Diastereoisomers Four variations of a given molecule, consisting of a pair of stereoisomers about a second asymmetric carbon atom for each of the two isomers of the first asymmetric carbon atom. See also

ISOMERS, CHIRAL COMPOUND.

Differentiation Refers to processes by which a single type of cells (stem cells, embryonic stem cells, etc.) become multiple, different types of (specialized) cells. See also CELL,

STEM CELLS, STEM CELL ONE, STEM CELL GROWTH

FACTOR (SCF), TOTIPOTENT STEM CELLS, COLONY

STIMULATING FACTORS (CSFs), EMBRYONIC STEM

CELLS, HUMAN EMBRYONIC STEM CELLS, GRANU-

LOCYTE-MACROPHAGE COLONY STIMULATING

FACTOR (GM-CSF), HEDGEHOG PROTEINS.

Digestion (within chemical production plants) Breakdown of feed stocks by various processes (chemical, mechanical, and biological) to yield their desired building-block components for inclusion as raw materials in subsequent chemical or biological processes.

Digestion (within organisms) The enzymeenhanced hydrolysis (breakdown) of major nutrients (food) in the gastrointestinal system to yield their building-block components (to the organism), such as amino acids, fatty acids, or other essential nutrients. See

also HYDROLYSIS, FATS, PROTEIN, AMINO ACID,

ESSENTIAL AMINO ACIDS, ESSENTIAL NUTRIENTS,

FATTY ACID, ESSENTIAL FATTY ACIDS, LIPASE,

“IDEAL PROTEIN” CONCEPT, ENZYME, PROTEASES,

PROTEOLYTIC ENZYMES, ABSORPTION, TRYPSIN,

LECITHIN, PROTEIN DIGESTIBILITY-CORRECTED

AMINO ACID SCORING (PDCAAS).

Diglycerides See TRIGLYCERIDES.

Diphtheria Antitoxin Discovered by Emil von Behring in 1900. See also ANTITOXIN,

ENTEROTOXIN.

Diploid The state of a cell in which each of the chromosomes, except for the sex chromosomes, is always represented twice (46 chromosomes in humans). In contrast to the haploid state in which each chromosome is represented only once. See also DIPLOPHASE,

CHROMOSOMES, HOMOZYGOUS, TRIPLOID.

Diplophase A phase in the life cycle of an organism in which the cells of the organism have two copies of each gene. When this

state exists the organism is said to be diploid.

See also DIPLOID, GENE, HOMOZYGOUS, CELL.

two covalently linked monosaccharide units; hence di for two. See also OLIGOSACCHARIDES,

MONOSACCHARIDES, POLYSACCHARIDES.

Dissimilation The breakdown of food material to yield energy and building blocks for cellular synthesis. See also DIGESTION (WITHIN

ORGANISMS).

Dissociating Enzymes See HARVESTING ENZYMES.

Distribution See “ADME” TESTS, PHARMACOKI-

NETICS.

Disulfide Bond An important type of covalent bond formed between two sulfur atoms of different cysteines in a protein. Disulfide bonds (linkages, bridges) contribute to holding proteins together and also help provide the internal structure (conformation) of the protein.

See also PROTEIN, CYSTEINE (cys), CYSTINE.

Diversity (within a species) R e f e r s t o t h e genetic variation that exists within a population (of organisms) in a species. For example, black cattle and white cattle; or both toxic and nontoxic strains/serotypes of Escherichia coliform (E. coli) bacteria. This diversity is due to one or more single-nucleotide polymorphisms (SNPs) in each individual’s genome (DNA) within the population of organisms. See also SPECIES, SINGLE-NUCLE-

OTIDE POLYMORPHISMS (SNPs), POLYMORPHISM

(GENETIC), NUCLEOTIDE, ORGANISM, STRAIN,

SEROTYPES, ESCHERICHIA COLIFORM (E. COLI),

ESCHERICHIA COLIFORM 0157:H7 (E. COLI 0157:H7).

Diversity Biotechnology Consortium A nonprofit U.S. organization formed in August of 1994 by a group of research institutions and companies. The consortium’s first president was Stuart A. Kauffman of the Santa Fe Institute. The consortium’s purpose is to further the use of molecular diversity as a tool in drug design, and in the study of mutating

ACID (DNA), METHYLATED, CELL, GENE, CANCER,

TRANSCRIPTION, GENETIC CODE, MESSENGER RNA

(mRNA), p53 GENE, TUMOR-SUPPRESSOR GENES.

