Nucleolus of maize chromosome 6

31 Mar

Nu body


nu ( ) body particles arranged like beads on a string along interphase chromosomes. These are most clearly seen in electron micrographs of negatively stained Miller spreads (q.v.). Nu bodies correspond to the nucleosomes (q.v.) of the biochemist. nuclear dimorphism in ciliated protozoa, the presence of two morphologically and functionally different types of nuclei. The macronucleus is large, highly polyploid, and it contains many nucleoli. Ma- cronuclear DNA functions analogously to the DNA of somatic cells. The micronucleus functions as the germline and is diploid. It is capable of undergoing meiosis during conjugation (q.v.).

The macronucleus develops from a micronucleus. See Appendix A, Pro- toctista, Ciliophora; Appendix C, 1876, Bu¨tschli. nuclear duplication mitosis (q.v.). nuclear emulsion a photographic emulsion espe- cially compounded to make visible the individual tracks of ionizing particles. nuclear envelope an envelope surrounding the nucleus, composed of two membranes enclosing a perinuclear cisterna. The outermost membrane is studded with ribosomes.

The perinuclear cisterna is traversed by nuclear pore complexes (q.v.). See lamins. nuclear family a pair of parents and their children. nuclear fission a transformation of atomic nuclei characterized by the splitting of a nucleus into at least two other nuclei and the release of amounts of energy far greater than those generated by conven- tional chemical reactions. nuclear fusion the coalescence of two or more atomic nuclei with the release of relatively vast amounts of energy. nuclear lamina See lamins. nuclear magnetic resonance (NMR) spectroscopy an instrumental technique used to determine the three-dimensional (3-D) structure of biological mol- ecules. NMR spectroscopy and x-ray crystallography (q.v.) are the only methods capable of analyzing the structures of proteins and nucleic acids at atomic resolution.

NMR spectroscopy exploits the behavior of certain atoms when they are placed in a strong static magnetic field and exposed to short pulses of energy in the radio-wave frequency range. For bio- logical samples, the important atoms are H-1, N-15, and C-13, and the magnets used are 10,000-15,000 times stronger than the earth’s magnetic field. To in- crease the level of N-15 and C-13 in the molecular targets, microorganisms from which the molecules are extracted are grown on media enriched with these isotopes. When placed in a strong magnetic field, the atomic nuclei of these atoms exhibit a property called nuclear spin, whereby they behave like tiny compass needles and orient themselves with respect to the magnetic field.

When exposed to pulses of radio waves of specific frequencies, the ori- ented nuclei jump to higher-energy states in which the spin is opposed to the magnetic field. The nuclei are now said to be in resonance, and they emit radio frequency radiation when they revert to their lower- energy states. The amount of energy needed to achieve resonance is dependent on the properties of each nucleus and its chemical environment, and plots of the strengths of the resonance signals versus radio-wave frequencies provide information about the nature of atoms and their proximity to one an- other. NMR data are coupled with computational tools to produce 3-D structures of biomolecules, which are stored in easily accessible databases.

The first protein structure determined by NMR spectros- copy was that of a bull seminal proteinase inhibitor. NMR spectroscopy techniques can also be extended to such areas as the study of molecular interactions, molecular motion, and the rate of chemical reac- tions. See Appendix C, 1985, Williamson et al.; 1966, Ernst and Anderson; 1991, Ernst; 2002, Wu¨thrich et al.; Appendix E, Individual Databases; Antennapedia, proteomics.

nuclear pore complex (NPC) an octagonally sym- metrical organelle that allows controlled passage of molecules from nucleus to cytoplasm and vice versa. A typical mammalian nucleus contains between 3,000 and 4,000 NPCs. Each complex is made up of a central core that is formed from eight spokelike structures which encircle a central plug and are sandwiched between two rings. Cytoplasmic fibrils attach to the subunits that make up the outer cyto- plasmic ring.

