S
,
Q P C R
,
R T
-
P C R
,
R A D I O I M M U N O A S S A Y
,
RADIOIMMUNOTECHNIQUE
.
Lac Operon An operon in Escherichia coli
(E. coli) that codes for three enzymes
involved in the metabolism of lactose. See
also
OPERON
,
CODING SEQUENCE
,
ESCHERICHIA
COLIFORM
 (
E
.
COLI
).
Lachrymal Fluid (tears) A salty solution
produced by the tear glands to bathe and
lubricate the eye. Possesses antimicrobial
properties.
Lactoferricin A protein compound that acts to
inhibit pathogenic (disease-causing) bacteria
and yeasts (e.g., in the human body). See
also
PROTEIN
,
PATHOGEN
,
BACTERIA
,
YEAST
,
LACTOFERRIN
.
Lactoferrin A protein compound that is natu-
rally produced in human breast milk. Also
produced in cow’s milk. Consumption of
lactoferrin by infants (e.g., via nursing) helps
strengthen their immune system. Consump-
tion of lactoferrin by older humans helps
their immune system to resist infectious dis-
eases. Lactoferrin binds free iron (e.g., in
body fluids), thereby denying that iron to
pathogenic baceria (which need that iron to
grow/infect). Pepsin and some other pro-
teases (enzymes) can convert lactoferrin to
lactoferricin. See also 
PROTEIN
,
PATHOGEN
,
BACTERIA
,
GROWTH
  (
MICROBIAL
),
LACTOFERRI-
CIN
,
PEPSIN
,
PROTEASE
,
HIGH
-
LACTOFERRIN RICE
,
LACTOPEROXIDASE
.
Lactonase An enzyme that “breaks open” the
lactone ring in (molecular structure of) the
mycotoxin zearalenone. See also 
ENZYME
,
MYCOTOXIN
,
ZEARALENONE
,
TOXIN
.
Lactoperoxidase A   p r o t e i n   c o m p o u n d
(enzyme) that acts to inhibit pathogenic bac-
teria (e.g., in human body). See also 
PROTEIN
,
ENZYME
,
PATHOGEN
,
BACTERIA
.
Lambda Phage A bacteriophage that infects
Escherichia coli  (E. coli). It is commonly
used as a vector in recombinant DNA
(deoxyribonucleic acid) research. See also
PHAGE
,
ESCHERICHIA COLIFORM
 (
E
.
COLI
).
Langerhans Cells See
DENDRITIC LANGERHANS
CELLS
,
ISLETS OF LANGERHANS
.
Lathyrism See
GLUCOSINOLATES
.
© 2002 by CRC Press LLC

L
Laurate A medium chain length (i.e., C12)
fatty acid that is naturally produced by coco-
nut trees, oil palm trees, and certain species
of wild plants. In 1992, some canola variet-
ies were genetically engineered so that they
could also produce (desirable) laurate in
their seeds. See also 
FATTY ACID
,
FATS
,
CANOLA
,
GENETIC ENGINEERING
,
GENETIC CODE
,
LPAAT
PROTEIN
,
ACP
,
LAUROYL
-
ACP THIOESTERASE
,
HIGH
-
LAURATE CANOLA
.
Lauric Acid See
LAURATE
.
Lauroyl-ACP Thioesterase The enzyme that
is required for the synthesis (manufacturing)
of laurate in plants. For example, the pres-
ence of this enzyme in the California bay
tree (Umbellularia californica) causes its
seed oil to contain as much as 45% laurate.
See also 
LAURATE
,
ENZYME
,
LPAAT PROTEIN
,
HIGH
-
LAURATE CANOLA
.
Lazaroids A class of drugs being developed to
“bring back from the dead” tissues that have
been (almost) killed due to a lack of oxygen
(e.g., Krebs Cycle L caused by a clot block-
ing a vital artery). See also 
HUMAN SUPER-
OXIDE DISMUTASE
 (
h
SOD
),
FIBRIN
,
REPERFUSION
.
LDL See
LOW
-
DENSITY LIPOPROTEINS
 (
LDLP
).
LDLP See
LOW
-
DENSITY LIPOPROTEINS
.
LDLP Receptors See
LOW
-
DENSITY LIPOPRO-
TEINS
 (
LDLP
).
Leader See
LEADER SEQUENCE
.
Leader Sequence The nontranslated sequence
at the 5
′ end of mRNA that precedes the
initiation codon. See also 
MESSENGER RNA
(
m
RNA
),
CODON
.
