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Terms and Definitions

These terms and their definitions have been developed from glossaries prepared by (1) U.S. Congressional Office of Technology Assessment, (2) National Institutes of Health's Understanding Gene Therapy On-line Glossary, and (3) Genetics Education Center, University of Kansas Medical Center.

To find a term select the first letter of the word you are seeking or browse the complete glossary.

Acronyms not appearing in the glossary, may be included in the List of Acronyms.

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  • J  | 
  • K  | 
  • L  | 
  • M  | 
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  • S  | 
  • T  | 
  • U  | 
  • V  | 
  • W  | 
  • X  | 
  • Y  | 
  • Z

  • Acquired mutations
    Gene changes that arise within individual cells and accumulate throughout a person's lifetime; also called somatic mutations. (See hereditary mutation.)
  • Adenine (A)
    A nitrogenous base, one member of the base pair A-T (adenine-thymine).
  • Advanced maternal age
    Women over age 34 (age 35 at delivery) at increased risk for genetic changes in fetus.
  • Alleles
    Variant forms of the same gene. Different alleles produce variations in inherited characteristics such as eye color or blood type.
  • Alpha-fetoprotein (AFP)
    A protein excreted by the fetus into the amniotic fluid and from there into the mother's bloodstream through the placenta.
  • Amino acid
    Any of a class of 20 molecules that are combined to form proteins in living things. The sequence of amino acids in a protein and hence protein function are determined by the genetic code.
  • Amino acid sequence
    The linear order of the amino acids in a protein or peptide.
  • Amniocentesis
    Prenatal diagnosis method using cells in the amniotic fluid to determine the number and kind of chromosomes of the fetus and, when indicated, perform biochemical studies.
  • Amniocyte
    Cells obtained by amniocentesis.
  • Amplification
    An increase in the number of copies of a specific DNA fragment; can be in vivo or in vitro. See cloning, polymerase chain reaction.
  • Aneuploidy
    State of having different chromosome numbers (too many or too few). (i.e. Down syndrome, Turner syndrome).
  • Artificial insemination
    The placement of sperm into a female reproductive tract or the mixing of male and female gametes by other than natural means.
  • Autoradiography
    A technique that uses X- ray film to locate radioactively labeled molecules or fragments of molecules; used in analyzing length and number of DNA fragments after they are separated by gel electrophoresis.
  • Autosome
    Any of the non-sex-determining chromosomes. Human cells have 22 pairs of autosomes.
  • B    Back to Top

    A virus whose host is a bacterium; commonly called phage.
  • Barr body
    The condensed single X-chromosome seen in the nuclei of somatic cells of female mammals.
  • Base pair (bp)
    The two complementary, nitrogen-rich molecules held together by weak chemical bonds. Two strands of DNA are held together in the shape of a double helix by the bonds between their base pairs. (See chemical base.)
  • Base sequence
    The order of nucleotide bases in a DNA molecule.
  • Base sequence analysis
    A method, sometimes automated, for determining the base sequence.
  • Benign (be-NINE)
    Not cancerous; does not invade nearby tissue or spread to other parts of the body.
  • Biological therapy (by-o-LOJ-i-kul)
    Treatment to stimulate or restore the ability of the immune system to fight infection and disease. Also called immunotherapy.
  • Biopsy (BY-op-see)
    The removal of a sample of tissue, which is then examined under a microscope to check for cancer cells. When only a sample of tissue is removed, the procedure is called incisional biopsy; when the whole tumor is removed, it is excisional biopsy. Removing tissue or fluid with a needle is called needle biopsy or needle aspiration.
  • Biotechnology
    A set of biological techniques developed through basic research and now applied to research and product development.
  • Bone marrow transplantation
    A procedure in which doctors replace marrow destroyed by treatment with high doses of anticancer drugs or radiation. The replacement marrow may be taken from the patient before treatment or may be donated by another person. When the patient's own marrow is used, the procedure is called autologous (aw-TOL-o-gus) bone marrow transplantation.
  • BRCA1
    A gene that normally helps to restrain cell growth.
  • BRCA1 breast cancer susceptibility gene
    A mutated version of BRCA1, which predisposes a person toward developing breast cancer.
  • C    Back to Top

