chorion 1. the insect egg shell. 2. see amnion. chorionic appendages the anterior, dorsal projec- tions of the Drosophila egg shell that serve as breath- ing tubes when the egg is submerged. chorionic gonadotropin duced by the placenta that continues to stimulate the production of progesterone by the corpus lu- teum and thus maintains the uterine wall in a glan- dular condition. This hormone is the one for which most pregnancy tests assay. For humans, it is abbre- viated HCG.
chorionic somatomammotropin See human growth hormone. chorionic villi sampling the harvesting of chori- onic cells by introducing a catheter through the va- gina and into the uterus until it touches the chorion, which surrounds the human embryo. The chorionic cells withdrawn can be used to detect enzymatic and karyotypic defects, since they have the same geno- type as embryonic cells. The sampling of chorionic villi can be performed between 10 and 11 weeks fol- lowing the last menstrual period, so this technique has the advantage over amniocentesis (q.v.) of allow- ing fetal disorders to be detected earlier. Thus, when necessary, the pregnancy can be terminated at an earlier stage with less risk to the mother.
See in- formed consent, genetic counseling, prenatal genetic testing. Christmas disease the B form of hemophilia (q.v.), named after a British patient, Stephen Christ- mas. In 1952, when only 5 years old, he was shown to be suffering from a type of hemophilia different from the classic form. He could not produce clotting factor 9. This is sometimes called the Christmas fac- tor in his honor. chromatid conversion a form of gene conversion (q.v.) rendered evident by identical sister-spore pairs in a fungal octad that exhibits a non-Mendelian ra- tio. For example, if an ordered octad of ascospores from a cross of + × m = (++) (++) (++) (mm), one chromatid of the m parental chromosome appears to have been converted to +.
In half-chromatid conver- sion, an octad of (++) (++) (+m) (mm) indicates that one chromatid of the m parental chromosome was “half-converted.” See ordered tetrad. chromatid interference a deviation from the ex- pected 1 : 2 : 1 ratio for the frequencies of 2-, 3-, and 4-strand double crossovers signaling a nonrandom participation of the chromatids of a tetrad in succes- sive crossovers.
chromatids the two daughter strands of a dupli- cated chromosome that are joined by a single centro- mere. Upon the division of the centromere, the sis- ter chromatids become separate chromosomes. See Appendix C, Flemming; meiosis, mitosis. chromatin the complex of nucleic acids (DNA and RNA) and proteins (histones and nonhistones) comprising eukaryotic chromosomes. See Appendix C, 1879, Flemming; 1976, Finch and Klug. chromatin diminution the elimination during em- bryogenesis of certain chromosomes or chromo- somal segments from the cells that form the somatic tissues. Germ cells, however, retain these chromo- somes. The process occurs in some ciliates, nema- todes, copepods, and insects.
The discarded DNA often contains highly repetitive sequences and some- times genes encoding ribosomal proteins. See Appen- dix C, 1887, Boveri; Ascarididae, C value paradox, genomic equivalence, Parascaris equorum. chromatin fibers fibers of 30 nm diameter seen when interphase nuclei are lysed and their contents viewed under an electron microscope. The fibers are composed of solenoids of nucleosomes held together by hexamers of histone 1 molecules. See solenoid structures.
chromatin immunoprecipitation a technique for identifying a specific DNA sequence in the genome (q.v.) to which a particular protein binds in vivo (q.v.). In this procedure, the protein in question is cross-linked to DNA with formaldehyde in vivo. The DNA is then extracted from cells and sheared into small fragments. Antibodies against the bound pro- tein are then used to isolate the protein-DNA com- plex, the protein is released, and the polymerase chain reaction (PCR) (q.v.) is used to amplify the DNA sequence to which the protein was bound. The amplified DNA can then be identified by se- quence analysis.
chromatin-negative an individual (normally a male) whose cell nuclei lack sex chromatin. chromatin-positive an individual (normally a fe- male) whose cell nuclei contain sex chromatin. See Barr body, sex chromatin. chromatograph the record produced by chroma- tography (q.v.). Generally applied to a filter paper sheet containing a grouping of spots that represent the compounds separated. chromatography a technique used for separating and identifying the components from mixtures of molecules having similar chemical and physical
properties. The population of different molecules is dissolved in an organic solvent miscible in water, and the solution is allowed to migrate through a station- ary phase. Since the molecules migrate at slightly different rates, they are eventually separated. In pa- per chromatography, filter paper serves as the station- ary phase.