DNA Microarray Initially developed by Patrick Brown during the 1980s, these microarrays enable analysis of the levels of expression of genes in an organism, or comparison of gene expression levels (e.g., between diseased and nondiseased tissues) via hybridization of messenger RNA (mRNA) to its counterpart DNA sequence, when biological samples containing DNA (e.g., in liquid) are passed over the array surface. To manufacture the DNA microarray, cellular mRNA is used to make segments of complementary DNA (cDNA) in lengths of approximately 500–5000 base pairs long, using the reverse transcriptase polymerase chain reaction (RT-PCR). These cDNA segments are then attached to a nylon or glass surface at known spots, so when hybridiza- tion-of-sample-DNA occurs, the location of the spot tells what DNA was in the sample. Another way to manufacture a type of DNA microarray is to similarly attach oligonucleotides or peptide nucleic acids of known sequence (composition) at known spots on the nylon or glass surface, and pass the biological sample containing DNA (e.g., in liquid) over that surface to identify the DNA in the sample by the spot to which it hybridizes. See also

GENE, ORGANISM, BIOCHIPS, MICROFLUIDICS,

DEOXYRIBONUCLEIC ACID (DNA), MESSENGER RNA

(mRNA), HYBRIDIZATION (MOLECULAR GENETICS),

EXPRESS, GENE EXPRESSION ANALYSIS, PROTEOM-

ICS, MICROARRAY (TESTING), OLIGONUCLEOTIDE,

NUCLEIC ACIDS, SEQUENCE (OF A DNA MOLECULE),

BIOINFORMATICS.

DNA Polymerase An enzyme that catalyzes the synthesis of DNA. The process is accomplished by catalyzing the addition of deoxyribonucleotide residues to the free 3′- hydroxyl end of a DNA chain, starting from a mixture of the appropriate triphosphorylated bases, which are dATP, dGTP, dCTP, and dTTP. This chemical reaction is reversible and, hence, DNA polymerase also functions as an exonuclease. See also ENZYME,

EXONUCLEASE, TAQ DNA POLYMERASE, DEOXY-

RIBONUCLEIC ACID (DNA), SYNTHESIZING (OF DNA

MOLECULES).

DNA Probe Also called gene probe or genetic probe. Short, specific (complementary to desired gene) artificially-produced segments of DNA are used to combine with and detect the presence of specific genes (or shorter DNA segments) within a chromosome. If a DNA probe of known composition and length is mingled with pieces of DNA (genes) from a chromosome, the probe will cling to its exact counterpart in the chromosomal DNA pieces (genes), forming a stable double-stranded hybrid. The presence of this (now) “labeled” probe is detected visually or with the aid of another detection instrument. Because the composition of the DNA probes is known, scientists can riffle through a chromosome, spotting segments of DNA (i.e., genes) that seem to be linked to genetic diseases. See also MUSCULAR DYSTROPHY (MD),

PROBE, POLYMERASE CHAIN REACTION (PCR),

GENE, POLYMERASE CHAIN REACTION (PCR) TECH-

NIQUE, CHROMOSOMES, DOUBLE HELIX, DUPLEX,

H Y B R I D I Z A T I O N (M O L E C U L A R G E N E T I C S ),

HYBRIDIZATION SURFACES, DEOXYRIBONUCLEIC

ACID (DNA), HOMEOBOX, RAPID MICROBIAL

DETECTION (RMD), SOUTHERN BLOT ANALYSIS.

DNA Profiling Invented in 1985 by Alec Jeffreys, this technique is used by forensic (i.e., crime-solving) chemists to match biological evidence (e.g., a blood stain) from a crime scene to the person (e.g., the assailant) involved in that particular crime. DNA profiling involves the use of RFLP (restriction fragment length polymorphism) analysis or ASO/PCR (allele-specific oligonucleotide/polymerase chain reaction) analysis to identify the specific sequence of bases (i.e., nucleotides) in a piece of DNA taken from the biological evidence. Since the specific sequence of bases in DNA molecules is different for each individual (due to DNA polymorphism), a criminal’s DNA can be matched to that of the evidence to prove guilt or innocence. Biological evidence may include, among other things, blood, hair, nail fragments, skin, and sperm. See also DEOXY-

RIBONUCLEIC ACID (DNA), RESTRICTION FRAG-

MENT LENGTH POLYMORPHISM (RFLP) TECHNIQUE,

POLYMORPHISM (CHEMICAL), POLYMERASE CHAIN

REACTION (PCR) TECHNIQUE, ALLELE, NUCLEO-

TIDE, NUCLEIC ACIDS, OLIGOMER, GENETIC CODE,