A basket-like structure protrudes from the inner nuclear ring. It is composed of struts that connect subunits of this ring to a smaller terminal ring. See nucleoporins (Nups). nuclear processing of RNA See posttranscriptional processing, RNA editing. nuclear reactor the apparatus in which nuclear fission may be sustained in a self-supporting chain reaction. A source of energy and radioisotopes. nuclear reprogramming modifications of DNA (e.g., by demethylation) and/or chromatin proteins (e.g., by dissociation from DNA) that allow a differ- entiated nucleus from larval or adult somatic cells to replace an egg nucleus and restore totipotency (q.v.) to the transplanted nucleus. See Appendix C, 2004, Simonssen and Gurdon; nuclear transfer. nuclear RNA RNA molecules found in the nucleus either associated with chromosomes or in the nu- cleoplasm. See chromosomal RNA, heterogeneous nuclear RNA. nuclear targeting signal See protein sorting, sort- ing signals. nuclear transfer the injection of a diploid somatic nucleus into an enucleated egg.

The nature of the ensuing development reveals the developmental po- tentialities of the implanted nucleus. Various am- phibian species were used in early experiments. The number of embryos that survived to tadpoles de- clined when donor cells were taken from animals at successively more advanced developmental stages. The recent birth of a lamb cloned from the somatic nucleus of an adult attracted worldwide attention. However, Dolly (q.v.) was the only successful out- come from 277 nuclear transfer procedures. See Appendix C, 1952, Briggs and King; 1962, 1967, Gurdon; 1997, Wilmut et al.; cloning, nuclear repro- gramming, sheep. nuclease any enzyme that breaks down nucleic acids. nucleic acid a nucleotide polymer. In the early lit- erature DNA and RNA were called thymonucleic acid and yeast nucleic acid, respectively. This is be- cause DNA was first isolated from beef thymus glands and RNA from bakers’ yeast cultures. See de- oxyribonucleic acid, ribonucleic acid. nucleic acid bases See bases of nucleic acids. nucleic acid fingerprinting a method for analyz- ing digests of DNA or RNA similar to the finger- printing method for fragmented proteins. See Ap- pendix C, 1965, Sanger, Brownlee and Barrell; DNA fingerprint technique, oligonucleotide fingerprinting (OFP). nuclein the acidic, phosphorus-rich substance iso- lated from human white blood cells by Miescher. We now know that nuclein was a mixture of nucleic acids and proteins. See Appendix C, 1871, Miescher.

nucleocapsid a virus nucleic acid and its sur- rounding capsid. See capsomere. nucleo-cytoplasmic ratio the ratio of the volume of nucleus to the volume of cytoplasm. nucleoid 1. a DNA-containing region within a prokaryote, mitochondrion, or chloroplast. 2. in an RNA tumor virus, the core of genetic RNA sur- rounded by an icosahedral protein capsid. nucleolin an acidic phosphoprotein synthesized in the dense fibrillar regions of the nucleolus. Human nucleolin is made up of 707 amino acids. The NCL gene resides at 2q12-qter. It consists of 14 exons with 13 introns and is about 11 kb long. Intron 11 encodes a small nucleolar RNA designated U20. This snoRNA has a region of perfect complementa- rity with a conserved sequence in the 18S rRNA. It follows that nucleolin is involved in the formation of the small ribosomal subunit. See Appendix C, 1989, Srivastava et al. nucleolus an RNA-rich, intranuclear domain found in eukaryotic cells that is associated with the nucleolus organizer (q.v.) and is the site of preribo- somal RNA (q.v.) synthesis and processing (q.v.) and of ribosomal particle assembly.

