are detected, and signals originating from above and below the focal plane are filtered out. The name con- focal indicates that the illumination, specimen, and detector all have the same focus. The specimen vol- ume assayed can be as small as 0.25 nanometers in diameter and 0.5 nanometers in depth. The two-di- mensional image is built up by collecting and inte- grating signals as the light beam moves horizontally and vertically across the specimen, and the image el- ements are converted into a video signal for display on a computer screen. Subsequently, stacks of opti- cal sections can be reconstructed to produce a view of the specimen in three dimensions.
confusing coloration a form of protective color- ation that tends to confuse the predator by having a different appearance according to whether its pos- sessor is at rest or in motion. congenic strains strains that differ from one an- other only with respect to a small chromosomal seg- ment. Several congenic mouse strains differ only in the major or minor histocompatibility loci they con- tain. Contrast with coisogenic. congenital existing at birth.
Congenital defects may or may not be of genetic origin. congression See chromosome congression. congruence in cladistics, congruent characters are shared features whose distribution among organisms fully corresponds to that in the same cladistic group- ing. The most likely cladogram is the one that pro- vides the maximum congruence between all of the characters involved.
conidium an asexual haploid spore borne on an ae- rial hypha. In Neurospora two types of conidia are found: oval macroconidia, which are multinucleate, and microconidia, which are smaller, spherical, and uninucleate. When incubated upon suitable me- dium, a conidium will germinate and form a new mycelium. conifers plants characterized by needle-shaped leaves and which bear their microsporangia and megasporangia in cones. The class contains 50 gen- era and 550 species, and among them are the largest (the Pacific coast redwood, Sequoia gigantia) and the longest-lived species.
See Appendix A, Trachaeophyta, Pteropsida, Gymnospermae, Coniferophyta. conjugation a temporary union of two single- celled organisms or hyphae with at least one of them receiving genetic material from the other. 1. In bac- teria, the exchange is unidirectional with the “male”
cell extruding all or a portion of one of its chromo- somes into the recipient “female” (see F factor, pilus). 2. In Paramecium, as shown in the illustration on page 98, entire nuclei are exchanged. See nuclear di- morphism. 3. In fungi, conjugation also occurs be- tween hyphae of opposite mating type to produce heterokaryons (q.v.). conjugation tube See pilus. conjugon a genetic element essential for bacterial conjugation. See fertility factor.
conodonts 1. the earliest known fossil elements that contain dentine, ranging in age from 510 to 220 million years old; thought to have functioned as teeth. 2. the earliest known vertebrates, intermedi- ate between the more primitive hagfish/lamprey lin- eages and the more complex armored jawless fishes (ostracoderms). Mineralization apparently began in the mouths of conodonts, rather than in the skin of ostracoderms. consanguinity genetic relationship. Consanguine- ous individuals have at least one common ancestor in the preceding few generations.
See isonymous marriage. consecutive sexuality the phenomenon in which most individuals of a species experience a functional male phase when young, and later change through a transitional stage to a functional female phase. A situation common in some molluscs. consensus sequence synonymous with canonical sequence (q.v.). conservative recombination breakage and re- union of preexisting strands of DNA in the absence of DNA synthesis.
conservative replication an obsolete model of DNA replication in which both old complementary polynucleotides are retained in one sibling cell, while the other gets the two newly synthesized strands. Compare with semiconservative replication. conservative substitution replacement of an amino acid in a polypeptide by one with similar characteristics; such substitutions are not likely to change the shape of the polypeptide chain, e.g., sub- stituting one hydrophobic amino acid for another. conserved sequence a sequence of nucleotides in genetic material or of amino acids in a polypeptide chain that either has not changed or that has changed only slightly during an evolutionary period of time. Conserved sequences are thought to gener- ally regulate vital functions and therefore have been selectively preserved during evolution.
Nuclear changes that accompany conjugation in Paramecium aurelia. (1) Two parental animals, each with one macronucleus and two diploid micronuclei. (2) Formation of eight haploid nuclei from the micronuclei of each conjugant. (3) Seven nuclei in each conjugant disappear, the remaining nucleus resides in the paroral cone; the macronucleus breaks up into fragments. (4-7) The nuclei in the paroral cones divide mitotically, forming “male” and “female” gamete nuclei. The female nuclei pass into the interior of the parental animals, while the male nuclei are transferred to the partners.