In column chromatography (q.v.), the sta- tionary phase is packed into a cylinder. In thin layer chromatography, the stationary phase is a thin layer of absorbent silica gel or alumina spread on a flat glass plate. See also affinity chromatography, counter- acting chromatographic electrophoresis, gas chroma- tography, ion exchange column. See Appendix C, 1941, Martin and Synge.
chromatophore submicroscopic particles isolated from photosynthetic bacteria that contain the pho- tosynthetic pigments. Compare with chromophore. chromatosome a DNA protein complex consist- ing of a nucleosome (q.v.), the linker DNA segment, and its H1 histone. See histones, nucleosome.
chromatrope See metachromasy. Chromista the name proposed by T. Cavalier- Smith for a kingdom to contain those eukaryotic species that show the following specific ultrastruc- tural similarities. They all posses, at some time dur- ing their life cycles, cells containing specialized un- dulopodia that have regularly spaced tufts of fibers extending from them. Such an undulopodium is of- ten called a hairy or tinsel flagellum. The lateral fi- bers are called mastigonemes, and they are tubules that are rooted in the flagellar axonemes.
The other organelle that characterizes the Chromista is the chloroplast ER. This refers to a chloroplast that is sur- rounded by four membranes, the last of which is continuous with the endoplasmic reticulum (q.v.). Species belonging to the phyla Chrysophyta, Crypt- ophyta, Xanthophyta, Eustimatophyta, Bacillano- phyta, Phaeophyta, Labyrinthulomycota, Hypochy- tridiomycota, and Oomycota fall into this group.
See Appendix A, Protoctista; Chrysophyta. chromocenter a central amorphous mass found in the nuclei of larval salivary gland cells of Drosophila. Since the chromocenter results from the fusion of the heterochromatic elements that surround the centromeres of all the chromosomes in each nucleus, the distal, banded, euchromatic arms of the somati- cally paired, polytene chromosomes radiate from the chromocenter. In mitotic cells the pericentromeric, heterochromatic elements are rich in highly repeti- tive, DNA sequences. These fail to replicate during polytenization (q.v.) and are therefore underrepre- sented in the chromocenter.
See Drosophila salivary gland chromosomes. chromomere one of the serially aligned beads or granules of a eukaryotic chromosome, resulting from local coiling of a continuous DNA thread; best seen when most of the rest of the chromosome is rela- tively uncoiled as in the leptotene and zygotene stages of meiosis (q.v.). In polytene chromosomes (q.v.), the chromomeres lie in register and give the chromosome its banded appearance. chromonema (plural chromonemata) the chro- mosome thread. chromoneme the DNA thread of bacteria and their viruses. chromophore
1. the part of a dye that gives the molecule its color.
2. any receptor molecule that ab- sorbs light (thereby producing a color) and is usually complexed with a protein.
For example, phyto- chromes (q.v.) consist of a linear tetrapyrole mole- cule that functions as a chromophore and a protein portion that anchors the chromophore to the inside surface of the plasma membrane and communicates a signal to one or more signal pathways. The same is true for retinal (q.v.) and its opsin (q.v.).
Compare with chromatophore. chromoplast a carotenoid-containing plastid that colors ripe fruits and flowers. chromosomal aberration an abnormal chromo- somal complement resulting from the loss, duplica- tion, or rearrangement of genetic material. Intra- chromosomal or homosomal aberrations involve changes that occur in but one chromosome. Such aberrations include deficiencies and duplications that result in a reduction or increase in the number of loci borne by the chromosome. Inversions and shifts involve changes in the arrangement of the loci, but not in their number.
In the case of an inversion a chromo- somal segment has been deleted, turned through 180°, and reinserted at the same position on a chro- mosome, with the result that the gene sequence for the segment is reversed with respect to that of the rest of the chromosome.