The illustration on page 304 shows chromosome 6 of maize (q.v.), which contains the nucleolus organizer and its nu- cleolus as they appear in meiotic prophase. The nu- cleolus is composed of the primary products of the ribosomal RNA genes (q.v.) and a variety of pro- teins, including RNA polymerases, ribonucleases, molecular chaperones (q.v.), helicases, ribosomal proteins, and proteins of unknown function. rRNA genes and their nascent transcripts were first seen as Miller trees (q.v.) in nucleoli from salamander oo- cytes. Under the electron microscope (q.v.), the nucleoli of most metazoans contain three major

Nucleolus organizer

Nucleolus of maize chromosome 6

morphological components: the fibrillar center (FC), the dense fibrillar component (DFC), and the granu- lar component (GC). These represent successive stages in the process of ribosome biogenesis. The FC contains tandem arrays of rRNA genes and is sur- rounded by the DFC, where newly synthesized pre- rRNA molecules and their associated proteins are found. Later events in posttranscriptional processing (q.v.) and assembly of preribosomal particles are as- sociated with the GC that surrounds the DFC. The nucleolus has also been implicated in some non-tra- ditional roles. For example, the yeast Cdc14 (q.v.), a protein that promotes the exit from mitosis (q.v.), localizes to the nucleolus during the G1 stage of the cell cycle (q.v.) and remains there until anaphase (q.v.), when it is liberated.

Sequestration of this pro- tein by the nucleolus thus prevents the cell from completing mitosis prematurely. See Appendix C, 1838, Schleiden; 1934, McClintock; 1965, Ritossa and Spiegelman; 1967, Birnstiel; 1969, Miller and Beatty; 1976, Chooi; 1989, Srivastava et al.; Cajal body, Cdc14, helicase, nucleolin, rDNA amplification, ribonuclease, ribosome, RNA polymerase, small nucle- olar RNAs. nucleolus organizer a region of one or more chro- mosomes that contains the ribosomal RNA genes (q.v.) and is associated with the nucleolus (q.v.). Also called nucleolus organizer region (NOR).

See Appendix C, 1934, McClintock; 1965, Ritossa and Spiegelman; 1967, Birnstiel; 1969, Miller and Beatty; 1976, Chooi; Cajal body, rDNA amplification, ribosome, RNA polymerase. nucleolus organizer region (NOR) nucleolus or- ganizer (q.v.). nucleomorph See cryptomonads. nucleon a constituent particle of an atomic nu- cleus. nucleoplasm the protoplasmic fluid contained in the nucleus. nucleoporins (Nups) a family of more than 100 different proteins that are localized in each nuclear pore complex (NPC) (q.v.). Some of these proteins are structural components of the organelle, while others facilitate the transport of proteins and RNAs through the NPCs. Some nucleoporins play a role in tethering telomeres (q.v.) to the nuclear envelope. nucleoprotein a compound of nucleic acid and protein. Either one of two main classes of basic pro- teins are found combined with DNA: one of low molecular weight (protamine) and one of high mo- lecular weight (histone). The basic amino acids of these proteins neutralize the phosphoric acid resi- dues of the DNA. See Appendix C, 1866, Miescher.

nucleosidase any enzyme that catalyzes the split- ting of nucleosides into bases and pentoses. nucleoside a purine or pyrimidine base attached to ribose or deoxyribose. The nucleosides commonly found in DNA or RNA are cytidine, cytosine deox- yriboside, thymidine, uridine, adenosine, adenine deoxyriboside, guanosine, and guanine deoxyribo- side. Note that thymidine is a deoxyriboside and cy- tidine, uridine, adenosine, and guanosine are ribo- sides.

