Microbiology and Immunology

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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