R
(1) RO¨ NTGEN.
(2) (in cell cycle) See cell cycle.
(3)
Arginine see amino acids
R antigen
Syn. R PROTEIN.
R body
A refractile, intracellular structure which occurs e.g. in certain
bacterial endosymbionts of protozoa and which appears to confer killer
characteristics on the
protozoan host cell. An R body consists of a proteinaceous ribbon, ca. 0.2–0.5
μm wide and ca. 10–15 μm in length, which is rolled up to form a cylinder;
under negative phase-contrast microscopy it appears as a bright ring or as a
pair of parallel rods. Its presence within a bacterial cell is apparently
associated with the presence
of plasmid(s) and/or phage(s). R bodies appear to unroll in the food
vacuoles of sensitive paramecia; unrolling appears to occur from the inside or
outside of the coil according to the species of the bacterium of origin. R
bodies have also been found in free-living strains of Pseudomonas which
are toxic for sensitive paramecia
and in a free-living Pseudomonas-like
bacterium.
r-chromatin
Chromatin
containing rRNA genes.
R factor
(1) See r plasmid.
(2) Release
factor: see protein synthesis.
R-glucan
Alkali-resistant (i.e. alkali-insoluble) glucans of fungal cell walls.
R loop (mol. biol.)
A single-stranded loop of DNA formed when a short ssRNA molecule pairs
with a complementary region of one strand of a dsDNA molecule, displacing the corresponding
region of the homologous strand (the R loop). An R loop can be observed by
electron microscopy using the kleinschmidt monolayer technique. If the ssRNA
usednis a mature mRNA derived from a split gene, an intron in the DNA – having
no homologous region in the mRNA – will appear as a loop of dsDNA extruded
between two R loops (each R loop corresponding to one of the two adjacent
exons); thus, R-looping can be used e.g. to detect introns in genes.
r plasmid
See R plasmid.
A
bacterial plasmid carrying genes that confer resistance to certain antibiotics.
r-selection
The
concept that in certain (r-selected) populations, a high reproductive rate is the
chief determinant of life history.
radial
cleavage
A type of
embryonic development in deuterostomes in which the planes of cell division
that transform the zygote into a ball of cells are either parallel or
perpendicular to the polar axis, thereby aligning tiers of cells one above the
other.
radial
symmetry
[L. radius, a
spoke of a wheel + Gk. summetros, symmetry]
Characterizing a body shaped like a pie or barrel, with many equal parts radiating outward like the spokes of a wheel; present in cnidarians and echinoderms.
Characterizing a body shaped like a pie or barrel, with many equal parts radiating outward like the spokes of a wheel; present in cnidarians and echinoderms.
radiation
[L. radius, a
spoke of a wheel, hence, a ray]
Energy emitted in the form of waves or particles.
Energy emitted in the form of waves or particles.
radiata
Members
of the radially symmetrical animal phyla, including cnidarians.
radicle
An
embryonic root of a plant.
radioactive
dating
[L. radius, a
spoke of a wheel, hence, a ray]
A method of determining the age of fossils and rocks using half-lives of radioactive isotopes.
A method of determining the age of fossils and rocks using half-lives of radioactive isotopes.
radioactive
isotope
An
isotope, an atomic form of a chemical element, that is unstable; the nucleus
decays spontaneously, giving off detectable particles and energy.
radiometric
dating
A method
paleontologists use for determining the ages of rocks and fossils on a scale of
absolute time, based on the half-life of radioactive isotopes.
reactant
A
starting material in a chemical reaction.
receptor
On or in
a cell, a specific protein molecule whose shape fits that of a specific
molecular messenger, such as a hormone.
receptor-mediated
endocytosis
(en-doh-sy-toh-sis)
The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances.
The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances.
receptor
potential
An
initial response of a receptor cell to a stimulus, consisting of a change in
voltage across the receptor membrane proportional to the stimulus strength. The
intensity of the receptor potential determines the frequency of action
potentials traveling to the nervous system.
recessive
allele
[L. recedere, to
recede]
In a heterozygote, the allele that is completely masked in the phenotype.
In a heterozygote, the allele that is completely masked in the phenotype.
reciprocal
altruism
(al-troo-iz-um)
Altruistic behavior between unrelated individuals; believed to produce some benefit to the altruistic individual in the future when the current beneficiary reciprocates.