See rare bases, inosine. nucleosome a beadlike structure of eukaryotic chromosomes, consisting of a core of eight histone molecules (two each of proteins H2A, H2B, H3, and H4) wrapped by a DNA segment about 150 base pairs in length and separated from adjacent nucleo- somes by a “linker” DNA sequence of about 50 base pairs). See Appendix C, 1974, Kornberg; 1977, Par- don et al., Leffak et al.; chromatosome, histones, so- lenoid structure. nucleotide one of the monomeric units from which DNA or RNA polymers are constructed, consisting of a purine or pyrimidine base, a pentose, and a phosphoric acid group. The nucleotides of DNA are deoxyadenylic acid, thymidylic acid, deoxyguanilic


acid, and deoxycytidylic acid. The corresponding nucleotides of RNA are adenylic acid, uridylic acid, guanylic acid, and cytidylic acid. nucleotide pair a hydrogen-bonded pair of purine- pyrimidine nucleotide bases on opposite strands of a double-helical DNA molecule. Normally, adenine pairs with thymine, and guanine pairs with cytosine; also called complementary base pairs. See Chargaff’s rules, deoxyribonucleic acid. nucleotide pair substitution the replacement of a given nucleotide pair by a different pair, usually through a transition or a transversion (both of which see). nucleotide sequence databases See Appendix E. nucleus the spheroidal, membrane-bounded struc- ture present in all eukaryotic cells which contains DNA, usually in the form of chromatin.

Two theo- ries explaining the origin of the nucleus appear be- low. See Appendix C, 1831, Brown; 1991, Sogin; 1992, Rivera and Lake; endokaryotic hypothesis, nu- clear envelope, nuclear pore complex (NPC), Sogin’s first symbiont. nuclide a species of atom characterized by the constitution of its nucleus. This is specified by the number of protons and neutrons it contains. nude mouse a laboratory mouse homozygous for the recessive mutation nu, which maps to chromo- some 11. Such mice are characterized by the com- plete absence of hair and thymus glands. Nude mice lack T lymphocytes (q.v.), but have natural killer cells (q.v.) and B lymphocytes (q.v.), and they are unable to reject homografts.

The nude mouse serves as a model system for the study of the immunologi- cal effects of thymus deprivation. See rejection. null allele an allele that produces no functional product and therefore usually behaves as a genetic recessive. For example, in the human ABO blood group system, the recessive allele (i) produces no de- tectable antigen, either in homozygous condition (blood group O) or in heterozygous condition with allele IA (blood group A) or with allele IB (blood group B). See silent allele. null hypothesis method the standard hypothesis used in testing the statistical significance of the dif- ference between the means of samples drawn from two populations. The null hypothesis states that there is no difference between the populations from which the samples are drawn. One then determines the probability that one will find a difference equal to or greater than the one actually observed. If this probability is .05 or less, the null hypothesis is re- jected, and the difference is said to be significant. nulliplex See autotetraploidy. nullosomic lacking both members of a pair of chromosomes. numerical taxonomy a system of classification us- ing a multitude of characteristics to determine over- all phenotypic similarity, each trait being given equal weight and without regard to phylogenetic relation- ships; also known as phenetic taxonomy. nu particles See nucleosomes. nurse cells cells in the insect ovary that function to nourish the oocyte. In Drosophila melanogaster there are 15 nurse cells, and their nuclei undergo en- domitosis (q.v.).

The chromatids generated by the cycles of DNA replication fall apart to produce a tangled mass unsuitable for cytological study. How- ever, in some alleles of the otu mutant (q.v.), ovarian nurse cells have banded polytene chromosomes suit- able for cytological mapping. The nurse-cell chro- mosomes are active in transcription of a variety of RNA molecules, which enter their cytoplasm and are eventually transported to the oocyte. The nurse cells degenerate after pumping almost all of their cy- toplasm to the oocyte. See cystocyte divisions, insect ovary types. nutritional mutant a mutation converting a proto- troph into an auxotroph. nutritive chord See insect ovary types. N value the haploid chromosome number; the number of chromosomes in each germ cell. See poly- ploidy. nystagmus a jerky twitching of the eye. See albi- nism.