    Carcinogen (kar-SIN-o-jin)
    A substance or agent that is known to cause cancer.
  • Carrier
    An individual heterozygous for a single recessive gene.
  • Catheter (KATH-e-ter)
    A thin plastic tube. When a catheter is place in a vein, it provides a pathway for drugs, nutrients, or blood products. Blood samples also can be removed through the catheter.
  • cDNA
    Complementary DNA produced from a RNA template by the action of RNA-dependent DNA polymerase.
  • Cell
    Small, watery, membrane-bound compartment filled with chemicals; the basic subunit of any living thing.
  • Centimorgan (cM)
    A unit of measure of recombination frequency. One centimorgan is equal to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing over in a single generation. In human beings, 1 centimorgan is equivalent, on average, to 1 million base pairs.
  • Centromere
    A specialized chromosome region to which spindle fibers attach during cell division.
  • Chemical base
    An essential building block. DNA contains four complementary bases: adenine, which pairs with thymine, and cytosine, which pairs with guanine. In RNA, thymine is replaced by uracil.
  • Chemotherapy (kee-mo-THER-a-pee)
    Treatment with anticancer drugs.
  • Chorionic villus sampling
    An invasive prenatal diagnostic procedure involving removal of villi from the human chorion to obtain chromosomes and cell products for diagnosis of disorders in the human embryo.
  • Chromosome banding
    A technique for staining chromosomes so that bands appear in a unique pattern particular to the chromosome.
  • Chromosomes
    Structures found in the nucleus of a cell, which contain the genes. Chromosomes come in pairs, and a normal human cell contains 46 chromosomes, 22 pairs of autosomes and two sex chromosomes, x and y.
  • Clinical trials
    Research studies that involve patients.
  • Clone
    A group of identical genes, cells, or organisms derived from a single ancestor.
  • Cloned DNA
    Any DNA fragment that passively replicates in the host organism after it has been joined to a cloning vector.
  • Cloning
    The process of making genetically identical copies.
  • Codon
    A sequence of three nucleotides in messenger mRNA that specifies an amino acid.
  • Colonoscopy
    Examination of the colon through a flexible, lighted instrument called a colonoscope.
  • Colony-stimulating factors
    Substances that stimulate the production of blood cells. Treatment with colony-stimulating factors (CSF) can help the blood- forming tissue recover from the effects of chemotherapy and radiation therapy. These include granulocyte colony-stimulating factors (G-CSF) and granulocyte-macrophage colony-stimulating factors (GM-CSF).
  • Consanguinity
    Genetic relationship. Consanguineous individuals have at least one common ancestor in the preceding few generations.
  • Conservative change
    An amino acid change that does not affect significantly the function of the protein.
  • Conserved sequence
    A base sequence in a DNA molecule (or an amino acid sequence in a protein) that has remained essentially unchanged throughout evolution.
  • Contiguous genes
    Genes physically close on a chromosome that when acting together express certain traits.
  • CT or CAT scan
    Detailed pictures of areas of the body created by a computer linked to an x-ray machine. Also called computed tomography scan or computed axial tomography scan.
  • Cytogenetics
    The study of chromosomes.
  • Cytoplasm
    The cellular substance outside the nucleus in which the cell's organelles are suspended.
  • D    Back to Top

    The loss of a segment of the genetic material from a chromosome.
  • Deletion mapping
    The use of overlapping deletions to localize the position of an unknown gene on a chromosome or linkage map.
  • Diploid
    A full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. Most animal cells except the gametes have a diploid set of chromosomes. The diploid human genome has 46 chromosomes. Compare haploid.
  • DNA
    The substance of heredity; a large molecule that carries the genetic information that cells need to replicate and to produce proteins. DNA is a double-stranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T). In nature, base pairs form only between A and T and between G and C; thus the base sequence of each single strand can be deduced from that of its partner.
  • DNA fingerprint technique
    A method employed to determine differences in amino acid sequences between related proteins; relies upon the presence of a simple tandem-repetitive sequences that are scattered throughout the human genome.
  • DNA hybridization
    A technique for selectively binding specific segments of single-stranded (ss) DNA or RNA by base pairing to complementary sequences on ssDNA molecules that are trapped on a nitrocellulose filter.
  • DNA probe
    Any biochemical used to identify or isolate a gene, a gene product, or a protein.
  • DNA repair genes
    Certain genes that are part of a DNA repair pathway; when altered, they permit mutations to pile up throughout the DNA.
  • DNA replication
    The use of existing DNA as a template for the synthesis of new DNA strands. In humans and other eukaryotes, replication occurs in the cell nucleus.
  • DNA sequence
    The relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome. See base sequence analysis.
  • Domain
    A discrete portion of a protein with its own function. The combination of domains in a single protein determines its overall function.
  • Dominant
    Alleles that determine the phenotype displayed in a heterozygote with another (recessive) allele.
  • Double helix
    The shape that two linear strands of DNA assume when bonded together.
  • E    Back to Top

    E. coli
    Common bacterium that has been studied intensively by geneticists because of its small genome size, normal lack of pathogenicity, and ease of growth in the laboratory.
  • Electrophoresis
    A method of separating large molecules (such as DNA fragments or proteins) from a mixture of similar molecules. An electric current is passed through a medium containing the mixture, and each kind of molecule travels through the medium at a different rate, depending on its electrical charge and size. Separation is based on these differences. Agarose and acrylamide gels are the media commonly used for electrophoresis of proteins and nucleic acids.
  • ELSI
    Ethical, legal and social implications (of Human Genome Project).
  • Endonuclease
    An enzyme that breaks the internal phosphodiester bonds in a DNA molecule.
  • Endoscopy (en-DOS-ko-pee)
    A procedure in which the doctor looks inside the body through a lighted tube called an endoscope.
  • Enzyme
    A protein that facilitates a specific chemical reaction.
  • Erythrocytes
    The hemoglobin-containing cell found in the blood of vertebrates, red blood cells.
  • Estrogen (ES-tro-jin)
    A female hormone.
  • Ethics
    The study of fundamental principles that defines values and determines moral duties and obligation.
  • Euchromatin
    The chromatin that shows the staining behavior characteristic of the majority of the chromosomal complement.
  • Eugenics
    The improvement of humanity by altering its genetic composition by encouraging breeding of those presumed to have desirable genes.
  • Eukaryote
    Cell or organism with membrane- bound, structurally discrete nucleus and other well- developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and blue- green algae. Compare prokaryote. See chromosomes.
  • Evolutionarily conserved
    See conserved sequence.
  • Exogenous DNA
    DNA originating outside an organism.
  • Exons
    The protein- coding DNA sequences of a gene. Compare introns.
  • Expressed gene
    See gene expression.
  • F    Back to Top

