Rhodophyta WORLD OF MICROBIOLOGY AND IMMUNOLOGY
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dose of anti-D immunoglobulin varies in different countries.
In the USA, it is standard practice for Rh– patients who
deliver Rh+ infants to receive an intramuscular dose of Rh
immune globulin within 72 hours after delivery. With this
treatment, the risk of subsequent sensitization deceases from
about 15% to 2%. However, in spite of the routine use of gam-
maglobulin for both antepartum and postpartum immunopro-
phylaxis, severe fetal Rh alloimmunization continues to be a
serious medical problem. In the presence of severe fetal ane-
mia, early intervention appears to offer substantial improve-
ment in clinical outcome.
Prenatal antibody screening is recommended for all
pregnant women at their first prenatal visit. Repeat antibody
screening at 24–28 weeks gestation is recommended for
unsensitized Rh-negative mothers. The goals of antepartum
care are to accurately screen the pregnant woman for Rh
incompatibility and sensitization, to start appropriate thera-
peutic interventions as quickly as possible, and to deliver a
mature fetus who has not yet developed severe hemolysis.
Frequent blood tests (indirect Coombs’ tests) are
obtained from the mother, starting at 16 to 20 weeks’ gestation.
These tests identify the presence of Rh-positive antibodies in
maternal blood. When the antibody titer rises to 1:16 or greater,
the fetus should be monitored by amniocentesis, cordocentesis,
or the delta optical density 450 test. Administration of a dose of
Rh immune globulin to Rh– patients at 28 weeks was found to
reduce the risk of sensitization to about 0.2%.
The early diagnosis of fetal Rh status represents the best
approach for the management of the disease, and a promising
non-invasive detection of incompatibility seems now possible
by means of the polymerase chain reaction(PCR) analysis of
cell-free fetal DNAcirculating in the mother’s blood.
See alsoAntibody and antigen; Antibody formation and
kinetics
RHIZOBIUM-AGROBACTERIUM GROUP•
seeECONOMIC USES AND BENEFITS OF MICROORGANISMS
RRhodophytaHODOPHYTA
The red algae phylum Rhodophyta synthesizes a class of
water-soluble pigments termed phycobilins, known to be pro-
duced only by another algae, the Cryptomonads. There are
approximately 6,000 species of Rhodophyta. Some of them
are unicellular species that grow as filaments or membrane-
like sheet cells, and some multicellular coralline species
deposit calcium carbonate inside and around their cell walls,
which are very similar in appearance to pink and red corals.
Some Rhodophyta have an important role in coral-reef forma-
tion in tropical seas due to the deposits of calcium carbonate
crystals they release in the environment, and are therefore
termed coralline algae.
Rhodophyta are ancient algae whose fossil remains are
found under the form of coralline algal skeletons in limestone
deposits of coral reef origin dating back to the Precambrian
Era. They use the blue spectrum of visible light to accomplish
photosynthesisthat allows them to live in deep waters, storing
energy under the form of Floridean starch. They make mostly
chlorophyll-a, and the pigments alpha and beta-carotene, phy-
coerythrin, as well as others similar to those made by
Cyanobacteria, such as allophycocyanin and r-phycocyanin.
The cell walls are made mainly of cellulose (but some species
use xylan), and colloidal substances, such as agars and
carageenan; and the cells may be multinucleated. The
Floridean starch, a carbohydrate molecule consisting of 15
units of glucose, is kept free in the cytoplasm, whereas in other
algae it is attached to the chloroplast. Some species are con-
sumed by humans such as the Japanese nori (Porphyra) and
others are utilized as components in processed food and by the
pharmaceutical industries, such as Chondrus,and Gelidium.
See alsoBlue-green algae; Petroleum microbiology; Protists;
Xanthophylls
RRibonucleic acid (RNA)IBONUCLEIC ACID(RNA)
Nucleic acids are complex molecules that contain a cell’s
genetic information and the instructions for carrying out cel-
lular processes. In eukaryotic cells, the two nucleic acids,
ribonucleic acid (RNA) and deoxyribonucleic acid(DNA),
work together to direct protein synthesis. Although it is DNA
that contains the instructions for directing the synthesis of spe-
cific structural and enzymatic proteins, several types of RNA
actually carry out the processes required to produce these pro-
teins. These include messenger RNA (mRNA), ribosomal
RNA (rRNA), and transfer RNA (tRNA). Further processing
of the various RNA’s is carried out by another type of RNA
called small nuclear RNA (snRNA). The structure of RNA is
very similar to that of DNA, however, instead of the base
thymine, RNA contains the base uracil. In addition, the pen-
tose sugar ribose is missing an oxygen atom at position two in
DNA, hence the name deoxy-.
Nucleic acids are long chain molecules that link
together individual nucleotides that are composed of a pentose
sugar, a nitrogenous base, and one or more phosphate groups.
The nucleotides, the building blocks of nucleic acids, in
ribonucleic acid are adenylic acid, cytidylic acid, guanylic
acid, and uridylic acid. Each of the RNA subunit nucleotides
carries a nitrogenous base: adenylic acid contains adenine (A),
cytidylic acid contains cytosine (C), guanylic acid contains
guanine (G), and uridylic acid contains uracil.
In humans, the DNA molecule is made of phosphate-
base-sugar nucleotide chains, and its three-dimensional shape
affects its genetic function. In humans and other higher organ-
isms, DNA is shaped in a two-stranded spiral helix organized
into structures called chromosomes. In contrast, most RNA
molecules are single-stranded and take various shapes.
Nucleic acids were first identified by the Swiss bio-
chemist Johann Miescher (1844–1895). Miescher isolated a
cellular substance containing nitrogen and phosphorus.
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