O 1. Ordovician. 2. oxygen. O antigens polysaccharide antigens of the cell walls of enterobacteria such as Escherichia or Salmonella; in contrast to the polysaccharide K antigens of bacte- rial capsules or the protein H antigens of bacterial flagella. oat See Avena. obese a gene, first identified in the mouse, from a strain where adults were double the normal weight and developed type 2 diabetes. These animals were ob−/ob−. The normal allele (ob+) encodes a 176- amino acid protein. When injected into overweight, ob− homozygotes, this protein causes a dose-depen- dent weight loss. For this reason, the protein has been named leptin (from the Greek root leptos, meaning thin). Leptin injection results in lowering of body weight, percentage of body fat, food intake, and serum concentrations of glucose and insulin. The ob+ gene is highly conserved among vertebrates, and its human homolog has been identified. See Ap- pendix C, 1994, Zhang et al.; 1995, Tartaglia et al.; diabetes mellitus. obligate restricted to a specified condition of life. For example, an obligate parasite cannot live in the absence of its host. See facultative. Occam’s razor a rule attributed to the medieval philosopher William of Occam. In modern times, the rule states that when there are several possible explanations of a phenomenon, one selects as most probable the explanation that is the simplest and most consistent with the data at hand.

Also called the parsimony principle. oceanic island an island that has risen from the sea. See continental island. ocellus one of the simple eyes located near the compound eyes of an insect; an eyespot in many in- vertebrates. ochre codon a triplet of mRNA nucleotides (UAA) usually not recognized by any tRNA molecules; one of three stop codons that normally signal termina- tion of translation. See amber codon, opal codon. ochre mutation one of a group of mutations re- sulting in abnormally short polypeptide chains. Be- cause of a base substitution, a codon specifying an amino acid is converted to UAA, which signals chain termination. UAA appears to be the codon primarily used for chain termination in E. coli. See amber mu- tation, nonsense mutation. ochre suppressor any mutant gene coding for a mutant tRNA whose anticodon can respond to the UAA stop codon by the insertion of an amino acid.

See amber suppressor. Ockham a variant spelling of Occam. See Occam’s razor. octad a fungal ascus containing eight linear asco- spores; produced in some ascomycete species when the tetrad of meiospores undergoes a mitotic divi- sion following meiosis. See ordered tetrad. octopine See opine. ocular albinism a hereditary eye disease of humans that occurs in two forms, one inherited as an autoso- mal recessive and the other X-linked. The X-linked condition is the most common form of ocular albi- nism. In males, the prevalence of the disease is 1/ 50,000. The normal gene is at Xp22.3, and it encodes a protein that contains 424 amino acids. This is lo- cated in the membranes of melanosomes but is not a tyrosinase. Males show a reduced pigmentation of the retina (q.v.) and iris but not of the hair and skin. They are extremely sensitive to light and have reduced vi- sual acuity.

Patients with ocular albinism or oculocuta- neous albinism have a misrouting of the optic tracts, which results in a loss of stereoscopic vision. In hetero- zygous females, retinas show a mosaic pattern of pig- ment distribution due to random inactivation of the X chromosomes during the early development of the eye. See albinism, dosage compensation. OD optical density. See Beer-Lambert law. OD260 unit one absorbance (OD260) unit is that amount of material per ml of solution that produces an absorbance of 1 in a 1.0-centimeter light path at a wavelength of 260 nanometers. See absorbance. odorant any one of a wide variety of molecules that produces an odor and that binds with an odor- ant receptor (q.v.) to trigger a cascade of signals that

eventually allows the brain to recognize the particu- lar odor. Usually, several different odorants combine to produce a single odor. Each type of odorant can bind to several different odorant receptors and each receptor to several different odorants (with varying affinity), thus forming the basis for the wide diver- sity in odors that can be detected by the brain. The term odorant usually precludes pheromones, which elicit endocrine or behavioral responses, and which are detected by two distinct families of odorant re- ceptors specific to cells residing in a distinct region of the olfactory system.