    Fecal occult blood test (FEE-kul-o-KULT)
    A test to check for hidden blood in stool. (Fecal refers to stool. Occult means hidden.)
  • FISH
    Florescent in situ hybridization: a technique for uniquely identifying whole chromosomes or parts of chromosomes using florescent tagged DNA.
  • Flow cytometry
    Analysis of biological material by detection of the light-absorbing or fluorescing properties of cells or subcellular fractions (i.e., chromosomes) passing in a narrow stream through a laser beam. An absorbance or fluorescence profile of the sample is produced.
  • Flow karyotyping
    Use of flow cytometry to analyze and/or separate chromosomes on the basis of their DNA content.
  • Fragile sites
    A non-staining gap of variable width that usually involves both chromatids and is always at exactly the same point on a specific chromosome derived from an individual or kindred.
  • Functional gene tests
    Biochemical assays for a specific protein, indicating a specific gene is not merely present but active.
  • G    Back to Top

    Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans).
  • Gene
    A unit of inheritance; a working subunit of DNA. Each of the body's 50,000 to 100,000 genes contains the code for a specific product, typically, a protein such as an enzyme.
  • Gene amplification
    Any process by which specific DNA sequences are replicated disproportionately greater than their representation in the parent molecules; during development, some genes become amplified in specific tissues.
  • Gene deletion
    The total loss or absence of a gene.
  • Gene expression
    The process by which a gene's coded information is translated into the structures present and operating in the cell (either proteins or RNAs).
  • Gene families
    Groups of closely related genes that make similar products.
  • Gene library
    See genomic library.
  • Gene mapping
    Determining the relative positions of genes on a chromosome and the distance between them.
  • Gene markers
    Landmarks for a target gene, either detectable traits that are inherited along with the gene, or distinctive segments of DNA.
  • Gene product
    The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease- causing alleles.
  • Gene testing
    Examining a sample of blood or other body fluid or tissue for biochemical, chromosomal, or genetic markers that indicate the presence or absence of genetic disease.
  • Gene therapy
    Treatment that alters genes (the basic units of heredity found in all cells in the body). In early studies of gene therapy for cancer, researchers are trying to improve the body's natural ability to fight the disease or to make the tumor more sensitive to other kinds of therapy. <br> Addition of a functional gene or group of genes to a cell by gene insertion to correct an hereditary disease.
  • Genetic code
    The sequence of nucleotides, coded in triplets (codons) along the mRNA, that determines the sequence of amino acids in protein synthesis. The DNA sequence of a gene can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.
  • Genetic counseling
    The educational process that helps individuals, couples, or families to understand genetic information and issues that may have an impact on them.
  • Genetic engineering technologies
    See recombinant DNA technologies.
  • Genetic linkage map
    A chromosome map showing the relative positions of the known genes on the chromosomes of a given species.
  • Genetic map
    See linkage map.
  • Genetic material
    See genome.
  • Genetic screening
    Testing groups of individuals to identify defective genes capable of causing hereditary conditions.
  • Genetic variation
    A phenotypic variance of a trait in a population attributed to genetic heterogeneity.
  • Genetics
    The scientific study of heredity how particular qualities or traits are transmitted from parents to offspring.
  • Genome
    All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs.
  • Genome maps
    Charts that indicate the ordered arrangement of the genes or other DNA markers within the chromosomes.
  • Genome projects
    Research and technology development efforts aimed at mapping and sequencing some or all of the genome of human beings and other organisms.
  • Genomic library
    A collection of clones made from a set of randomly generated overlapping DNA fragments representing the entire genome of an organism.
  • Genotype
    Genetic constitution of an organism.
  • Germ cells
    The reproductive cells of the body, either egg or sperm cells.
  • Germline mutation
    See Hereditary mutation.
  • H    Back to Top