Leaky Mutants A   m u t a n t   i n   w h i c h   t h e
mutated gene product, such as an enzyme,
still possesses a fraction of its normal bio-
logical activity. See also 
MUTATION
,
GENE
,
PROTEIN
,
BIOLOGICAL ACTIVITY
,
ENZYME
.
Lear See
CANOLA
.
Lecithin See
LECITHIN
  (
crude, mixture
),
LECITHIN
(
refined, specific
).
Lecithin (crude, mixture) A mixture of phos-
pholipids (i.e., lecithin-phosphatidylcholine,
cephalin, inositol phosphatides, glycerides,
tocopherols, glucosides, and certain pig-
ments). Historically, crude (mixture) lecithin
has often been utilized commercially in food
processing as an emulsifier, instantizing
agent, and lubricating agent. Because leci-
thin-phosphatidylcholine naturally contains
a high content of linoleic acid, consumption
by humans of lecithin-phosphatidylcholine
results in similar impact (e.g., lowered cho-
lesterol levels in blood) as consumption of
linoleic acid. Because dietary fats are gen-
erally not absorbed directly through the
intestinal wall (when eaten), they must first
be emulsified, to form micelles that can pass
through the intestinal wall and thus be
absorbed by the body. That emulsifica-
tion/micelle-formation is aided by lecithin,
since it is an emulsifier. See also 
LECITHIN
(
refined, specific
),
LIPOPROTEIN
,
LIPIDS
,
CONJU-
GATED PROTEIN
,
HIGH
-
DENSITY LIPOPROTEINS
(
HDLP
),
LOW
-
DENSITY LIPOPROTEINS
  (
LDLP
),
SOYBEAN PLANT
,
SOYBEAN OIL
,
CHOLINE
,
SIGNAL
TRANSDUCTION
,
LINOLEIC ACID
,
ACETYLCHOLINE
,
FATS
,
MICELLE
,
DIGESTION
 (
WITHIN ORGANISMS
),
BILE ACIDS
.
Lecithin (refined, specific) A by-product of
the refining process for soybean oil (deoiled
lecithin from processed soybeans is com-
posed of approximately 20–25% phosphati-
dyl choline by weight). The lecithin
molecule (i.e., phosphatidyl choline) natu-
rally contains a high content of linoleic acid,
so consumption of lecithin by humans
results in similar impact (e.g., lowered cho-
lesterol levels in blood) as consumption of
linoleic acid. Because dietary fats are gen-
erally not absorbed directly through the
intestinal wall (when eaten), they must first
be emulsified to form micelles that can pass
through the intestinal wall and be absorbed
by the body. That emulsification/micelle-for-
mation is aided by lecithin, since it is an
emulsifier.
Lecithin (also known as phosphatidylcho-
line) is a source of choline when digested,
and is a critical component of the lipopro-
teins that transport fat and cholesterol mol-
ecules in the bloodstream (e.g., from the
digestive system, to body cells, to the liver,
etc.). Lecithin (phosphatidylcholine) pro-
motes synthesis of high-density lipoproteins
(HDLP, also known as “good” cholesterol)
by the liver, when it is consumed by humans.
Phosphatidyl choline (PC) is involved in cell
signal transduction (e.g., via which a cell
reacts to an external chemical “signal”).
Some other common dietary sources of
© 2002 by CRC Press LLC

L
lecithin include eggs, red meats, spinach,
and nuts. See also 
LIPOPROTEIN
,
LIPIDS
,
CONJU-
GATED PROTEIN
,
HIGH
-
DENSITY LIPOPROTEINS
(
HDLP
),
LOW
-
DENSITY LIPOPROTEINS
  (
LDLP
),
SOYBEAN PLANT
,
SOYBEAN OIL
,
CHOLINE
,
SIGNAL
TRANSDUCTION
,
LINOLEIC ACID
,
ACETYLCHOLINE
,
LECITHIN
  (
crude, mixture
),
FATS
,
MICELLE
,
DIGES-
TION
 (
WITHIN ORGANISMS
).
Lectins A class of proteins that have the capa-
bility to rapidly (and reversibly) combine
with specific sugar molecules (e.g., those
sugar molecules or glycoproteins on the sur-
face of adjacent cells, within an organism).
