Part I: Glossary and References


These terms have special technical meanings as they are used in this guide. Key words that denote fundamental genetics concepts are printed in boldface and cross-referenced to operational definitions or examples of how they are used in the text.

adenine -- A purine, one of the nitrogenous bases found in DNA and RNA. gone

aerobic -- Occurring in the presence of oxygen.

agar -- A polysaccharide produced by a red algae. It is used to gel culture media for growing microorganisms.

AIDS -- Acquired Immune Deficiency Syndrome. A viral disease of the human immune system.

AIR -- The abbreviation for P-ribosylaminoimidazole, formerly called aminoimidazoleribotide, precursor of the red pigment that accumulates in ade1 and ade2 yeast mutants.

allele -- A particular form of a gene. (see page 9)

AMP -- The abbreviation for adenosine monophosphate, precursor of adenosine triphosphate (ATP) and nucleic acids.

anaerobic -- Occurring in the absence of free oxygen.

ascomycetes -- A broad group of fungi, including yeast and sordaria, with spores formed in a membranous spore sac.

ascospore -- One of the spores contained in an ascus of an ascomycete such as yeast; a differentiated meiotic product.

ascus -- The spore sac produced by an ascomycete.

ATP -- (adenosine triphosphate); an energy storage molecule in intermediary metabolism.

autoclave -- A device to sterilize media and equipment using high pressure steam; a pressure cooker.

autolysis -- The self-digestion of plant, animal, or fungal tissue.

auxotroph -- A mutant organism requiring a specific growth substance not normally required by its species.

biopolymer -- Large molecules such as proteins and nucleic acids, made of repeating smaller units (monomers) in organisms.

biosynthesis -- Any process that produces some type of molecule or structure in an organism.

biosynthetic pathway -- The specific series of reactions in an organism (between enzymes and their substrates) that result in the production of a specific end product.

biotin -- One of the B vitamins; the sole complex growth substance required by yeast.

Candida albicans -- A species of yeast found in most people, pathogenic in individuals with impaired immune systems.

catalyst -- A substance, such as an enzyme, which alters the rate of a chemical reaction or allows it to take place under milder conditions than otherwise possible.

centromere -- The specialized region of a chromosome to which spindle fibers are attached, and which therefore always segregates at the first meiotic division. (see page 15)

chromatid -- One of two duplicated subunits of a chromosomes, joined at the centromere. (see page 15)

chromosome -- A structure composed of protein and DNA containing the nuclear genetic material of an organism. (see page 9)

clone -- The asexually produced progeny of an individual. To cause to grow as a clone.

complementation -- Interaction between two allelic or nonallelic genes resulting in a product or function that neither is capable of producing alone. (see page 12)

conjugation -- Sexual reproduction; fusion of gametes, in which the nuclei fuse, to produce a zygote.

crossing over -- The reciprocal exchange of chromatid segments during meiosis or mitosis.

Cryptococcus neoformans -- A basidiomycetous species of yeast which can be an opportunistic pathogen, causing a life-threatening meningitis.

culture -- a population of microorganisms growing under artificial conditions.

cytosine -- A pyrimidine, one of the basic nitrogenous bases found in DNA and RNA.

dextrose -- Also called glucose -- A six carbon sugar used by many organisms as a major energy source (also is a monomer of some polysaccharides, including starch, cellulose, and glycogen).

differentiation -- The modification of cells, through gene regulation, to change their structure and function.

dimer -- A molecule consisting of two similar subunits.

diploid -- Any cell or organism that has two copies of each kind of chromosome. (see page 6)

disjunction -- The process in mitosis or meiosis in which the centromere splits and the chromatids separate, going to opposite poles of the cell.

DNA -- The abbreviation for deoxyribonucleic acid, a biopolymer that encodes the genetic information in higher organisms. (see page 8)

dominant allele and trait -- An allele of a gene, and the corresponding trait, that is phenotypically observed in preference to a recessive alternative. (see page 13)

dosimeter -- An instrument that measures and indicates the amount of radiation energy received.

enzyme -- A catalytic protein.

epistasis -- Expression of the phenotypic trait of a gene obscuring or preventing the expression of a trait of a nonallelic gene.

eukaryote -- An organism having a true nucleus.

excision repair -- A system of repairing single-strand damage in DNA, involving several enzymes, which first remove the damaged portion and then fill the gap by copying the remaining strand. Can operate in the dark.

feedback inhibition -- Inhibition of an enzyme controlling an early step in a pathway, usually at a branching point, by the end-product that has reached a critical concentration.

fermentation -- The process by which yeasts and other microorganisms convert sugars to alcohol and carbon dioxide in the absence of oxygen.