    A single set of chromosomes (half the full set of genetic material), present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells. Compare diploid.
  • Hardy-Weinberg Law
    The concept that both gene frequencies and genotype frequencies will remain constant from generation to generation in an infinitely large, interbreeding population in which mating is at random and there is no selection, migration or mutation.
  • Hereditary mutation
    A gene change in the body's reproductive cells (egg or sperm) that becomes incorporated in the DNA of every cell in the body; also called germline mutation.
  • Heterogeneity
    The production of identical or similar phenotypes by different genetic mechanisms.
  • Heterozygosity
    The presence of different alleles at one or more loci on homologous chromosomes.
  • Heterozygote
    Having two alleles that are different for a given gene.
  • HGP
    Human Genome Project.
  • HHMI
    Howard Hughes Medical Institute.
  • Homeobox
    A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. It has been found in many organisms from fruit flies to human beings. In the fruit fly, a homeobox appears to determine when particular groups of genes are expressed during development.
  • Homologies
    Similarities in DNA or protein sequences between individuals of the same species or among different species.
  • Homologous chromosomes
    A pair of chromosomes containing the same linear gene sequences, each derived from one parent.
  • Homozygote
    Having identical alleles at one or more loci in homologous chromosome segments.
  • Hormone therapy
    Treatment that prevents certain cancer cells from getting the hormones they need to grow.
  • Hormones
    Chemicals produced by glands in the body. Hormones control the actions of certain cells or organs.
  • Housekeeping genes
    Those genes expressed in all cells because they provide functions needed for sustenance of all cell types.
  • HUGO
    Human Genome Organization.
  • Human gene therapy
    Insertion of normal DNA directly into cells to correct a genetic defect.
  • Human gene therapy
    Insertion of normal DNA directly into cells to correct a genetic defect.
  • Human genome
    The full collection of genes needed to produce a human being.
  • Human Genome Initiative
    Collective name for several projects begun in 1986 to (1) create an ordered set of DNA segments from known chromosomal locations, (2) develop new computational methods for analyzing genetic map and DNA sequence data, and (3) develop new techniques and instruments for detecting and analyzing DNA. This DOE initiative is now known as the Human Genome Program. The national effort, led by DOE and NIH (now including the National Human Genome Research Institute), is known as the Human Genome Project.
  • Human Genome Project
    An international research effort (led in the United States by the National Institutes of Health and the Department of Energy) aimed at identifying and ordering every base in the human genome.
  • Hybridization
    The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double- stranded molecule.
  • I    Back to Top

    Procedures that produce pictures of areas inside the body.
  • Immune system
    The complex group of cells and organs that defends the body against infection and disease.
  • Immunotherapy (IM-yoo-no-THER-a-pee)
    See Biological therapy.
  • Impotence (IM-po-tens)
    Inability to have an erection.
  • Imprinting
    A biochemical phenomenon that determines, for certain genes, which one of the pair of alleles, the mother's or the father's, will be active in that individual.
  • In situ hybridization
    Use of a DNA or RNA probe to detect the presence of the complementary DNA sequence in cloned bacterial or cultured eukaryotic cells.
  • In vitro
    Outside a living organism.
  • Inborn errors of metabolism
    Inherited diseases resulting from alterations in genes that code for enzymes.
  • Incomplete penetrance
    The gene for a condition is present, but not obviously expressed in all individuals in a family with the gene.
  • Informatics
    The study of the application of computer and statistical techniques to the management of information. In genome projects, informatics includes the development of methods to search databases quickly, to analyze DNA sequence information, and to predict protein sequence and structure from DNA sequence data.
  • Interferon (in-ter-FEER-on)
    A type of biological response modifier (a substance that can improve the body's natural response to disease). It slows the rate of growth and division of cancer cells, causing them to become sluggish and die.
  • Interleukin-2 (in-ter-LOO-kin)
    A type of biological response modifier (a substance that can improve the body's natural response to disease). It stimulates the growth of certain disease-fighting blood cells in the immune system. Also called IL-2.
  • Interphase
    The period in the cell cycle when DNA is replicated in the nucleus; followed by mitosis.
  • Introns
    The DNA base sequences interrupting the protein- coding sequences of a gene; these sequences are transcribed into RNA but are cut out of the message before it is translated into protein. Compare exons.
  • Isochromosome
    A metacentric chromosome produced during mitosis or meiosis when the centromere splits transversely instead of longitudinally; the arms of such chromosome are equal in length and genetically identical, however, the loci are positioned in reverse sequence in the two arms.
  • K    Back to Top

    A set of photographed, banded chromosomes arranged in order from largest to smallest.
  • Kb
    See kilobase.
  • Kilobase (kb)
    Unit of length for DNA fragments equal to 1000 nucleotides.
  • L    Back to Top