See pheromone. odorant receptor a protein molecule that resides on the cell surface of an olfactory receptor neuron (q.v.) and which binds an odorant (q.v.). Odorant receptors are encoded by distinct families of odorant receptor genes, which encode G protein-coupled re- ceptors (GPCRs) (q.v.). The binding of an odorant to an odorant receptor (q.v.) causes a structural change in the latter, which leads to the activation of the G protein (q.v.) attached to it. The G protein then causes the activation of different intracellular signaling events, which result in the production of an electrical impulse that is transmitted to specific regions of the brain via nerve processes. Here the information from different types of odorant recep- tors is sorted out, and specific odors are perceived by the organism.

Also called olfactory receptor. See Appendix C, 1991, Buck and Axel; cellular signal transduction, G protein-coupled receptors, G proteins, odorant receptor gene. odorant receptor gene any one of a family of genes expressed in an olfactory sensory organ and encoding an odorant receptor (q.v.). C. elegans and several mammalian species have large odorant recep- tor gene families, which encode approximately 1,000 different genes. This corresponds to approxi- mately 1%-5% of the genes in the euchromatic (q.v.) genomes of these organisms. By comparison, Drosophila has a family of only 60 such genes (i.e., 0.5% of the total genes). In each organism odorant receptor genes encode a family of related proteins, which have in common the fact that they are G pro- tein-coupled receptors (q.v.).

Members of a family differ from one another in the extent of sequence similarity. Between organisms, odorant receptor gene families differ vastly in size and sequence com- position. Perception of olfaction therefore involves gene families that meet the unique needs of each species. Two additional gene families, one with ap- proximately 35 and another with 150 members, have been identified in some mammals, which are thought to encode pheromone receptors.

See Appen- dix C, 1991, Buck and Axel. Oenothera lamarckiana the evening primrose. During meiosis, plants of this and related species, such as O. grandiflora, have their chromosomes ar- ranged in rings rather than pairs. The evolution of this atypical cytogenetic behavior, the result of the accumulation of reciprocal translocations, has been extensively studied. See Appendix A, Angiospermae, Dicotyledoneae, Myrtales, Appendix C, 1901, de Vries; 1930, Cleland and Blakeslee; Renner complex. Ohno hypothesis the proposal advanced by S.

Ohno that the unique regulatory features of the X chromosomes dictate the evolutionary conservation of the primordial X-linkage group among mammals. Any translocation between the X chromosome and an autosome would disturb the dosage compensa- tion mechanism, and therefore offspring bearing such a translocation would be eliminated. Therefore, if any gene is found to be sex-linked in a given spe- cies, such as Homo sapiens, it is likely to be X-linked in all other mammals. See Appendix C, 1967, Ohno; dosage compensation. oil-immersion objective the objective lens system used for highest resolution with the light micro- scope. The space between the coverslip over the ob- ject to be examined and the lens is filled with a drop of oil of the same refractive index as the glass.

Okazaki fragments See replication of DNA. olfactory epithelium in mammals, the tissue lo- cated in the nasal cavity that contains olfactory re- ceptor neurons (q.v.), which detect and transmit ol- factory signals to the brain. In addition to these neurons, the olfactory epithelium also contains sup- porting cells and stem cells that divide regularly to replace olfactory neurons that die. The correspond- ing olfactory sensory organs in Drosophila are in the antenna and the maxillary palp, where the fly’s ol- factory receptor neurons are located. olfactory receptor neurons nerve cells that reside in the olfactory sensory organs and are the primary cells for the detection and transduction of olfactory


signals to the brain. In mammalian species, the den- drites of these neurons contain cilia (q.v.), on which reside odorant receptors. The binding of an odorant (q.v.) to a receptor causes intracellular biochemical changes which result in the generation of an electri- cal impulse that travels along the axon (q.v.) of the olfactory neuron to the olfactory bulb, where the signal is transferred to additional nerve cells for transport to the brain. In rodents and in Drosophila, each olfactory receptor neuron is highly specific, in that it expresses only one odorant receptor gene (q.v.), but multiple neurons collectively produce a pattern of neuronal activation, which is interpreted by the brain into distinct odors that are perceived by the organism. Also called olfactory sensory neuron. See anosmia, dendrite, odorant receptor. Oligocene the third epoch in the Tertiary period. Old World monkeys and apes evolved. Further con- tinental drift (q.v.) left South America separated from North America, and Australia separated from Antarctica to which it was fused previously.

