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Glossary A: Angstrom; unit of length. achondrite: Differentiated meteorite. anticodon: Triplet of bases in transfer RNA complementary to the codon. archaebacteria: Organisms constituting one of the three biological king- doms. Archean: Period of Earth's history from 3.8 to 2.4 billion years ago. arc see: Arc second; unit of angular measurement in astronomy. ATF: Astrometric telescope facility. AU: Astronomical unit; mean distance between the Earth and the Sun. biogenic elements: Elements making up the bulk of living organisms. CAI: Calcium-aluminum inclusion, found in meteorites. carbonaceous chondrite: Meteorite with granules containing carbon-rich matter. CCD: Charge coupled device. CIT: Circumstellar Imaging Telescope. cm: Centimeter. codon: Triplet code of bases in DNA specifying an amino acid in protein synthesis. COMPLEX: Committee on Planetary and Lunar Exploration. CRAF: Comet Rendezvous Asteroid Flyby mission. Cretaceous: Period of Earth's history from 145 to 65 million years ago. D/H: Deuterium-to-hydrogen ratio. DNA: Deoxyribonucleic acid. DOE: Department of Energy. 135

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136 ECHO: Evolution of Complex and Higher Organisms; report. EIRP: Effective isotropic radiated power. ESA: European Space Agency. eubacteria: All other bacteria besides the archaebacteria. eukaryote: Cells with true nucleus and other internal organelles. GLOSSARY FGS: Fine guidance sensor. FIRST: Far-Infrared Space Telescope. Fischer-Tropsch reaction: Process in which carbon monoxide and hydro- gen mixtures are converted into hydrocarbons and related compounds. FOS: Faint object spectrograph. genome: The complete set of genes in an organism. GHz: Gigahertz; unit of frequency. HD/H2: Ratio of deuterated hydrogen to hydrogen. heterocyclic organic polymers: Compounds consisting of monomeric units of organic ring molecules in which not all atoms in the rings are alike. heterotroph: Organism requiring organic compounds as food source. HIMS: Hubble imaging Michelson spectrometer. IR: Infrared region of electromagnetic spectrum. IRAS: Infrared Astronomical Satellite. ISO: Infrared Space Observatory. ISOCAM: Infrared Space Observatory camera. I: Joule; unit of heat energy. K: Kelvin; unit of temperature. KAO: Kuiper Airborne Observatory. L183: Interstellar cloud. LDR: Large deployable reflector. m: Meter. MAP: Multichannel astrometic photometer. MHz: Megahertz; unit of frequency. MIPS: Multiband imaging photometer for SIRTF. mRNA: Messenger RNA; directs the synthesis of proteins. NAB: Nicotinamide adenine dinucleotide; coenzyme involved in redox reactions. NASA: National Aeronautics and Space Administration.

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GLOSSARY 137 NICMOS: Near-infrared camera and multiobject spectrometer. NIH: National Institutes of Health. NMR: Nuclear magnetic resonance. NRAO: National Radio Astronomy Observatory. NRC: National Research Council. NSF: National Science Foundation. nucleoside: Precursor of nucleic acids; consists of an organic base and a sugar. nucleosynthesis: Production of elements heavier than hydrogen. oligonucleotide: Short chain of nucleic acid monomers. Oligopeptide: Short chain of amino acids. PAH: Polyaromatic hydrocarbon. Paleozoic: Period in Earth's history from 670 to 245 million years ago. Permian: Period of Earth's history from 285 to 245 million years ago. Phanerozoic: Period of Earth's history from 670 million years ago to present. phenotype: Observable physiological behavior of an organism. phosphomonoesterase: Hydrolytic enzyme; releases inorganic phosphate. pllototroph: Organism deriving its energy from light. phylogeny: Ordering of biological species based on their evolutionary relationships. planetesimal: Solar-system body; of the order of a kilometer in size. planetoid: Solar-system body; tens to hundreds of kilometers in size. prebiotic: Before the appearance of life on Earth. Precambrian: Period of Earth's history from its formation to 600 million years ago. prokaryote: Organism lacking a true nucleus. Proterozoic: Period of Earth's history from 2.5 billion to 600 million years ago. pyrolysis: Destruction of organic compounds by combustion. regolith: Surface debris on solar-system objects produced by impacting bodies. RFI: Radio frequency interferences. ribonucleotide: Monomeric unit of RNA. ribooligonucleotide: Short chain of ribonucleotides. ribosome: Cellular particle; site of protein synthesis. RNA: Ribonucleic acid. RNA polymerase: Enzyme that polymerizes ribonucleotides. RNase P: tRNA-processing enzyme containing a catalytic RNA subunit. rRNA: Ribosomal RNA; involved in protein synthesis.

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38 GLOSSARY S.: Svedberg unit; sedimentation constant used in ultracentrifugation. SAO: Smithsonian Astrophysical Observatory. SETI: Search for extraterrestrial intelligence. SIRTF: Space Infrared Telescope Facility. SNC: Shergottite, nakhlite, and chassignite meteorites; possibly from Mars. SSB: Space Science Board/Space Studies Board. T4 RNA ligase: Enzyme causing ribonucleic acid fragments to join to- gether. template: Molecule that is copied to form its complement in nucleic acid synthesis. thiol ester: Sulfur-containing ester. TMC-1: Interstellar cloud. translation: Process by which DNA code specifies sequencing of amino acids. tRNA: Transfer RNA; combines with specific amino acid in protein syn- thesis. . . UV: Ultraviolet region of electromagnetic spectrum. Van der Waals force: Weak attractive force between nonpolar molecules. Viking: U.S. mission to Mars in 1975. W: Watt; unit of power. WFC: Wide-field camera.