Altruistic behavior between unrelated individuals; believed to produce some benefit to the altruistic individual in the future when the current beneficiary reciprocates.
recognition
sequence
A
specific sequence of nucleotides at which a restriction enzyme cleaves a DNA
molecule.
recognition
species concept
The idea
that specific mating adaptations become fixed in a population and form the
basis of species identification.
recombinant
An
offspring whose phenotype differs from that of the parents.
recombinant
DNA
A DNA
molecule made in vitro with segments from different sources.
recombination
The
formation of new gene combinations; in eukaryotes, may be accomplished by new
associations of chromosomes produced during sexual reproduction or crossing
over; in prokaryotes, may be accomplished through transformation, conjugation, or transduction.
redox
reaction
(ree-doks)
A chemical reaction involving the transfer of one or more electrons from one reactant to another; also called oxidation-reduction reaction.
A chemical reaction involving the transfer of one or more electrons from one reactant to another; also called oxidation-reduction reaction.
reducing
agent
The
electron donor in a redox reaction.
reduction
[L. reducere, to
lead back]
The gaining of electrons by a substance involved in a redox reaction.
The gaining of electrons by a substance involved in a redox reaction.
reflex
[L. reflectere, to
bend back]
An automatic reaction to a stimulus, mediated by the spinal cord or lower brain.
An automatic reaction to a stimulus, mediated by the spinal cord or lower brain.
refractory
period
(ree-frak-tor-ee)
The short time immediately after an action potential in which the neuron cannot respond to another stimulus, owing to an increase in potassium permeability.
The short time immediately after an action potential in which the neuron cannot respond to another stimulus, owing to an increase in potassium permeability.
regulative
development
A pattern
of development, such as that of a mammal, in which the early blastomeres retain
the potential to form the entire animal.
relative
fitness
The
contribution of one genotype to the next generation compared to that of
alternative genotypes for the same locus.
relay
neuron
Neuron
that transmits signals between different regions of the central nervous system.
releaser
A signal
stimulus that functions as a communication signal between individuals of the
same species.
releasing
hormone
A hormone
produced by neurosecretory cells in the hypothalamus of the vertebrate brain
that stimulates or inhibits the secretion of hormones by the anterior
pituitary.
renal
[L. renes, kidneys]
Pertaining to the kidney.
Pertaining to the kidney.
repetitive
DNA
Nucleotide
sequences, usually noncoding, that are present in many copies in a eukaryotic
genome. The repeated units may be short and arranged tandemly (in series) or
long and dispersed in the genome.
replication
The
process of making a copy of something.
replication
fork
A
Y-shaped point on a replicating DNA molecule where new strands are growing.
repressible
enzyme
An enzyme
whose synthesis is inhibited by a specific metabolite.
repressor
[L. reprimere, to
press back, keep back]
A protein that suppresses the transcription of a gene.
A protein that suppresses the transcription of a gene.
reproductive
isolation
Two
populations of organisms are isolated if their members are unable to interbreed
and produce fertile offspring. Various structural, behavioral, and biochemical
features can prevent interbreeding and thus reproductively isolate populations
as distinct species.
Reptilia
[L. reprimere, to
press back, keep back]
The vertebrate class of reptiles, represented by lizards, snakes, turtles, and crocodilians.
The vertebrate class of reptiles, represented by lizards, snakes, turtles, and crocodilians.
resolving
power
[L. resolvere, to
loosen, unbind]
A measure of the clarity of an image; the minimum distance that two points can be separated and still be distinguished as two separate points.
A measure of the clarity of an image; the minimum distance that two points can be separated and still be distinguished as two separate points.
resource
partitioning
The division
of environmental resources by coexisting species populations such that the
niche of each species differs by one or more significant factors from the
niches of all coexisting species populations.
respiration
[L. respirare, to
breathe]
(1) In aerobic organisms, the intake of oxygen and the liberation of carbon dioxide. (2) In cells, the oxygen-requiring stage in the breakdown and release of energy from fuel molecules.
(1) In aerobic organisms, the intake of oxygen and the liberation of carbon dioxide. (2) In cells, the oxygen-requiring stage in the breakdown and release of energy from fuel molecules.
resting
potential
The
membrane potential characteristic of a nonconducting, excitable cell, with the
inside of the cell more negative than the outside.
restriction
enzyme
A
degradative enzyme that recognizes and cuts up DNA (including that of certain
phages) that is foreign to a bacterium.
restriction
fragment length polymorphisms (RFLPs)
Differences
in DNA sequence on homologous chromosomes that result in different patterns of
restriction fragment lengths (DNA segments resulting from treatment with
restriction enzymes); useful as genetic markers for making linkage maps.
restriction
site
A
specific sequence on a DNA strand that is recognized as a "cut site"
by a restriction enzyme.
reticular
formation
[L. reticulum, a
network]
A brain circuit involved with alertness and direction of attention to selected events; consists of a loose network of interneurons running through the brainstem, plus certain neurons in the thalamus that function as an extension of this network.