In the case of shift a chro- mosomal segment has been removed from its normal position and inserted (in the normal or reversed se- quence) into another region of the same chromo- some. Interchromosomal or heterosomal aberrations arise from situations in which nonhomologous chro- mosomes are broken, and interchange occurs between the resulting fragments, producing a translocation. See radiation-induced chromosomal aberrations.
chromosomal mutation See chromosomal aberra- tion. chromosomal polymorphism the existence with- in a population of two or more different structural arrangements of chromosomal material (e.g., inver- sions, translocations, duplications, etc.). chromosomal puff a localized swelling of a spe- cific region of a polytene chromosome due to local- ized synthesis of DNA or RNA. Extremely large RNA puffs are called Balbiani rings (q.v.). See Ap- pendix C, 1952, Beerman; Clever and Karlson; 1961, Beerman; 1980, Grone- meyer and Pongs; heat-shock puffs, insulator DNAs, Rhynchosciara, Sciara.
chromosomal RNA ribonucleic acid molecules as- sociated with chromosomes during either division (e.g., primer, q.v.) or interphase (e.g., incomplete transcripts). chromosomal sterility sterility from the lack of homology between the parental chromosomes in a hybrid. chromosomal tubules microtubules of the spindle apparatus that originate at kinetochores of centro- meres. The chromosomal tubules interpenetrate on the spindle with polar tubules (q.v.) and are hypoth- esized to slide by one another during anaphase as a consequence of making and breaking of cross bridges between them.
chromosome 1. in prokaryotes, the circular DNA molecule containing the entire set of genetic instruc- tions essential for life of the cell. See genophore. 2. in the eukaryotic nucleus, one of the threadlike structures consisting of chromatin (q.v.) and carrying genetic information arranged in a linear sequence.
See Appendix C, 1883, Roux; 1888, Waldeyer. chromosome arms the two major segments of a chromosome, whose length is determined by the po- sition of the centromere. See acrocentric, metacen- tric, submetacentric, telocentric. chromosome banding techniques there are four popular methods for staining human chromosomes. To produce G banding, chromosomes are usually treated with trypsin and then stained with Giemsa. Most euchromatin stains lightly, and most hetero- chromatin stains darkly under these conditions. C bands are produced by treating chromosomes with alkali and controlling the hydrolysis in a buffered salt solution. C banding is particularly useful for staining and highlighting centromeres and polymorphic bands (especially those of meiotic chromosomes). With Q banding, chromosomes are stained with a fluorochrome dye, usually quinacrine mustard or quinacrine dihydrochloride, and are viewed under ultraviolet light.
The bright bands correspond to the dark G bands (with the exception of some of the polymorphic bands). Q banding is especially useful for identifying the Y chromosome and polymor- phisms that are not easily demonstrated by the G- banding procedure. R bands are produced by treat- ing chromosomes with heat in a phosphate buffer. They can then be stained with Giemsa to produce a pattern that is the reverse (hence the R in the term) of G bands, thereby allowing the evaluation of ter- minal bands that are light after G banding. Alterna- tively, chromosomes can be heated in buffer and then stained with acridine orange. When viewed un- der ultraviolet light, the bands appear in shades of red, orange, yellow, and green.
They can also be photographed in color, but printed in black and white to reveal more distinctive R bands. Q-dark, R- positive bands have a low AT:CG ratio and are rich in SINE repeats and Alu sequences. Q-bright, R- negative bands have a high AT:CG ratio and are rich in LINE repeats. See Appendix C, 1970, Caspersson et al.; 1971, O’Riordan et al.; repetitious DNA. chromosome bridge a bridge formed between the separating groups of anaphase chromosomes because the two centromeres of a dicentric chromosome are being drawn to opposite poles (see illustration on page 82). Such bridges may form as the result of sin- gle- or three-strand double-exchanges within the re- verse loop of a paracentric inversion heterozygote (see inversion).
They may also arise as radiation-in- duced chromosome aberrations (q.v.). Such chro- mosome bridges are always accompanied by acentric chromosome fragments. See breakage-fusion-bridge cycle. chromosome condensation the process whereby eukaryotic chromosomes become shorter and thick- er during prophase as a consequence of coiling and supercoiling of chromatic strands. See Appendix C, 1970, Johnson and Rao; nucleosome, solenoid struc- ture. chromosome congression the movement of chro- mosomes to the spindle equator during mitosis. chromosome diminution or elimination See chromatin diminution. chromosome jumping See chromosome walking. chromosome loss failure of a chromosome to be included in a daughter nucleus during cell division. See anaphase lag.