four-strand stage -- In meiosis, the stage which results in the formation of a tetrad from synapsis of homologous chromosomes, each consisting of two chromatids.

gamete -- Differentiated reproductive cells, generally haploid. (see page 6)

gene -- The inheritable unit of information coded in nucleic acid. (see page 8)

gene conversion -- Nonreciprocal recombination.

genome -- The total set of genes of any organism. (see page 14)

genotype -- The specific arrangement of the genes in an organism. (see page 9)

germ line -- Reproductive cells that form the gametes which carry genetic information to the next generation.

germination -- Initiation of growth by a spore.

glucose -- A simple 6-carbon sugar (dextrose).

guanine -- One of the four nitrogenous bases found in DNA.

haploid -- Having a single copy of the basic chromosome set of an organism. (see page 6)

heterozygous -- Having two different alleles of a particular gene. (see page 14)

homologs -- Chromosomes composed of corresponding genes.

homozygous -- Having identical alleles of a particular gene on a homologous chromosome pair. (see page 14)

inoculating loop -- A microbiological utensil that consists of a small wire, with a loop on one end, attached at the other end to a small rod. It is used to transfer cells.

incubate -- To maintain an organism in an artificial environment that contributes to its development.

inhibition -- The stopping of a physiological process by deactivating one of the enzymes needed in that process.

inoculate -- To introduce a microorganism in order to immunize, cure, or experiment.

ionizing radiation -- Particles or photons that have sufficient energy to produce ionization during their passage through matter.

linkage -- Failure of genes to segregate independently because they reside on the same chromosome.

lipid -- Any of a number of fats or oils insoluble in water.

locus -- The position on a chromosome occupied by a gene.

macromolecule -- A large polymeric molecule.

mating type -- One of two alternate cell type traits manifesting the ability to conjugate.

medium -- (pl. media) -- A substance in or on which microorganisms grow.

meiosis -- A mode of nuclear division which results in reduction of the ploidy by a factor of two. (see page 6)

Mendel, Gregor -- Austrian monk generally considered the "Father" of genetics, who first demonstrated simple algebraic rules of heredity and formulated the concept of the gene.

metabolism -- The chemical processes of life.

micromanipulation -- The technique of moving small objects while viewing them through a microscope, usually with a mechanical device.

mitochondria -- Cell organelle which produces energy through respiration.

mitosis -- The process by which the chromosomes duplicate themselves and separate into two identical groups just before a cell divides. (see page 15)

morphology -- The form and structure of an organism.

mutagens -- Radiation, chemicals, or other agents which increase the rate of mutation.

mutant -- An offspring different from its parents due to a change in its genetic material. (see page 13)

mutation -- Any heritable change in a gene. (see page 9)

MV medium -- A chemically defined minimal growth medium for yeast supplemented with vitamins.

Neurospora crassa -- A species of filamentous fungi (ascomycete) widely used in genetics.

non-reciprocal recombination -- A process in which genetic information is transferred from one member of a heterozygous region to the other without the reciprocal transfer occurring.

nucleic acid -- Either DNA or RNA, two closely-related members of a class of organic compounds, which encode genetic information.

nucleotides -- The repeating subunits of nucleic acids.

objective (microscope) -- The lens closest to the object being viewed.

ocular (microscope) -- The eye piece.

opportunistic pathogen -- An organism which, although not normally disease-causing, can cause disease in individuals whose resistance has been weakened in some way.

organelle -- A cell substructure.

oxidative metabolism -- Energy-releasing processes which require free oxygen.

parasexual cycle -- A series of events leading to genetic recombination in vegetative or somatic cells.

peptide -- A short chain of chemically bonded amino acids.

petite colony mutation ("petites") -- Respiration deficient variants of yeast resulting from lack of or defective mitochondria.

PETITE medium -- A yeast growth medium containing yeast extract and glycerol, used to distinguish petite colony mutants, which cannot assimilate glycerol.

Petri plate -- A small shallow culture dish of thin glass or plastic with a loosely fitting, over- lapping cover, usually used to contain agar medium.

phenotype -- The expression of the genotype in the appearance and functions of an organism; the observable traits. (see page 9)

pheromone -- A chemical messenger acting between different organisms to alter function or behavior.

photoreactivating enzyme -- An enzyme which uses visible light energy to split pyrimidine dimers, thereby preventing damage by UV radiation.

phylogenetic -- Pertaining to relationships among organisms which share common ancestry.

plating -- The process of transferring yeast cells from an original source to a petri dish (plate) containing growth medium.

ploidy -- Referring to the number of sets of chromosomes a cell contains (see haploid and diploid).

pressure cooker -- A kitchen utensil for cooking foods by means of steam superheated under pressure.

progeny -- Offsprings or descendants.

protein -- An organic compound composed of one or more polypeptide chains of amino acids, the chemical composition of enzymes and most structural materials in a cell.

prototroph -- An organism which has no additional nutritional requirements other than those of the wild type.