Lectins are a common component of the sur-
face (membranes) of plant and animal cells,
and are so specific (regarding sugar mole-
cules that they will or won’t combine
with/attach to) that they discriminate
between different monosaccharides and dif-
ferent oligosaccharides (i.e., on the surfaces
of adjacent cells within an organism). This
capability to reversibly combine with sugar
(i.e., carbohydrate) molecules (on the sur-
face of adjacent cells) is utilized by:
• Bacteria and other microorganisms, to
adhere to (sugar molecules on surface
of ) host cells, as the first step in the
process of infecting those host cells
• White blood cells (e.g., lymphocytes),
to adhere to the walls of blood vessels
(endothelium), as the first step to leav-
ing the bloodstream to go fight infection
(pathogens, trauma) in tissue adjacent to
that blood vessel. The lectin (glycopro-
tein) that adheres to the (endothelial
sugar molecule on) blood vessel wall is
called L-selectin, or the homing receptor.
The two sugar molecules (glycoproteins)
on the blood vessel wall (endothelium)
are called P-selectin and E-selectin (also
known as ELAM-1)
• Cancerous tumor cells, to adhere to the
walls of blood vessels (endothelium) as
part of the tumor-proliferation process
known as metastasis (i.e., new tumors
are “seeded” throughout the body via
this process).
Separate and apart from the above impacts,
some plant lectins (e.g., in the seeds of certain
plants) are toxic to some of the animals that
consume those seeds. See also 
PROTEIN
,
SUGAR
MOLECULES
,
GLYCOPROTEIN
,
LEUKOCYTES
,
SELECTINS
,
LYMPHOCYTES
,
MONOCYTES
,
NEUTRO-
PHILS
,
ENDOTHELIAL CELLS
,
ENDOTHELIUM
,
CAN-
CER
,
METASTASIS
,
SIGNAL TRANSDUCTION
.
Leptin A protein hormone that is produced by
fat cells (adipose tissue) in the body. When
leptin is produced and travels to cells whose
surface bears leptin receptors (e.g., in the
brain), those (brain) cells receive signal
(transduction) indicating fullness/satiety.
Leptin has been found to be present in the
bloodstream of obese humans at a concen-
tration of approximately four times the con-
centration found in bloodstreams of lean
humans. High levels of leptin present in the
bloodstream disrupt some of the activities of
insulin (hormone which regulates blood
sugar levels), and may possibly lead to dia-
betes. See also 
HORMONE
,
PROTEIN
,
BIOLOGICAL
ACTIVITY
,
INSULIN
,
ADIPOSE
.
Leptin Receptors Cellular receptors which
are specific to leptin. In 1996, H. Ralph
Snodgrass discovered that leptin receptors
are involved in the “sorting” of immature
blood cells (from bone marrow) to create
subpopulations. See also 
LEPTIN
,
RECEPTORS
.
Lethal Mutation Mutation of a gene to yield
no, or a totally defective, gene product (pro-
tein), thereby making it unable to function,
and hence unable to sustain the life of the
organism.
Leucine (leu) A monocarboxylic essential
amino acid. See also 
AMINO ACID
,
ESSENTIAL
AMINO ACIDS
,
ALS GENE
.
Leukocytes (white blood cells) A diverse fam-
ily of nucleated cells that has many immu-
nological functions. See also 
NEUTROPHILS
,
EOSINOPHILS
,
BASOPHILS
,
LYMPHOCYTE
,
B LYM-
PHOCYTES
,
MONOCYTES
,
GRANULOCYTES
.
Leukotrienes Lipid mediator molecules (syn-
thesized from arachidonic acid) released by
certain cells (T cells), which “signal” leuko-
cytes (white blood cells) during the initial
stages of an infection or an allergic reaction.
When thus activated, the leukocytes migrate
to the site of infection to combat the patho-
gens (or allergens), and mediate the inflam-
mation. See also 
LIPIDS
,
LEUKOCYTES
,
MAST
CELLS
,
SIGNALING
,
SIGNAL TRANSDUCTION
,
© 2002 by CRC Press LLC

L
T CELLS
,
PATHOGEN
,
ARACHIDONIC ACID
,
ALLER-
GIES
,
SIGNALING MOLECULE
.
Levorotary (L) Isomer An isomer of an opti-
cally active compound; rotates (when illu-
minated) the plane of plane-polarized light
to the left. See also 
STEREOISOMERS
,
DEX-
TROROTARY
 (
D
)
ISOMER
.
LH See
LUTEINIZING HORMONE
.
Library A set of cloned DNA fragments
together representing the entire genome. See
also
DEOXYRIBONUCLEIC ACID
 (
DNA
),
GENOME
.