    An unordered collection of clones (i.e., cloned DNA from a particular organism), whose relationship to each other can be established by physical mapping. See Genomic Library.
  • Ligase
    An enzyme that functions in DNA repair.
  • Linkage
    Analysis of pedigree the tracking of a gene through a family by following the inheritance of a (closely associated) gene or trait and a DNA marker.
  • Linkage
    The greater association in inheritance of two or more nonallelic genes than is to be expected from independent assortment; genes are linked because they reside on the same chromosome.
  • Linkage analysis
    A gene-hunting technique that traces patterns of heredity in large, high-risk families, in an attempt to locate a disease-causing gene mutation by identifying traits that are co-inherited with it.
  • Linkage map
    A map of the relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together. Distance is measured in centimorgans (cM).
  • Local treatment
    Treatment that affects the tumor and the area close to it.
  • Localize
    Determination of the original position (locus) of a gene or other marker on a chromosome.
  • Locus (plural is loci)
    The position on a chromosome of a gene or other chromosome marker; also, the DNA at that position. The use of locus is sometimes restricted to mean regions of DNA that are expressed. See gene expression.
  • Lod score
    Logarithm of the odd score; a measure of the likelihood of two loci being within a measurable distance of each other.
  • Lymph (limf)
    An almost colorless fluid that travels through the lymphatic system and carries cells that help fight infection and disease.
  • Lymph nodes
    Small, bean-shaped organs located along the channels of the lymphatic system. Bacteria or cancer cells that enter the lymphatic system may be found in the nodes. Also called lymph glands.
  • Lymphatic system (lim-FAT-ik)
    The tissues and organs, including the bone marrow, spleen, thymus, and lymph nodes, that produce and store cells that fight infection and disease. This system also has channels that carry lymph.
  • M    Back to Top

    Macrorestriction map
    Map depicting the order of and distance between sites at which restriction enzymes cleave chromosomes.
  • Malignant (ma-LIG-nant)
  • Mammogram (MAM-o-gram)
    An x-ray of the breast.
  • Mapping
    See gene mapping, linkage map, physical map.
  • Marker
    A gene with a known location on a chromosome and a clear-cut phenotype, used as a point of reference when mapping a new mutant.
  • Mb
    See megabase.
  • Megabase (Mb)
    Unit of length for DNA fragments equal to 1 million nucleotides and roughly equal to 1 cM.
  • Meiosis
    The doubling of gametic chromosome number. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes.
  • Melanoma
    Cancer of the cells that produce pigment in the skin. Melanoma usually begins in a mole.
  • Messenger RNA (mRNA)
    RNA that serves as a template for protein synthesis. (See genetic code.)
  • Metaphase
    A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.
  • Metastasis (meh-TAS-ta-sis)
    The spread of cancer from one part of the body to another. Cells in the metastatic (secondary) tumor are like those in the original (primary) tumor.
  • Missense mutation
    A change in the base sequence of a gene that alters or eliminates a protein.
  • Mitochondrial DNA
    The mitochondrial genome consists of a circular DNA duplex, with 5 to 10 copies per organelle.
  • Mitosis
    The process of nuclear division in cells that produces daughter cells that are genetically identical to each other and to the parent cell.
  • Molecule
    A group of atoms arranged to interact in a particular way; one molecule of any substance is the smallest physical unit of that particular substance.
  • Monoclonal antibodies
    Substances that can locate and bind to cancer cells wherever they are in the body. They can be used alone, or they can be used to deliver drugs, toxins, or radioactive material directly to the tumor cells.
  • MRI
    A procedure using a magnet linked to a computer to create pictures of areas inside the body. Also called magnetic resonance imaging.
  • mRNA
    See messenger RNA.
  • Multifactorial
    A characteristic influenced in its expression by many factors, both genetic and environmental.
  • Mutation
    A change in the number, arrangement, or molecular sequence of a gene.
  • N    Back to Top

    Newborn screening (for genetic disorders)
    Examining blood samples from a newborn infant to detect disease-related abnormalities or deficiencies in gene products. There are other purposes for , and methods of, screening newborns.
  • NIH
    National Institutes of Health.
  • Nitrogenous base
    A nitrogen containing molecule having the chemical properties of a base.
  • Nonsense mutation
    A mutation in which a codon is changed to a stop codon, resulting in a truncated protein product.
  • Nucleic acid
    A large molecule composed of nucleotide subunits.
  • Nucleotide
    A subunit of DNA or RNA, consisting of one chemical base plus a phosphate molecule and a sugar molecule.
  • Nucleus
    The cell structure that houses the chromosomes.
  • O    Back to Top

    Genes that normally play a role in the growth of cells but, when over expressed or mutated, can foster the growth of cancer.
  • Oncologist (on-KOL-o-jist)
    A doctor who specializes in treating cancer.
  • Overlapping clones
    See genomic library.
  • P    Back to Top