PRPP -- The abbreviation for phosphoribosylpyrophosphate, the first compound in the AMP biosynthetic pathway.

purine -- A double-ring nitrogen base such as adenine or guanine.

pyrimidine -- A single-ring nitrogen base such as cytosine, thymine, or uracil.

recessive allele and trait -- An allele of a gene, and the corresponding trait, that is not phenotypically observed in the presence of a dominant alternative. (see page 13)

recombination -- Production through a genetic event of a combination of parental genes not found in the parent organisms.

recombinant -- An offspring that carries combinations of parental genes not found in either of the parents.

replica plating -- A rapid method of transferring cultures from one agar medium to one or several others, analogous to a rubber stamp.

replication (gene) -- The process in which DNA (or RNA) is synthesized, using an existing molecule as a template.

respiration -- The process of cellular metabolism in which organic molecules of foodstuffs are broken down and oxidized to carbon dioxide and water.

reversion -- A subsequent mutation that restores the original or wild-type phenotype that was altered by a prior mutation.

RNA -- The abbreviation for ribonucleic acid, the hereditary material of certain viruses, and the material coded by DNA of other cells to carry out specific genetic functions (for example, messenger-RNA and transfer-RNA).

Saccharomyces cerevisiae -- Baker's Yeast.

Schizosaccharomyces pombe -- An ascomycetous species of yeast that divides by binary fission.

sectored colony -- A colony which is not uniform in color or growth due to the presence of variant clones.

segregants -- Progeny that result from the separation of alleles and homologous chromosomes during meiosis or mitosis.

shmoo -- Transient gamete forms assumed by haploid yeast in response to mating pheromone produced by the opposite mating type, named for their resemblance to the Lovable animals in the "Li'l Abner" comic strip.

small molecule -- The building blocks of the macromolecules.

snail enzyme -- The digestive juice from the crop of land snails such as Helix pomatia, used to dissolve the ascus wall to liberate the ascospores.

somatic -- Pertaining solely to the bodily part of an organism, as opposed to the germinal part.

spindle -- The fine threads formed between the poles of the nucleus during cell division.

spontaneous mutation -- A change in an organism's DNA occurring in the absence of a known mutagen.

spore -- A one-celled, dormant sexual or asexual form that is usually resistant to environmental stress.

sporulation -- The act and process of spore formation.

strain -- A group or type of organism having a similar ancestry or line of descent.

strand -- A chromatid.

streaking -- Transferring cells to an agar surface to start a culture. May be used to dilute the cells on the surface to produce isolated colonies growing from single cells.

subunits -- The small molecules, or building blocks, of the macromolecules.

syndrome -- A pattern of signs and symptoms that occur together to characterize an abnormal condition.

tester -- Standard strains used as type representatives to define particular alleles or genes.

thymine -- One of the four nitrogen bases in DNA.

trait -- Observable consequence of gene expression.

transient differentiation (gametogenesis) -- The variety of physiological and shape changes of a haploid yeast cells, in response to the mating pheromones of the opposite mating type, resulting in the formation of a cell stage that is capable of mating or of resuming cell division.

transmission -- Inheritance of genetic traits.

two-strand stage -- When a chromosome has been replicated and consists of two chromatids joined at a centromere.

ultraviolet -- Light having wavelengths shorter than visible violet light.

unicellular -- One-celled.

UV -- Abbreviation for ultraviolet.

velveteen -- The cloth material used as the "velveteen stamp" for replica plating.

viable -- Alive; capable of reproducing.

wet mount -- A temporary preparation of a sample for microscopic examination, suspended in water under a cover glass on a microscope slide.

wild type -- The normal, unmutated representative type of an organism, not always a truly wild form in the case of domesticated creatures, such as yeast.

x-rays -- Electromagnetic radiations, of shorter wavelength than visible light, produced when high-speed electrons interact with matter.

YEAD medium -- A yeast growth medium containing yeast extract, adenine, and dextrose.

YED medium -- A yeast growth medium containing yeast extract and dextrose.

YEKAC medium -- A yeast sporulation medium containing yeast extract and potassium acetate.

YEPAD medium -- A yeast growth/storage medium containing yeast extract, peptone, and dextrose.

zygote -- The cell produced by the fusion of two gamete cells and their nuclei, such as the peanut shaped diploid cells formed by conjugation of haploid yeast of opposite mating types (shmoos). (see page 7)

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Last updated Friday July 11 1997