Ligand (in biochemistry) In general, a mole-
cule or ion that can bind to (interact with) a
protein molecule. For example, a pharma-
ceutical that binds to a receptor protein mol-
ecule on the surface of a cell may be called
a ligand. See also 
PROTEIN
,
RECEPTORS
,
T CELL
RECEPTORS
,
ENDOCYTOSIS
,
CD
4
PROTEIN
,
INVA-
SIN
,
LIGAND
 (
IN CHROMATOGRAPHY
),
CHELATION
.
Ligand (in chromatography) A term used to
describe a substance (the ligand) that has the
capacity for specific and noncovalent
(reversible) binding to some protein. A
ligand may be a coenzyme for a specific
enzyme. The ligand can be covalently
attached (immobilized) by means of the
appropriate chemical reaction to the surface
of certain porous column material. When a
mixture of proteins containing the enzyme
to be isolated is passed through the column,
the enzyme, which is capable of tightly bind-
ing to the ligand, does so, and is in this
manner held to the column. The other pro-
teins present, which have no specific affinity
for the ligand, pass on through the column.
The protein/ligand complex is then dissoci-
ated and the enzyme eluted from the column,
which may be accomplished by passing
more free (unbound) coenzymes through the
column. The ligand may be hormones (i.e.,
used to isolate receptor molecules) or any
other type of molecule that is capable of
binding specifically and reversibly to the
desired protein or protein complex. See also
AFFINITY CHROMATOGRAPHY
,
SUBSTRATE
  (
IN
CHROMATOGRAPHY
),
CHROMATOGRAPHY
,
PROTEIN
,
PEPTIDE
,
ANTIBODY
,
MONOCLONAL ANTIBODIES
(
MA
b
).
Ligase An enzyme used to catalyze the joining
of single-stranded DNA segments. See also
DEOXYRIBONUCLEIC ACID
 (
DNA
).
Ligation The formation of a phosphodiester
bond to link two adjacent bases separated by
a nick in one strand of a double helix of DNA
(deoxyribonucleic acid). The term can also be
applied to blunt-end ligation and to the join-
ing of RNA (ribonucleic acid) strands. See
also
DEOXYRIBONUCLEIC ACID
 (
DNA
),
LIGASE
.
Light-Chain Variable (VL) Domains The
regions (domains) of the antibody (mole-
cule’s) “light chain” that vary in their amino
acid sequence. The “chains” (of atoms) com-
prising the antibody (immunoglobulin) mol-
ecule consist of a region of variable (V)
amino acid sequence and a region in which
the amino acid sequence remains constant
(C). An antibody molecule possesses two
antigen binding sites, and it is the variable
domains of the light (VL) and heavy (VH)
chains which contribute to this (antigen
binding ability). See also 
ANTIBODY
,
IMMUNO-
GLOBULIN
,
PROTEIN
,
SEQUENCE
  (
OF A PROTEIN
MOLECULE
),
ANTIGEN
,
AMINO ACID
,
COMBINING
SITE
,
DOMAIN
  (
OF A PROTEIN
),
HEAVY
-
CHAIN
VARIABLE
 (
VH
)
DOMAINS
.
Lignans A category of phytochemicals that
play defensive roles (e.g., against infections
by bacteria, fungi, etc.) within land plants
(e.g., those grown by man for crops). Lig-
nans are also sometimes referred to by some
people as “phytoestrogens,” and are typi-
cally beneficial to the health of humans that
consume them. Lignans are found in virtu-
ally all fruits, vegetables, and cereals
(grains); generally within the seed coats,
stems, leaves, or flowers. One of the benefi-
cial lignans commonly consumed by
humans is sesamin, found in seeds of the
sesame plant (Sesamum indicum); which
acts as an antioxidant. See also 
PHYTOCHEM-
ICALS
,
PHYTOESTROGENS
,
ISOFLAVONES
,
ANTI-
OXIDANTS
,
OXIDATIVE STRESS
.
Lignins A category of phenolic (“ring-shaped”
molecules) polymeric (i.e., composed of
more than one molecular unit) compounds
produced by land plants within the cell walls
(i.e., exterior of cell’s plasma membrane) of
those plants, to reinforce/strengthen those
cell walls. See also 
CELL
,
POLYMER
,
PLASMA
MEMBRANE
.
Lignocellulose A complex biopolymer com-
prising the bulk of woody plants. It consists
© 2002 by CRC Press LLC

L
of polysaccharides and polymer phenols.
See also 
POLYSACCHARIDES
,
LIGNINS
.
