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race  A distinguishable group of organisms of a particular species, that are geographically, ecologically, physiologically, physically and/or chromosomically distinct from other members of the species.

raceme  An inflorescence in which the main axis is elongated but the flowers are borne on pedicels that are about equal in length.

rachilla (Gr. rhachis, a backbone + L. diminutive suffix -illa) Shortened axis of a spikelet.

rachis (Gr. rhachis, a backbone)  Main axis of a spike; axis of fern leaf (frond) from which pinnae arise; in compound leaves, the extension of the petiole corresponding to the midrib of an entire leaf.

radicle (L. radix, root)  That portion of the plant embryo which develops into the primary or seed root.

radioactive isotope; radioisotope  An unstable isotope that emits ionizing radiation.

raft culture  See nurse culture.

ramet  An individual member of a clone. cf ortet.

random amplified polymorphic DNA (RAPD; pronounced 'rapid') A technique using single, short (usually 10-mer) synthetic oligonucleotide primers for PCR. The primer, whose sequence has been chosen at random, initiates replication at its complementary sites on the DNA, producing fragments up to about 2 kb long, which can be separated by electrophoresis and stained with ethidium bromide. A primer can exhibit polymorphism between individuals, and polymorphic fragments can be used as markers.

random genetic drift  See genetic drift..

random mutagenesis  A non-directed change of one or more nucleotide pairs in a DNA molecule.

random primer method  A protocol for labelling DNA in vitro. A sample of random oligonucleotides (6 or 14 nucleotides long) containing all possible combinations of nucleotide sequences is hybridized to a DNA probe. Then, in the presence of a DNA polymerase and the four deoxyribonucleotides - one of which is labelled - the 3´ hydroxy ends of the hybridized oligonucleotides provide initiation sites for DNA synthesis that uses the separated strands of the probe DNA as a template. This reaction produces labelled copies of portions of the probe DNA.

RAPD  See random amplified polymorphic DNA.

reading frame A series of triplets beginning from a specific nucleotide. Each triplet is represented by a single amino acid in the protein synthesized. The reading frame defines which sets of three nucleotides are read as triplets in the DNA, and hence as codons in the corresponding mRNA; this is determined by the initiation codon, AUG. Thus the sequence
AUG/GCA/AAA/UUU/CCC/ and not as
Depending on where one begins, each DNA strand contains three different reading frames. See open reading frame, overlapping reading frames.

read-through  Transcription or translation that proceeds beyond the normal stopping point because of the absence of the transcription or translation termination signal of a gene.

reca  A protein in most bacteria, and that is essential for DNA repair and DNA recombination.

recalcitrant  Of seeds: unable to survive drying and subsequent storage at low temperature. See field gene bank.

receptacle (L. receptaculum, a reservoir)  Enlarged end of the pedicel or peduncle, to which other flower parts are attached.

receptor  A molecule that can accept the binding of a ligand.

receptor-binding screening  One of the biotechnology-based methods for discovering conventional drugs. The method relies on the fact that many drugs act by binding to specific proteins (receptors) on or in cells: these proteins usually bind to hormones or to other cells, and control the cell's behaviour, although they may be enzymes or structural elements of the cell. The drug interferes with the normal role of the protein.

recessive  Describing an allele whose effect with respect to a particular trait is not evident in heterozygotes. Opposite to dominant.

recessive-acting oncogene  See recessive oncogene.

recessive oncogene; recessive-acting oncogene; anti-oncogene A single copy of this gene is sufficient to suppress cell proliferation; the loss of both copies of the gene contributes to cancer formation. See oncogene.

reciprocal crosses Crosses between the same two strains, but with the sexes reversed; e.g., female A × male B and male A × female B.

reciprocating shaker  A platform shaker used for agitating culture flasks, with a back and forth action at variable speeds.

recognition sequence  See recognition site.

recognition site  A nucleotide sequence - composed typically of 4, 6 or 8 nucleotides - that is recognized by and to which a restriction endonuclease (restriction enzyme) binds. For type II restriction enzymes (those used in gene-cloning experiments) it is also the sequence within which the enzyme specifically cuts (and their corresponding enzymes methylate) the DNA, i.e., for type II enzymes, the recognition site and the target site are the same sequence. Type I enzymes bind to their recognition site and then cleave the DNA at some more or less random position outside that recognition site. cf restriction site.

recombinant  A term used in both classical and molecular genetics.