    Pap test
    Microscopic examination of cells collected from the cervix. It is used to detect changes that may be cancer or may lead to cancer, and it can show noncancerous conditions, such as infection or inflammation. Also called Pap smear.
  • Parthenogenesis
    The development of an individual from an egg without fertilization.
  • Pathologist (path-OL-o-jist)
    A doctor who identifies diseases by studying cells and tissues under a microscope.
  • PCR
    Polymerase chain reaction; a technique for copying the complementary strands of a target DNA molecule simultaneously for a series of cycles until the desired amount is obtained.
  • Pedigree
    A diagram of the heredity of a particular trait through many generations of a family.
  • Pelvic
    Having to do with the pelvis, the lower part of the abdomen, located between the hip bones.
  • Penetrance
    A term indicating the likelihood that a given gene will actually result in disease.
  • Peripheral stem cell support (per-IF-er-ul)
    A method of replacing blood-forming cells destroyed by cancer treatment. Certain cells (stem cells) in the blood that are similar to those in bone marrow are removed from the patient's blood before treatment. The cells are given back to the patient after treatment.
  • Phage
    A virus for which the natural host is a bacterial cell.
  • Phenotype
    Observable characteristics of an organism produced by the organism's genotype interacting with the environment.
  • Physical map
    Map where the distance between markers is the actual distance, such as the number of base pairs.
  • Plasmid
    Autonomously replicating, extrachromosomal circular DNA molecules, distinct from the normal bacterial genome and nonessential for cell survival under nonselective conditions. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors.
  • Pleiotropy
    The phenomenon of variable phenotypes for a number of distinct and seemingly unrelated phenotypic effects.
  • Polygenic disorders
    Genetic disorders resulting from the combined action of alleles of more than one gene (e.g., heart disease, diabetes, and some cancers). Although such disorders are inherited, they depend on the simultaneous presence of several alleles; thus the hereditary patterns are usually more complex than those of single- gene disorders. Compare single-gene disorders.
  • Polymerase
    Any enzyme that catalyzes the formation of DNA or RNA from deoxyribonucleotides or ribonucleotides.
  • Polymerase chain reaction (PCR)
    A method for amplifying a DNA base sequence using a heat- stable polymerase and two 20- base primers, one complementary to the (+)- strand at one end of the sequence to be amplified and the other complementary to the (- )- strand at the other end. Because the newly synthesized DNA strands can subsequently serve as additional templates for the same primer sequences, successive rounds of primer annealing, strand elongation, and dissociation produce rapid and highly specific amplification of the desired sequence. PCR also can be used to detect the existence of the defined sequence in a DNA sample.
  • Polymerase, DNA or RNA
    Enzymes that catalyze the synthesis of nucleic acids on preexisting nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.
  • Polymorphism
    Difference in DNA sequence among individuals. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for genetic linkage analysis. Compare mutation.
  • Predictive gene tests
    Tests to identify gene abnormalities that may make a person susceptible to certain diseases or disorders.
  • Predisposition
    To have a tendency or inclination towards something in advance.
  • Prenatal diagnosis
    Examining fetal cells taken from the amniotic fluid, the primitive placenta (chorion), or the umbilical cord for biochemical, chromosomal, or gene alterations.
  • Presymptomatic diagnosis
    Diagnosis of a genetic condition before the appearance of symptoms.
  • Primer
    Nucleotides used in the polymerase chain reaction to initiate DNA synthesis at a particular location.
  • Probability
    The long term frequency of an event relative to all alternative events, and usually expressed as decimal fraction.
  • Proband
    Individual in a family who brought the family to medical attention.
  • Probe
    Single-stranded DNA labeled with radioactive isotopes or tagged in other ways for ease in identification.
  • Progesterone (pro-JES-ter-own)
    A female hormone.
  • Prognosis (prog-NOS-sis)
    The probable outcome or course of a disease; the chance of recovery.
  • Prokaryote
    Cell or organism lacking a membrane- bound, structurally discrete nucleus and other subcellular compartments. Bacteria are prokaryotes. Compare eukaryote. See chromosomes.
  • Promoter
    A site on DNA to which RNA polymerase will bind and initiate transcription.
  • Proofreader genes
    See DNA repair genes.
  • Prophylactic surgery
    Surgery to remove tissue that is in danger of becoming cancerous, before cancer has the chance to develop. Surgery to remove the breasts of women at high risk of developing breast cancer is known as prophylactic mastectomy.
  • Protein
    A large, complex molecule composed of amino acids. The sequence of the amino acids, and thus the function of the protein, is determined by the sequence of the base pairs in the gene that encodes it. Proteins are essential to the structure, function, and regulation of the body. Examples are hormones, enzymes, and antibodies.
  • Protein product
    The protein molecule assembled under the direction of a gene.
  • Purine
    A nitrogen-containing, single- ring, basic compound that occurs in nucleic acids. The purines in DNA and RNA are adenine and guanine.
  • Pyrimidine
    A nitrogen- containing, double- ring, basic compound that occurs in nucleic acids. The pyrimidines in DNA are cytosine and thymine; in RNA, cytosine and uracil.
  • R    Back to Top