A brain circuit involved with alertness and direction of attention to selected events; consists of a loose network of interneurons running through the brainstem, plus certain neurons in the thalamus that function as an extension of this network.
reticulum
[L.
network]
A fine network (e.g., endoplasmic reticulum).
A fine network (e.g., endoplasmic reticulum).
retina
(reh-tin-uh)
[L. dim. of rete, net]
The innermost layer of the vertebrate eye, containing photoreceptor cells (rods and cones) and neurons; transmits images formed by the lens to the brain via the optic nerve.
The innermost layer of the vertebrate eye, containing photoreceptor cells (rods and cones) and neurons; transmits images formed by the lens to the brain via the optic nerve.
retinal
The
light-absorbing pigment in rods and cones of the vertebrate eye.
retrovirus
(reh-troh-vy-rus)
[L. turning back]
An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome; an important class of cancer-causing viruses.
An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome; an important class of cancer-causing viruses.
reverse
transcriptase
(trans-krip-tase)
An enzyme encoded by some RNA viruses that uses RNA as a template for DNA synthesis.
An enzyme encoded by some RNA viruses that uses RNA as a template for DNA synthesis.
rhizoid
[Gk. rhiza, root]
Rootlike anchoring structure in fungi and nonvascular plants.
Rootlike anchoring structure in fungi and nonvascular plants.
rhizome
[Gk. rhizoma, mass
of roots]
In vascular plants, a horizontal stem growing along or below the surface of the soil; may be enlarged for storage or may function in vegetative reproduction.
In vascular plants, a horizontal stem growing along or below the surface of the soil; may be enlarged for storage or may function in vegetative reproduction.
rhodopsin
[Gk. rhizoma, mass
of roots]
A visual pigment consisting of retinal and opsin. When rhodopsin absorbs light, the retinal changes shape and dissociates from the opsin, after which it is converted back to its original form.
A visual pigment consisting of retinal and opsin. When rhodopsin absorbs light, the retinal changes shape and dissociates from the opsin, after which it is converted back to its original form.
ribonucleic
acid (RNA)
(ry-boh-noo-klay-ik)
A type of nucleic acid consisting of nucleotide monomers with a ribose sugar and the nitrogenous bases adenine (A), cytosine (C), guanine (G), and uracil (U); usually single-stranded; functions in protein synthesis and as the genome of some viruses.
A type of nucleic acid consisting of nucleotide monomers with a ribose sugar and the nitrogenous bases adenine (A), cytosine (C), guanine (G), and uracil (U); usually single-stranded; functions in protein synthesis and as the genome of some viruses.
ribose
The sugar
component of RNA.
ribosomal
RNA (rRNA)
The most
abundant type of RNA. Together with proteins, it forms the structure of
ribosomes that coordinate the sequential coupling of tRNA molecules to the
series of mRNA codons.
ribosome
A cell
organelle constructed in the nucleolus, functioning as the site of protein
synthesis in the cytoplasm. Consists of rRNA and protein molecules, which make
up two subunits.
ribozyme
An
enzymatic RNA molecule that catalyzes reactions during RNA splicing.
RNA
Abbreviation
of ribonucleic acid.
RNA
polymerase
(pul-im-ur-ase)
An enzyme that links together the growing chain of ribonucleotides during transcription.
An enzyme that links together the growing chain of ribonucleotides during transcription.
RNA
processing
Modification
of RNA before it leaves the nucleus, a process unique to eukaryotes.
RNA
splicing
The
removal of noncoding portions (introns) of the RNA molecule after initial
synthesis.
rod cell
One of
two kinds of photoreceptors in the vertebrate retina; sensitive to black and
white and enables night vision.
root
The
descending axis of a plant, normally below ground and serving both to anchor
the plant and to take up and conduct water and dissolved minerals.
root cap
A cone of
cells at the tip of a plant root that protects the apical meristem.
root hair
A tiny
projection growing just behind the root tips of plants, increasing surface area
for the absorption of water and minerals.
root
pressure
The
upward push of water within the stele of vascular plants, caused by active
pumping of minerals into the xylem by root cells.
rough ER
That
portion of the endoplasmic reticulum studded with ribosomes.
rubisco
Ribulose
carboxylase, the enzyme that catalyzes the first step (the addition of CO2 to
RuBP, or ribulose bisphosphate) of the Calvin cycle.
ruminant
An animal,
such as a cow or a sheep, with an elaborate, multicompartmentalized stomach
specialized for an herbivorous diet.
Tags:
R plasmid
reciprocal altruism
repetitive DNA
Reverse transcriptase
RNA processing
RNA splicing
