controlled by two closely linked genes on chromosome
- Can lead to serious medical complications between a
mother and her developing fetus if they have different
Rh blood types (+ or –). This can occur when the par-
ents have different Rh blood types, e.g., the mother is
Rh– and the father is Rh+.
See alsoBLOOD.
rhizome A modified underground stem (has nodes
and scalelike leaves) that grows horizontal along or
below the ground and sends out roots and shoots.
rhodopsin The red photosensitive pigment found in
the retina’s rod cells; contains the protein opsin linked
to retinene (or retinal), a light-absorbing chemical
derived from vitamin A and formed only in the dark.
Light striking the rhodopsin molecule forces it to
change shape and generate signals as the retinene splits
from the opsin, which is then reattached in the dark to
complete the visual cycle.
ribonucleic acid(RNA) Linear polymer molecule
composed of a chain of ribose units linked between
positions 3 and 5 by phosphodiester groups. The bases
adenine or guanine (via atom N-9) or uracil or cyto-
sine (via atom N-1) are attached to ribose at its atom
C-1 by ß-N-glycosidic bonds. The three most impor-
tant types of RNAs in the cell are messenger RNA
(mRNA), transfer RNA (tRNA), and ribosomal RNA
(rRNA).
See alsoNUCLEOTIDES.
ribonucleotide reductases ENZYMEs that catalyze
the reduction of ribonucleotide diphosphates or
triphosphates to the corresponding deoxyribonu-
cleotides by a RADICAL-dependent reaction. The
enzyme of animal, yeast, and AEROBIC ESCHERICHIA
COLI cells contains an oxo-bridged DINUCLEAR iron
center and a tyrosyl radical cation, and uses thioredox-
in, a thiol-containing protein, as reductant. At least
three other ribonucleotide reductases are known from
bacteria, containing an IRON–SULFUR CLUSTERwith a
glycyl radical, adenosyl COBALAMIN, and a dinuclear
manganese CLUSTER.
ribose The sugar component of RNA; a five-carbon
(pentose) aldose.ribosomal RNA(rRNA) The most common form
of RNA. When combined with certain proteins, it
forms ribosomes that are responsible for translation of
MESSENGER RNA(mRNA) into protein chains.ribosome A subcellular unit composed of specific
rRNA and proteins that is responsible for the TRANSLA-
TIONof MESSENGER RNA(mRNA) into protein synthesis.ribozyme RNA with enzymatic or catalytic ability to
specifically cleave (break down) or bind RNA molecules.
Also known as autocatalytic or catalytic RNA.ribulose-1,5-bisphosphate carboxylase/oxygenase
(rubisco) Amagnesium-dependent ENZYME. The pri-
mary enzyme of carbon dioxide fixation in plants and294 rhizome
Uracil is one of the four bases in RNA. The others are adenine,
guanine, and cytosine. Uracil replaces thymine, which is the
fourth base in DNA. Like thymine, uracil always pairs with
adenine.(Courtesy of Darryl Leja, NHGRI, National Institutes of
Health)