    1. In classical genetics: An organism or cell that is the result of recombination (crossing-over), e.g., Parents: AB/ab and ab/ab; recombinant offspring: Ab/ab.

    2. In molecular genetics: A molecule containing DNA from different sources. The word is typically used as an adjective, e.g., recombinant DNA.

recombinant DNA  The result of combining DNA fragments from different sources.

recombinant DNA technology  A set of techniques which enable one to manipulate DNA. One of the main techniques is DNA cloning (because it produces an unlimited number of copies of a particular DNA segment), and the result is sometimes called a DNA clone or gene clone (if the segment is a gene), or simply a clone. An organism manipulated using recombinant DNA techniques is called a genetically modified organism (GMO).

    Among other things, recombinant DNA technology involves:
    - identifying genes;
    - cloning genes;
    - studying the expression of cloned genes; and
    - producing large quantities of the gene product.

recombinant protein  A protein whose amino acid sequence is encoded by a cloned gene.

recombinant RNA  A term used to describe RNA molecules joined in vitro by T4 RNA ligase.

recombinant toxin  A single multifunctional toxic protein that has been created by combining the coding regions of various genes.

recombinant vaccine  A vaccine produced from a cloned gene.

recombination  The process of crossing over, which occurs during meiosis I. It involves breakage in the same position of each of a pair of non-sister chromatids from homologous chromosomes, followed by joining of non-sister fragments, resulting in a reciprocal exchange of DNA between non-sister chromatids within an homologous pair of chromosomes. recombination fraction; recombination frequency  The proportion of gametes that have arisen from recombination between two loci. It is estimated as the number of recombinant individuals among a set of offspring of a particular mating, divided by the total number of offspring from that mating. Represented by the Greek letter theta (q). Linkage maps are created from estimates of recombination fraction between all pair-wise combinations of loci. See map distance.

reconstructed cell  A viable transformed cell resulting from genetic engineering.

re-differentiation  Cell or tissue reversal from one differentiated type to another differentiated type of cell or tissue. See de-differentiation.

reduction division  Phase of meiosis in which the maternal and paternal chromosomes of the bivalent separate. See equational division.

regeneration (L. re, again + generate, to beget)  The growth of new tissues or organs to replace those injured or lost. In tissue culture, regeneration is used to define the development of organs or plantlets from a tissue, callus culture or from a bud. See conversion; micropropagation; organogenesis.

regulator  Substance regulating growth and development of cells, organs, etc.

regulatory gene  A gene whose protein controls the activity of other genes or metabolic pathways.

rejuvenation  Reversion from adult to juvenile stage.

relaxed circle  See nicked circle.

relaxed circle plasmid  See plasmid.

relaxed plasmid  A plasmid that replicates independently of the main bacterial chromosome and is present in 10-500 copies per cell. See plasmid.

release factors  1. Soluble protein that recognizes termination codons in mRNAs and terminate translation in response to these codons.

    2. A hormone that is produced by the hypothalamus and stimulates the release of a hormone from the anterior pituitary gland into the bloodstream.

re-naturation  The re-association of two nucleic acid strands after denaturation. The restoration of a molecule to its native form. In nucleic acid biochemistry, this term usually refers to the formation of a double-stranded helix from complementary single-stranded molecules. Some simple proteins can also be re-natured and regain their function.

rennin  An enzyme secreted by cells lining the stomach in mammals, and that is responsible for clotting milk.

repeat unit  A sequence of bases that occurs repeatedly in the genome, often end-on-end, i.e., tandemly.

repetitive DNA  DNA sequences that are present in a genome in multiple copies, sometimes a million times or more.

replacement; gene replacement  A method of substituting a cloned gene, or part of a gene, which may have been mutated in vitro, for the wild-type copy of the gene within the host's chromosome. See homogenotization.