    Radiation therapy (ray-dee-AY-shun)
    Treatment with high-energy rays to kill or damage cancer cells. External radiation therapy is the use of a machine to aim high- energy rays at the cancer. Internal radiation is the placement of radioactive material inside the body as close as possible to the cancer.
  • Radioactive (RAY-dee-o-AK-tiv)
    Giving off radiation.
  • Radionuclide scanning
    An exam that produces pictures (scans) of internal parts of the body. The patient is given an injection or swallows a small amount of radioactive material. A machine called a scanner then measures the radioactivity in certain organs.
  • Recessive
    A gene that is phenotypically manifest in the homozygous state but is masked in the presence of a dominant allele.
  • Recessive allele
    A gene that is expressed only when its counterpart allele on the matching chromosome is also recessive (not dominant). Autosomal recessive disorders develop in persons who receive two copies of the mutant gene, one from each parent who is a carrier. (See Dominant allele.)
  • Recombinant DNA molecules
    A combination of DNA molecules of different origin that are joined using recombinant DNA technologies.
  • Recombinant DNA technologies
    Procedures used to join together DNA segments in a cell- free system (an environment outside a cell or organism). Under appropriate conditions, a recombinant DNA molecule can enter a cell and replicate there, either autonomously or after it has become integrated into a cellular chromosome.
  • Recombination
    The natural process of breaking and rejoining DNA strands to produce new combinations of genes and, thus, generate genetic variation. Gene crossover during meiosis.
  • Regulatory regions or sequences
    A DNA base sequence that controls gene expression.
  • Remission
    Disappearance of the signs and symptoms of cancer. When this happens, the disease is said to be "in remission". Remission can be temporary or permanent.
  • Repeat sequences
    The length of a nucleotide sequence that is repeated in a tandem cluster.
  • Reproductive cells
    Egg and sperm cells. Each mature reproductive cell carries a single set of 23 chromosomes.
  • Resolution
    Degree of molecular detail on a physical map of DNA, ranging from low to high.
  • Restriction enzyme cutting site
    A specific nucleotide sequence of DNA at which a particular restriction enzyme cuts the DNA. Some sites occur frequently in DNA (e.g., every several hundred base pairs), others much less frequently (rare- cutter; e.g., every 10,000 base pairs).
  • Restriction enzymes
    Enzymes that can cut strands of DNA at specific base sequences.
  • Restriction fragment length polymorphism (RFLP)
    Variation between individuals in DNA fragment sizes cut by specific restriction enzymes; polymorphic sequences that result in RFLPs are used as markers on both physical maps and genetic linkage maps. RFLPs are usually caused by mutation at a cutting site. See marker.
  • Ribonucleic acid (RNA)
    A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.
  • Ribosomal RNA (rRNA)
    A class of RNA found in the ribosomes of cells.
  • Ribosomes
    Small cellular components composed of specialized ribosomal RNA and protein; site of protein synthesis. See ribonucleic acid (RNA).
  • Risk factor
    Something that increases a person's chance of developing a disease.
  • RNA
    Ribonucleic acid, a chemical similar to DNA. The several classes of RNA molecules play important roles in protein synthesis and other cell activities.
  • S    Back to Top

    Sanger sequence
    "Plus and minus" or "primed synthesis" method; DNA is synthesized so it is radioactively labeled and the reaction terminates specifically at the position corresponding to a given base.
  • Screening
    Looking for evidence of a particular disease such as cancer in persons with no symptoms of disease. Checking for disease when there are no symptoms.
  • Sequence
    See base sequence.
  • Sequence tagged site (STS)
    Short (200 to 500 base pairs) DNA sequence that has a single occurrence in the human genome and whose location and base sequence are known. Detectable by polymerase chain reaction, STSs are useful for localizing and orienting the mapping and sequence data reported from many different laboratories and serve as landmarks on the developing physical map of the human genome. Expressed sequence tags (ESTs) are STSs derived from cDNAs.
  • Sequencing
    Determination of the order of nucleotides (base sequences) in a DNA or RNA molecule or the order of amino acids in a protein.
  • Sex chromosomes
    The chromosomes that determine the sex of an organism. Human females have two X chromosomes; males have one X and one Y.
  • Sex determination
    The mechanism in a given species by which sex is determined; in many species sex is determined at fertilization by the nature of the sperm that fertilizes the egg.
  • Shotgun method
    Cloning of DNA fragments randomly generated from a genome. See library, genomic library.
  • Side effects
    Problems that occur when treatment affects healthy cells. Common side effects of cancer treatment are fatigue, nausea, vomiting, decreased blood cell counts, hair loss, and mouth sores.
  • Sigmoidoscopy (sig-moy-DOS-ko-pee)
    A procedure in which a doctor looks inside the rectum and the lower part of the colon (sigmoid colon) through a lighted tube. The doctor may collect samples of tissue or cells for closer examination. Also called proctosigmoidoscopy.
  • Single-gene disorder
    Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease). Compare polygenic disorders.
  • Somatic cell hybrid
    Hybrid cell line derived from two different species; contains a complete chromosomal complement of one species and a partial chromosomal complement of the other; human/hamster hybrids grow and divide, losing human chromosomes with each generation until they finally stabilize, the hybrid cell line established is then utilized to detect the presence of genes on the remaining human chromosome.
  • Somatic cells
    All body cells except the reproductive cells.
  • Somatic mutation
    A mutation occurring in any cell that is not destined to become a germ cell; if the mutant cell continues to divide, the individual will come to contain a patch of tissue of genotype different from the cells of the rest of the body.
  • Southern blotting
    Transfer by absorption of DNA fragments separated in electrophoretic gels to membrane filters for detection of specific base sequences by radiolabeled complementary probes.
  • Sperm banking
    Freezing sperm before cancer treatment for use in the future. This procedure can allow men to father children after loss of fertility.
  • Stage
    The extent of a cancer, especially whether the disease has spread from the original site to other parts of the body.
  • Stool
    The waste matter discharged in a bowel movement; feces.
  • STS
    See sequence tagged site.
  • Surgery
    An operation.
  • Syndrome
    A recognizable pattern or group of multiple signs, symptoms or malformations that characterize a particular condition; syndromes are thought to arise from a common origin and result from more than one developmental error during fetal growth.
  • Systemic treatment (sis-TEM-ik)
    Treatment that reaches cells all over the body by traveling through the bloodstream.
  • T    Back to Top