Male and female haploid nuclei fuse. (8) Each fusion nucleus divides twice mitotically. (9) Two of the four nuclei so formed differentiate into macronuclear anlagen (white circles) while the other two become micronuclei. (10-12) Each micronucleus divides, and transverse fission of the exconjugants produces four animals (two of which are shown in 11). Fragments of the old macronucleus are gradually lost. In (12) transverse fission begins in the animals seen previously in (11).
conspecific belonging to the same species. constant region See immunoglobulin. constitutive enzyme an enzyme that is always produced irrespective of environmental conditions. See Appendix C, 1937, Karstrom. constitutive gene a gene whose activity depends only on the efficiency of its promoter in binding RNA polymerase.
constitutive gene expression See gene expres- sion. constitutive heterochromatin See heterochro- matin. constitutive mutation a mutation that results in an increased constitutive synthesis by a bacterium of several functionally related, inducible enzymes. Such a mutation either modifies an operator gene so
that the repressor cannot combine with it or modi- fies the regulator gene so that the repressor is not formed. See regulator gene. constriction an unspiralized region of a metaphase chromosome. Kinetochores and nucleolus organizers are located in such regions. contact inhibition the cessation of cell movement on contact with another cell. It is often observed when freely growing cells, tissue cultured on a petri plate, come into physical contact with each other.
Cancer cells lose this property and tend to pile up in tissue culture to form multilayers called foci. containment in microbiology, measures taken to diminish or prevent the infection of laboratory workers by the products of recombinant DNA tech- nology and the escape of such products from the laboratory. Biological containment is accomplished by using genetically altered bacteria, phages, and plas- mids that are unable to carry out certain essential functions (e.g., growth, DNA replication, DNA transfer, infection, and propagation) except under specific laboratory conditions.
Physical containment is accomplished by design and use of special facilities and laboratory procedures such as limited access, safety hoods, aerosol control, protective clothing, pi- peting aids, etc. contig an abbreviation for contiguous. When a col- lection of cloned DNA fragments can be arranged so that they overlap to provide gap-free coverage of a chromosome, one has assembled the fragments (contigs) into a contig map.
See genomic library, reads, scaffold, shotgun sequencing. contiguous gene syndrome a situation where a patient suffers from two or more genetic diseases si- multaneously because of the simultaneous deletion of neighboring genes. A classic example is an unfor- tunate boy who had a deletion in Xp 21. This re- moved a contiguous set of genes and resulted in a complex array of syndromes, including Duchenne muscular dystrophy (DMD) and retinitus pigmento- sum (both of which see). DNA from this patient pro- vided investigators with a means to clone the DMD gene and subsequently to isolate dystrophin.
See Ap- pendix C, 1987, Hoffman, Brown, and Kunkel. continental drift the concept that the world’s con- tinents once formed a part of a single mass and have since drifted into their present positions. The mod- ern concept of plate tectonics (q.v.) has refined the theory of continental drift by placing the continents on larger sections of the earth’s crust plates) that are in motion.
Continental drift began in the Proterozoic era; the continents collided to form a giant land mass named Pangaea in the late Permian period, and redispersed in the Mesozoic era. See Appendix C, 1912, Wegener; 1927, du Toit; geological time divisions, sea floor spreading. continental island an island assumed to have been once connected to a neighboring continent. Contrast with oceanic island. continuous distribution a collection of data that yield a continuous spectrum of values. For example, measurements such as height of plant or weight of fruit, carried out one or more decimal places.
See discontinuous distribution. continuous fibers the microtubules that connect the two poles of the mitotic apparatus, as distinct from traction fibers and astral fibers. See mitotic ap- paratus. continuous variation the phenotypic variation ex- hibited by quantitative traits that vary by impercep- tible degrees from one extreme to another. In hu- man populations, phenotypes like body weight, height, and intelligence show continuous variation. See discontinuous variation, quantitative inberitance. contractile ring a transitory organelle that, during late anaphase and telophase, assumes the form of a continuous equatorial annulus beneath the plasma membrane of the cleavage furrow. The ring is com- posed of an array of actin microfilaments aligned cir- cumferentially along the equator of the cell.
An in- teraction between cytoplasmic myosin and these actin molecules causes them to slide past one an- other, closing down the contractile ring, and produc- ing the cleavage furrow. contractile vacuole an organelle found in fresh water protozoans, like Paramecium. Contractile vac- uoles are pumping structures that fill and then con- tract to expel excess water from the cell. control a standard of comparison; a test or experi- ment established as a check of other experiments, performed by maintaining identical conditions ex- cept for the one varied factor, whose causal signifi- cance can thus be inferred. controlled pollination a common practice in plant hybridization of bagging the pistillate flowers to protect them from undesired pollen.
When the pistillate flowers are in a receptive condition, they are dusted with pollen of a specified type. controlling elements a class of genetic elements that renders target genes unstably hypermutable, as in the Dissociation-Activator system of corn (q.v.). They include receptors and regulators. The receptor