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octoploid Cell or organism with eight sets of chromosomes, i.e., chromosome number 2n = 8x.
oestrogen; estrogen The generic term for a group of female sex hormones which control the development of sexual characteristics and control oestrus.
oestrous cycle (from oestrus) The cycle of reproductive activity shown by most sexually mature non-pregnant female mammals.
oestrus (adj: oestrous) In female mammals, the period of sexual excitement and acceptance of the male. a.k.a. rut, heat.
offset Young plant produced at the base of a mature plant.
offshoot Short, usually horizontal, stem produced near the crown of a plant.
offspring; progeny (both same in plural) New individual organisms that result from the process of sexual or asexual reproduction.
Okazaki fragment Since DNA can be replicated in only one direction, i.e., nucleotides can be added only at the 3´ end, only one of the two strands of a double helix can be replicated continuously. The other strand is replicated in small segments (Okazaki fragments) that are subsequently joined together by DNA ligase. See primosome.
OLA See oligonucleotide ligation assay.
oligomer A molecule formed from a small number of monomers.
oligonucleotide A short molecule (usually 6 to 100 nucleotides) of single-stranded DNA.
oligonucleotide ligation assay (OLA) A diagnostic technique for determining the presence or absence of a specific nucleotide pair within a target gene, often indicating whether the gene is wild type (normal) or mutant (defective).
oligonucleotide-directed mutagenesis; oligonucleotide-directed site-specific mutagenesis. See site-specific mutagenesis.
oligosaccharide Carbohydrate consisting of several linked sugar units.
oncogene A gene that causes cells to grow in an uncontrolled manner, i.e., that causes cancer. Oncogenes are mutant forms of normal functional genes (called proto-oncogenes) that have a role in normal cell proliferation. Oncogenes are found in tumours and in retroviruses; in the latter case, having been picked up along with retroviral genes during retroviral replication in the host. Once incorporated as part of the retroviral genome, they are activated at inappropriate times and places by retroviral promoters, thereby becoming oncogenes.
See cellular oncogene; dominant oncogene; immortalizing oncogene; recessive oncogene.
oncogenesis The progression of cytological, genetic and cellular changes that culminate in a tumour.
oncogenic Oncogenic genes are responsible for the transformation of normal cells into tumour cells. See Agrobacterium.
onco-mouse A mouse that has been genetically modified to incorporate an oncogene; oncogenes cause cells to undergo cancerous transformation.
ontogeny Developmental life history of an organism.
oocyte The egg mother cell; it undergoes two meiotic divisions (oogenesis) to form the egg cell. The primary oocyte is before completion of the first meiotic division; the secondary oocyte is after completion of the first meiotic division.
oogenesis The formation and growth of the egg or ovum in an animal ovary.
oogonium (pl: oogonia) 1. A germ cell of the female animal, that gives rise to oocytes by mitotic division.
2. The female sex organ of algae and fungi.
oosphere The non-motile female gamete in plants and some algae.
oospore (Gr. oion, an egg + spore) A resistant spore developing from a zygote, resulting from the fusion of heterogametes in certain algae and fungi.
open continuous culture A continuous culture in which inflow of fresh medium is balanced by outflow of a corresponding volume of culture. Cells are constantly washed out with the outflowing liquid. In a steady state, the rate of cell wash-out equals the rate of formation of new cells in the system. See continuous culture; batch culture; closed continuous culture.
open pollination Pollination by wind, insects or other natural mechanisms.
open reading frame (ORF) A sequence of nucleotides in a DNA molecule that has the potential to encode a peptide or protein: it starts with a start triplet (ATG), is followed by a string of triplets each of which encodes an amino acid, and ends with a stop triplet (TAA, TAG or TGA). This term is often used when, after the sequence of a DNA fragment has been determined, the function of the encoded protein is not known. The existence of open reading frames is usually inferred from the DNA (rather than the RNA) sequence.
operational definition An operation or procedure that can be carried out to define or delimit something.
operator The region of DNA that is upstream from a gene or genes and to which one or more regulatory proteins (repressor or activator) binds to control the expression of the gene(s).
operon A functionally integrated genetic unit for the control of gene expression in bacteria. It consists of one or more genes that encode one or more polypeptide(s) and the adjacent site (promoter and operator) that controls their expression by regulating the transcription of the structural genes.
opine The condensation product of an amino acid with either a keto-acid or a sugar, produced by the crown gall tissues. Opine synthesis is a unique characteristic of tumour cells.