See geo- logic time divisions, Indrichotherium. oligo dA (oligo dT) a homopolymer chain of deoxyriboadenylate (or deoxyribothymidylate) sub- units of unspecified length, but generally 100-400 residues. oligogene a gene producing a pronounced pheno- typic effect as opposed to a polygene (q.v.), which has an individually small effect. oligomer a molecule made up of a relatively few monomeric subunits. oligonucleotide a linear sequence of up to 20 nu- cleotides joined by phosphodiester bonds. See allele- specific oligonucleotide testing, polynucleotide. oligonucleotide-directed mutagenesis a tech- nique that allows a specific mutation to be inserted in a gene at a selected site. An olignucleotide se- quence complementary to the segment of interest, but containing an alteration at a selected site, is chemically synthesized. Next this is hybridized to a complementary wild-type target gene contained in a single-stranded phage such as M13.

The hybridized oligonucleotide fragment is then used as a primer by DNA polymerase I, which extends the molecule while taking instructions from the wild-type com- plementary strand. The result is a double helix con- taining a mutant and a wild-type strand. The hetero- duplex is then used to transform bacterial cells. From these colonies, strains that contain the mutant homoduplexes can be recovered and propagated. This procedure is also called site-specified mutagene- sis. See Appendix C, 1978, Hutchison et al. oligonucleotide fingerprinting (OFP) any tech- nique which produces a “fingerprint” consisting of a distinct oligonucleotide (q.v.) pattern representing nucleic acids from a particular source. In the sim- plest example, the genomes of different strains of an organism may be compared by enzymatic digestion of their genomic DNA (or RNA) to generate oligo- nucleotide fragments, which, when resolved on a gel by electrophoresis (q.v.), produce banding patterns representing fingerprints unique to each strain.

A relatively more complex technique uses fingerprints generated by hybridization of oligonucleotides to cDNA (q.v.) or genomic libraries to characterize ex- pressed genes at the genome-wide scale, to compare different cDNA libraries, and to select shotgun clones for sequencing. In this method, hundreds of labeled, synthetic oligonucleotides of known se- quences, usually 6-10 bp in length, are hybridized to PCR-amplified cDNA or genomic library se- quences that have been spotted on parallel DNA mi- croarrays. Each oligonucleotide probe (q.v.) is used in a separate hybridization experiment. The extent of hybridization across microarray filters is recorded by a laser scanner and image analysis software. This produces a unique fingerprint of each arrayed DNA fragment, based on the extent of similarity to each oligonucleotide sequence. Using this approach, hun- dreds of thousands of individual library fragments can be comparatively examined. cDNAs with simi- lar fingerprints are grouped into clusters, and this provides information about the number of expressed genes and their relative expression levels. Individual fingerprints are used for database searches for se- quence matches to known genes or to identify new genes.

DNA fragments having maximum dissimilar- ity in their fingerprints (i.e., minimum sequence overlap) are used for genomic sequencing with re- duced redundancy. See DNA fingerprint technique, DNA microarray technology, genomic library, poly- merase chain reaction. oligopyrene sperm See sperm polymorphism. oligosaccharide a polymer made up of a few (2- 10) monosaccharide units. Oligosaccharides are attached to many secreted proteins, such as immu- noglobulins and clotting factors. They are also found on the extracellular face of proteins that extend through cell membranes. The lipids of the red cell plasma membrane contain oligosaccharides that specify blood types. Such complex carbohydrates re- quire a different enzyme for each step in their syn-

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