replacement therapy   The administration of metabolites, co-factors or hormones that are deficient as the result of a genetic disease.

replica plating  A procedure for duplicating the bacterial colonies growing on agar medium in one Petri plate to agar medium in another Petri plate.

replication  The synthesis of duplex (double-stranded) DNA by copying from a single-stranded template.

replicative form (RF) The molecular configuration of viral nucleic acid that is the template for replication in the host cell.

replicon  The portion of a DNA molecule which is replicable from a single origin. Plasmids and the chromosomes of bacteria, phages and other viruses usually have a single origin of replication and, in these cases, the entire DNA molecule constitutes a single replicon. Eukaryotic chromosomes have multiple internal origins and thus contain several replicons. The word is often used in the sense of a DNA molecule capable of independent replication, e.g., "The shuttle vector pJDB219 is a replicon in both yeast and E. coli."

replisome  The complete replication apparatus present at a replication fork that carries out the semi-conservative replication of DNA.

reporter gene  A gene that encodes a product that can readily be assayed. Thus reporter genes are used to determinate whether a particular DNA construct has been successfully introduced into a cell, organ or tissue.

repressible enzyme  An enzyme whose synthesis is diminished by a regulatory molecule.

repression  Inhibition of transcription by preventing RNA polymerase from binding to the transcription initiation site: a repressed gene is "turned off."

repressor  A protein which binds to a specific DNA sequence (the operator) upstream from the transcription initiation site of a gene or operon and prevents RNA polymerase from commencing mRNA synthesis. Examples of repressors are the C1 protein of bacteriophage and the lac1 protein of the lac operon.

reproduction  The production of an organism, cell or organelle like itself (self propagation).

    1. Sexual reproduction: the regular alternation (in the life-cycle of haplontic, diplontic and diplohaplontic organisms) of meiosis and fertilization (karyogamy) which provides for the production of offspring. The main biological significance of sexual reproduction lies in the fact that it achieves genetic recombination.

    2. Asexual or agamic reproduction: the development of a new individual from either a single cell (agamospermy) or from a group of cells (vegetative reproduction) in the absence of any sexual process. See also apomixis.

repulsion  The phase state in which a dominant (or wild-type) allele at one locus and a recessive (or mutant) allele at a second locus occur on the same chromosome. Also called trans configuration. See coupling.

residues  The components of macromolecules, e.g., amino acids, nucleotides.

resistance  Term commonly used to describe the ability of an organism to withstand a stress, a force or an effect of a disease, or its agent or a toxic substance.

resistance factor  A plasmid that confers antibiotic resistance to a bacterium.

rest period  An endogenous physiological condition of viable seeds, buds or bulbs that prevents growth even in the presence of otherwise favourable environmental conditions. By some seed physiologists, this is referred to as dormancy.

restitution nucleus  A nucleus with unreduced or doubled chromosome number that results from the failure of a meiotic or mitotic division.

restriction endonuclease [enzyme]  A class of endonucleases that cleaves DNA after recognizing a specific sequence, e.g., BamH1 (5´GGATCC3´), EcoRI (5´GAATTC3´), and HindIII (5´AAGCTT3´). There are three types of restriction endonuclease enzymes:

    Type I: Cuts non-specifically a distance greater than 1000 bp from its recognition sequence and contains both restriction and methylation activities.

    Type II: Cuts at or near a short, and often palindromic (q.v.) , recognition sequence. A separate enzyme methylates the same recognition sequence. They may make the cuts in the two DNA strands exactly opposite one another and generate blunt ends, or they may make staggered cuts to generate sticky ends. The type II restriction enzymes are the ones commonly exploited in recombinant DNA technology.