    Tandem repeat sequences
    Multiple copies of the same base sequence on a chromosome; used as a marker in physical mapping.
  • Technology transfer
    The process of converting scientific findings from research laboratories into useful products by the commercial sector.
  • Telomere
    The ends of chromosomes. These specialized structures are involved in the replication and stability of linear DNA molecules. See DNA replication.
  • Teratogens
    Any agent that raises the incidence of congenital malformations.
  • Thymine (T)
    A nitrogenous base, one member of the base pair A- T (adenine-thymine).
  • Tissue (TISH-oo)
    A group or layer of cells that together perform a specific function.
  • Toxins
    Poisons produced by certain animals, plants, or bacteria.
  • Trait
    Any detectable phenotypic property of an organism.
  • Transcription
    The process of copying information from DNA into new strands of messenger RNA (mRNA). The mRNA then carries this information to the cytoplasm, where it serves as the blueprint for the manufacture of a specific protein.
  • Transduction
    The transfer of bacterial genetic material from one bacterium to another using a phage as a vector.
  • Transfer RNA (tRNA)
    A class of RNA having structures with triplet nucleotide sequences that are complementary to the triplet nucleotide coding sequences of mRNA. The role of tRNAs in protein synthesis is to bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by mRNA.
  • Transferase
    Enzymes that catalyze the transfer of functional groups between donor and acceptor molecules.
  • Transformation
    A process by which the genetic material carried by an individual cell is altered by incorporation of exogenous DNA into its genome.
  • Transgenic organism
    One into which a cloned genetic material has been experimentally transferred, a subset of these foreign gene express themselves in their offspring.
  • Translation
    The process of turning instructions from mRNA, base by base, into chains of amino acids that then fold into proteins. This process takes place in the cytoplasm, on structures called ribosomes.
  • Translocation
    A chromosome aberration which results in a change in position of a chromosomal segment within the genome, but does not change the total number of genes present.
  • Triplet code
    A code in which a given amino acid is specified by a set of three nucleotides.
  • tRNA
    See transfer RNA.
  • Tumor
    A mass of excess tissue.
  • Tumor markers
    Substances found in abnormal amounts in the blood, in other body fluids, or in tumor tissue of some patients with certain types of cancer.
  • Tumor suppressor gene
    Genes that normally restrain cell growth but, when missing or inactivated by mutation, allow cells to grow uncontrolled.
  • U    Back to Top

    Ultrasonography (ul-tra-son-OG-ra-fee)
    An exam in which sound waves are bounced off tissues and the echoes are converted into a picture (sonogram).
  • Ultraviolet radiation (ul-tra-VI-o-let ray-dee-AY-shun)
    Invisible rays that are part of the energy that comes from the sun. Ultraviolet radiation can burn the skin and cause skin cancer. It is made up of two types of rays, UVA and UVB. Skin specialists recommend that people use sunscreens that block both kinds of radiation.
    United National Educational, Scientific, and Cultural Organization.
  • V    Back to Top

    A self-replicating DNA molecule that transfers a DNA segment between host cells.
  • Virus
    A noncellular biological entity that can reproduce only within a host cell. Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus.
  • VNTR
    Variable number tandem repeats; any gene whose alleles contain different numbers of tandemly repeated oligonucleotide sequences.
  • W    Back to Top

    Western blotting analysis
    A technique used to identify a specific protein; the probe is a radioactively labeled antibody raised against the protein in question.
  • White blood cells
    Cells that help the body fight infection and disease.
  • Wilms' tumor
    A kidney cancer (tumor) that occurs in children, usually before age 5.
  • X    Back to Top

    X chromosome
    A sex chromosome; normal females carry two X chromosomes.
  • X-inactivation
    The repression of one of the two X-chromosomes in the somatic cells of females as a method of dosage compensation; at an early embryonic stage in the normal female, one of the two X-chromosomes undergoes inactivation, apparently at random, from this point on all descendent cells will have the same X-chromosome inactivated as the cell from which they arose, thus a female is a mosaic composed of two types of cells, one which expresses only the paternal X-chromosome, and another which expresses only the maternal X-chromosome.
  • X-rays
    High-energy radiation used in low doses to diagnose disease or injury, and in high doses to treat cancer.
  • XYY syndrome
    Genetic condition in males with extra Y chromosome (in 1 in 1000 male births). Symptoms: tall stature (over 6'), may including sterility, developmental delay, learning problems.
  • Y    Back to Top

    Y chromosome
    A sex chromosome; normal males carry one Y and one X chromosome.
  • YAC
    Yeast artificial chromosome; a linear vector into which a large fragment of DNA can be inserted; the development of YAC's in 1987 has increased the number of nucleotides which can be cloned.
  • Z    Back to Top

    Zoo blot
    Northern analysis of mRNA from different organisms.

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