OPU See ovum pickup.
ordinate The vertical axis of a graph. Opposite: absciss; abscissa.
ORF See open reading frame.
organ A tissue or group of tissues that constitute a morphologically and functionally distinct part of an organism.
organ culture The aseptic culture of complete living organs of animals and plants outside the body in a suitable culture medium. Animal organs must be small enough to allow the nutrients in the culture medium to penetrate all the cells.
organellar genes Genes located on organelles outside the nucleus.
organelle A membrane-bounded specialized region within a cell, such as the mitochondrion or dictyosome, that carries out a specialized function in the life of a cell.
organic Referring in chemistry to compounds containing carbon, many of which have been in some manner associated with living organisms.
organic complex A chemically undefined compound added to nutrient media to stimulate growth, e.g., coconut milk; yeast extract; casein hydrolysate.
organic co-solvent A compound used to dissolve some neutral organic substances, such as in media preparation. Organic co-solvents include alcohols (usually ethanol), acetone and dimethylsulphoxide (DMSO).
organic evolution The process by which changes in the genetic composition of populations of organisms occur in response to environmental changes.
organized growth The development under tissue culture conditions of organized explants (meristem tips or shoot tips, floral buds or organ primordia). See unorganized growth.
organized tissue Composed of regularly differentiated cells.
organizer An inductor; a chemical substance in a living system that determines the fate in development of certain cells or groups of cells.
organogenesis The initiation of adventitious or de novo shoots or roots from callus, meristem or suspension cultures. See micropropagation; regeneration.
organoid An organ-like structure produced in culture, such as leaves, roots or callus.
organoleptic Having an effect on one of the organs of sense, such as taste or smell.
organism An individual living system, such as animal, plant or micro-organism, that is capable of reproduction, growth and maintenance.
origin of replication The nucleotide sequence at which DNA synthesis (replication) is initiated.
ortet The plant from which a clone is obtained.
osmic acid (= osmium tetroxide) (OsO4; f.w. 254.20) A fixing agent commonly used to prepare tissue samples for electron microscopy.
osmolarity The total molar concentration of the solutes. Osmolarity affects the osmotic potential of solution or nutrient medium.
osmosis (Gr. osmos, a pushing) Diffusion from areas of high concentration to areas of lower concentration of a solvent through a differentially permeable membrane.
osmotic potential [value] Potential brought about by dissolving a substance, especially in water.
osmoticum An agent, such as PEG, mannitol, glucose or sucrose, employed to maintain the osmotic potential of a nutrient medium equivalent to that of the cultured cells (isotonic). Because of this osmotic equilibrium, cells are not damaged in vitro.
outbreeding A mating system characterized by the breeding of genetically unrelated or dissimilar individuals. Since genetic diversity (q.v.) tends to be enhanced and since vigour or fitness of individuals can be increased by this process, it is often used to counter the detrimental effects of continuous inbreeding.
outflow The volume of growing cells that is removed from a bioreactor during a continuous fermentation process.
oxygen-electrode-based sensor Sensor in which an oxygen electrode - a standard electrochemical cell which measures the amount of oxygen in a solution - is coated with a biological material which generates or absorbs oxygen. When the biological coating is active, the amount of oxygen next to the electrode changes and the signal from the electrode changes.
ovary 1. Enlarged basal portion of the pistil of a plant flower that contains the ovules.
2. The reproduction organ in female animals in which eggs (ova) are produced.
overdominance A condition in which heterozygotes are superior (on some scale of measurement) to either of the associated homozygotes.
overhang See extension.
overlapping reading frames Start triplets in different reading frames generate different polypeptides from the same DNA sequence. See reading frame.
ovulation In mammals, the process of escape of the ovum (egg cell) from the ovary.
ovule (L. ovulum, diminutive of ovum, egg) The part of the reproductive organs in seed plants that consists of the nucellus, the embryo sac and integuments.
ovum (L. ovum, egg; pl: ova) 1. A gamete of female animals, produced by the ovary.
2. The oosphere in plants.
ovum pickup (OPU) The non-surgical collection of ova from a female.
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