    Type III: Cuts 24-26 bp downstream from a short, asymmetrical recognition sequence. Requires ATP and contains both restriction and methylation activities.

restriction enzyme  See restriction nuclease.

restriction exonuclease [enzyme]  A class of nucleases that degrades DNA or RNA, starting from an end either 5´ or 3´.

restriction fragment  A fragment of DNA produced by cleaving (digesting, cutting) a DNA molecule with one or more restriction endonucleases.

restriction fragment length polymorphism (RFLP)  The occurrence of variation in the length of DNA fragments that are produced after cleavage with a type II restriction endonuclease. The differences in DNA lengths are due to the presence or absence of recognition site(s) for that particular restriction enzyme. RFLPs were initially detected using hybridization with DNA probes after separation of digested genomic DNA by gel electrophoresis (Southern analysis). Now they are typically detected by electrophoresis of digested PCR product.

restriction map  The linear array of restriction endonuclease sites on a DNA molecule. See mapping.

restriction nuclease  A bacterial enzyme that cuts DNA at a specific site.

restriction site  The specific nucleotide sequence in DNA that is recognized by a type II restriction endonuclease and within which it makes a double-stranded cut. Restriction sites usually comprise four or six base pairs that typically are palindromic (q.v.),
e.g., 5´GGCC3´


    The two strands may be cut either opposite to one another, to create blunt ends, or in a staggered manner, giving sticky ends, depending on the enzyme involved. See restriction endonuclease.

reticulocyte  A young red blood cell.

retro-element  Any of the integrated retroviruses or the transposable elements that resemble them.

retro-poson; retro-transposon   A transposable element that moves via reverse transcription (i.e., from DNA to RNA to DNA) but lacks the long terminal repeat sequences.

retroviral vectors  Gene transfer systems based on viruses that have RNA as their genetic material.

retrovirus  A class of eukaryotic RNA viruses that can form double-stranded DNA copies of their genomes by using reverse transcription; the double-stranded forms integrate into chromosomes of an infected cell. Many naturally occurring cancers of vertebrate animals are caused by retroviruses. Also, the AIDS virus is a retrovirus.

reversal transfer  Transfer of a culture from a callus-supporting medium to a shoot-inducing medium.

reverse genetics  Use positional cloning.

reverse mutation  See reversion.

reverse transcriptase; RNA-dependent DNA polymerase  An enzyme that uses RNA molecule as a template for the synthesis of a complementary DNA strand.

reverse transcription  The synthesis of DNA on a template of RNA, accomplished by reverse transcriptase.

reversion; reverse mutation  Restitution of a mutant gene to the wild-type condition, or at least to a form that gives the wild phenotype; more generally, the appearance of a trait expressed by a remote ancestor.

RF  See replicative form.

RFLP  See restriction fragment length polymorphism.

rhizobacterium  A micro-organism whose natural habitat is near, on or in plant roots.

Rhizobium (pl: rhizobia)  Prokaryote able to establish symbiotic relationship with leguminous plants, as a result of which elemental nitrogen is fixed or converted to ammonia. See nitrogen fixation.

rhizosphere  The soil region in the immediate vicinity of growing plant roots.

ribose  A monosaccharide (C5H10O5) rarely occurring free in nature, but important as a component of RNA.

ribonuclease  Any enzyme that hydrolyses RNA.

ribonucleic acid  See RNA.

ribosomal binding site  A sequence of nucleotides near the 5´ end of a bacterial mRNA molecule that facilitates the binding of the mRNA to the small ribosomal sub-unit. Also called the Shine-Delgarno sequence.

ribosomal RNA  See rRNA.

ribosome (ribo, from RNA + Gr. soma, body)  The sub-cellular structure that contains both RNA and protein molecules and mediates the translation of mRNA into protein. Ribosomes comprise large and small sub-units. See organelle; translation.

ribozyme; gene shears  RNA molecule that can catalyse chemical reactions, often cutting other RNAs.

ribulose  A keto-pentose sugar (C5H11O5) that is involved in carbon dioxide fixation in photosynthesis.

ribulose biphosphate (RuBP)  A five-carbon sugar that is combined with carbon dioxide to form a six-carbon intermediate in the first stage of the dark reaction of photosynthesis.

rinderpest  Cattle plague; a viral infection of cattle, sheep and goats.

Ri plasmid  A class of large conjugative plasmids found in the soil bacterium Agrobacterium rhizogenes. Ri plasmids are responsible for hairy root disease of certain plants. A segment of the Ri plasmid is found in the genome of tumour tissue from plants with hairy root disease.