Limonene See
PHYTOCHEMICALS
.
Linkage A phenomenon discovered by Tho-
mas Hunt Morgan in the early 1900s via his
experiments with fruit flies. This term
describes the tendency of genes to be inher-
ited together as a result of their locations
being physically close to each other on the
same chromosome; measured by percent
recombination between loci. Because the
locus (location of gene on the chromosome)
determines the likelihood that two genes will
go together into offspring, “marker genes”
that are linked to a gene (e.g., for a given
trait or disease) of interest can be utilized to
predict the presence of that (trait or disease-
causing) gene. See also 
GENE
,
LOCUS
,
CHRO-
MOSOMES
,
LINKAGE GROUP
,
MARKER
  (
GENETIC
MARKER
),
MAP DISTANCE
,
LINKAGE MAP
.
Linkage Group Includes all loci (in DNA
molecule) that can be connected (directly or
indirectly) by linkage relationships; equiva-
lent to a chromosome. See also 
LOCUS
,
CHRO-
MOSOMES
,
LINKAGE
,
CHROMATIDS
,
CHROMATIN
,
LINKAGE MAP
,
DEOXYRIBONUCLEIC ACID
 (
DNA
).
Linkage Map A depiction of gene loci (on
chromosomes) based on the frequency of
recombination (of linked genes) in the off-
spring’s genome. See also 
LINKAGE
,
LINKAGE
GROUP
,
GENE
,
LOCUS
,
MARKER
  (
GENETIC
MARKER
).
Linker A short synthetic duplex oligonucle-
otide containing the target site for some
restriction enzyme. It may be added to the
ends of a DNA (deoxyribonucleic acid) frag-
ment prepared by cleavage with some other
enzyme reconstructions of recombinant
DNA.
Linking The process of “attaching” a drug or
a toxin to a monoclonal antibody, or another
homing molecule of the immune system.
Because this attachment must be reversible,
so that the homing molecule can release the
drug or toxin after delivering that drug or
toxin to the desired site in the body (e.g.,
delivery of a toxin to a tumor, to kill the
tumor), linking is a difficult process to reli-
ably achieve. See also 
IMMUNOTOXIN
,
CONJU-
GATE
,
MONOCLONAL ANTIBODIES
 (
MA
b
),
TOXIN
.
Linoleic Acid One of the so-called “omega-6”
(n-6) polyunsaturated fatty acids (PUFA), it
has historically comprised approximately
53% of the total fatty acid content of soybean
oil. It is an essential fatty acid for humans.
When consumed by humans, linoleic acid
causes LDLP cholesterol levels in the blood
to decrease, which reduces risk of coronary
heart disease (CHD). The human body con-
verts linoleic acid to the n-6 highly unsatur-
ated fatty acid (HUFA) arachidonic acid. See
also
POLYUNSATURATED FATTY ACIDS
  (
PUFA
),
N-
6
FATTY ACIDS
,
FATS
,
UNSATURATED FATTY
ACIDS
,
ESSENTIAL FATTY ACIDS
,
LOW DENSITY
LIPOPROTEINS
  (
LDLP
),
CHOLESTEROL
,
LECITHIN
,
CONJUGATED LINOLEIC ACID
  (
CLA
),
CORONARY
HEART DISEASE
 (
CHD
),
VOLICITIN
,
SOYBEAN OIL
,
ARACHIDONIC ACID
,
COSUPPRESSION
.
Linolenic Acid Also known as 
α-linolenic
acid. One of the so-called “omega-3” (n-3)
polyunsaturated fatty acids (PUFA), it has
historically comprised approximately 8% of
the total fatty acid content of soybean oil. It
is an essential fatty acid for humans (i.e.,
required by the human body). The human
body converts linolenic acid to the n-3 highly
unsaturated fatty acids (HUFA) docosahex-
anoic acid (DHA) and eicosapentanoic acid
(EPA). When consumed by humans, both
DHA and EPA confer various health benefits
to the human body. See also 
N-
3
FATTY ACIDS
,
POLYUNSATURATED FATTY ACIDS
 (
PUFA
),
UNSAT-
URATED FATTY ACIDS
,
ESSENTIAL FATTY ACIDS
,
CORONARY HEART DISEASE
  (
CHD
),
CANCER
,
HIGHLY UNSATURATED FATTY ACIDS
  (
HUFA
),
DOCOSAHEXANOIC ACID
  (
DHA
),
EICOSAPEN-
TANOIC ACID
 (
EPA
),
FATS
.