R-loops  Single-stranded DNA regions in RNA-DNA hybrids formed in vitro under conditions where RNA-DNA duplexes are more stable than DNA-DNA duplexes.

RNA  Ribonucleic acid. An organic acid composed of repeating nucleotide units of adenine, guanine, cytosine and uracil, whose ribose components are linked by phospho-diester bonds. The information-carrying material in some viruses. More generally, a molecule derived from DNA by transcription that may carry information (messenger RNA (mRNA)), provide sub-cellular structure (ribosomal RNA (rRNA)), transport amino acids (transfer RNA (tRNA)) or facilitate the biochemical modification of itself or other RNA molecules. See antigen RNA; gene splicing; heterogeneous nuclear RNA (hnRNA); mRNA; ribosomal RNA; RNA polymerase; small nuclear RNA; transfer RNA.

RNA-dependent DNA polymerase  See reverse transcriptase.

RNA editing  Post-trancriptional processes that alter the information encoded in gene transcripts (RNAs).

RNA polymerase  An enzyme that catalyses the synthesis of RNA from a DNA template. See polymerase; RNA.

RNAase  See RNase.

RNase  Ribonuclease. A group of enzymes that catalyse the cleavage of nucleotides in RNA.

Roentgen (symbol: r)  Obsolete unit of ionizing radiation. The SI unit is the sievert (symbol: Sv; 1 Sv  8.4 r)

root  The descending axis of a plant, normally below ground, which serves to anchor the plant and to absorb and conduct water and mineral nutrients.

root apex  The apical meristem of a root; very similar to the shoot apical meristem in that it forms the three meristematic areas: the protoderm (developing into the epidermis); the procambium (which develops into the stele); and the growth meristem (which forms the cortex).

root cap  A thimblelike mass of cells covering and protecting the apical meristem of a root.

root culture  The culture of isolated root tips of apical or lateral origin to produce in vitro root systems with indeterminate growth habits. Root culture was among the first kinds of plant tissue cultures, and is still largely used in the study of developmental phenomena, and mycorrhizal, symbiotic and plant-parasitic relationships.

root cutting  Cutting made from sections of roots alone.

root hairs   Outgrowths from epidermal cell walls of the root specialized for water and nutrient absorption.

root nodule  A small round mass of cells that is located on the roots of plants and contains nitrogen-fixing bacteria.

root tuber  Thickened root that stores carbohydrates.

root zone  The volume of soil or growing medium containing the roots of a plant. In soil science, the depth of the soil profile in which roots are normally found. See rhizosphere.

rootstock  The trunk or root material to which buds or scions are inserted in grafting. See stock.

rotary shaker  Rotating apparatus with a platform on which, containers can be shaken, such as Erlenmeyer flasks containing cells in liquid nutrient medium.

rRNA; ribosomal RNA  The RNA molecules which are essential structural and functional components of ribosomes, the organelles responsible for protein synthesis. The different rRNA molecules are known by their sedimentation (Svedberg; symbol S) values. E. coli ribosomes contain one 16S rRNA molecule (1541 nucleotides long) in the same (small) sub-unit and a 23S rRNA (2904 nucleotides) and a 5S rRNA (120 nucleotides) in the large sub-unit. These three rRNA molecules are synthesized as part of a large precursor molecule which also contains the sequences of a number of tRNAs. Special processing enzymes cleave this large precursor to generate the functional moieties. See RNA.

RuBP  See ribulose biphosphate.

ruminant animals; ruminants Animals having a rumen - a large digestive vat in which fibrous plant material is partially broken down by microbial fermentation, prior to digestion in a "true" stomach (the abomasum). There are also two other stomachs - the reticulum and the omasum. Typical ruminants are cattle and sheep.

runner  A lateral stem that grows horizontally along the ground surface and gives rise to new plants either from axillary or terminal buds. cf stolon.

rust  A generic descriptor for various plant diseases, especially those caused by a group of parasitic fungi of the phylum Basidiomycota, that attack the leaves and stems of crops.

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