gamma). The host bacterium regulates the expres- sion of the gene. No syntheses of the toxin occur until the intracellular level of iron falls below a cer- tain threshold. See Appendix C, 1888, Roux and Yer- sin; 1971, Freeman; prophage-mediated conversion. diploblastic having a body made of two cellular layers only (ectoderm and endoderm), as the coelen- terates. diplochromosome a chromosome arising from an abnormal duplication in which the centromere fails to divide and the daughter chromosomes fail to move apart.
The resulting chromosome contains four chromatids. Diplococcus pneumoniae the former designation given for Streptococcus pneumoniae, the cause of bac- terial pneumonia. See Streptococcus. diplo-haplont an organism (such as an embryo- phyte) in which the products of meiosis form hap- loid gametophytes that produce gametes. Fertili- zation generates a diploid sporophyte in which meiosis takes place. Thus, diploid and haploid gener- ations alternate.
Contrast with diplont, haplont. diploid or diploidy referring to the situation or state in the life cycle where a cell or organism has two sets of chromosomes: one from the mother and one from the father. Diploidy results from the fusion of the haploid egg nucleus and a haploid sperm nu- cleus. See autosome, C value, merozygote, N value, polyploidy, sex chromosome, syngamy. diplonema See meiosis. diplont an organism (such as any multicellular ani- mal) characterized by a life cycle in which the prod- ucts of meiosis function as gametes. There is no hap- loid multicellular stage as in a diplo-haplont and haplont (q.v.).
diplophase the diploid phase of the life cycle be- tween the formation of the zygote and the meiosis. diplospory a type of apomixis in plants in which a diploid gametophyte is formed after mitotic divi- sions of the spore-forming cells. diplotene See meiosis. Dipodomys ordii a species of jumping rodent found in arid and desert regions of North America. This kangaroo rat is famous for the large amount of repetitious DNA (q.v.) in its genome. dipole a molecule carrying charges of opposite sign at opposite poles. Diptera an insect order containing midges, mos- quitoes, and flies.
See Appendix A, Animalia, Arthro- poda. directional selection selection resulting in a shift in the population mean in the direction desired by the breeder or in the direction of greater adaptation by nature. For example, the breeder might select for a number of generations seeds from only the longest ear of corn in the population. See disruptive selec- tion. direct repeats identical or closely related DNA se- quences present in two or more copies in the same orientation in the same molecule, although not nec- essarily adjacent. DIS Drosophila Information Service (q.v.). discoidal cleavage cleavage occurring at the sur- face of an enormous yolk mass.
discontinuous distribution a collection of data re- corded as whole numbers, and thus not yielding a continuous spectrum of values; e.g., the number of leaves per plant in a population of plants. See contin- uous distribution. discontinuous replication See replication of DNA. discontinuous variations variations that fall into two or more non-overlapping classes. discordant twins are said to be discordant with re- spect to a trait if one shows the trait and the other does not. disequilibrium See gametic disequilibrium, linkage disequilibrium. disjunction the moving apart of chromosomes during anaphase of mitotic or meiotic divisions.
disomy the presence in a cell of a pair of chromo- somes of a specified kind. The normal condition for a diploid cell is heterodisomy, where one member of each autosomal pair is of maternal and the other of paternal origin.
If both chromosomes are inherited from the same parent, the term uniparental disomy is used. There are instances reported where a child suffering from cystic fibrosis (q.v.) has two copies of chromosome 7, both containing the CF gene from a heterozygous mother. Here it is assumed that a diso- mic egg produced by nondisjunction (q.v.) was fer- tilized by a normal sperm to produce a trisomic, but the paternal chromosome 7 was lost early in devel- opment and only the disomic, diploid cell line sur- vived. dispersal mechanism any means by which a spe- cies is aided in extending its range. For example, sticky seeds can cling to animals and be transported by them to new regions.
dispersive replication an obsolete model of DNA replication in which parental and newly synthesized daughter molecules are interspersed in an essentially random fashion. disruptive selection the selection of divergent phenotypic extremes in a population until, after sev- eral generations of selection, two discontinuous strains are obtained. For example, the breeder might select for a number of generations seeds from the longest and the shortest ears of corn in a population. See directional selection. disseminule a plant part that gives rise to a new plant. Dissociation-Activator system See Activator-Dis- sociation system.
distal situated away from the place of attachment. In the case of a chromosome, the part farthest from the centromere. distributive pairing the pairing of chromosomes at metaphase I of meiosis that leads to their proper distribution to daughter cells. Synaptonemal com- plexes play no role in this type of chromosomal asso- ciation. distylic species a plant species composed of two types of individuals each characterized by a different flower morphology. disulfide linkage the sulfur-to-sulfur bonding of adjacent cysteine residues in or between protein molecules. diurnal 1. pertaining to the daytime. 2. recurring in the period of a day; daily.
divergence in molecular biology, the percent dif- ference between nucleotide sequences of two related DNA segments or between amino acid sequences of the two related polypeptide chains. divergence node the branching point in an evolu- tionary tree. The place where two lineages diverge from a common ancestor. See cladogram, node. divergent transcription the transcriptional orien- tation of different DNA segments in opposite direc- tions from a central region. diversity in ecology, the number of species or other taxa in a particular ecological unit. Division See Appendix A: Classification. dizygotic twins See twins.
D loop 1. a displacement loop formed early in the replication of duplex DNA (either circular or linear) consisting of a single, unreplicated, parental strand on one side, and a double-stranded branch (com- posed of one parental strand paired with the leading strand, q.v.) on the other side. Because the leading strand displaces the unreplicated parental strand, the replication “bubble” or “eye” is called a displacement or D loop. 2. a region of vertebrate mtDNA that is noncoding but contains promoters and an origin for the replication of mtDNA. Shortly after replication is initiated, a temporary arrest in DNA elongation cre- ates this displacement loop.
The D loop is a bubble in which one strand of the control region has been copied and the other displaced. This D loop has been used as a target region for sequence comparisons when erecting phylogenetic trees. See Neandertal. DNA deoxyribonucleic acid (q.v.). Also see insula- tor DNA, promiscuous DNA. DNA adduct See adduct. DNA-agar technique a technique for testing the degree of homology between nucleic acid molecules from different sources by allowing fragments of radioactive nucleic acid from one source to react with nonlabeled nucleic acids from another source trapped in an agar gel. This procedure binds to the gel radioactive polynucleotide fragments that are complementary to those trapped in the agar. See Ap- pendix C, 1963, McCarty and Bolton; hybrid duplex molecule.
DNA amplification See amplicon, polymerase chain reaction. DNAase symbol for deoxyribonuclease (q.v.). DNAase footprinting a technique for determining the sequence of a DNA segment to which a DNA- binding protein binds. In this technique, a double- stranded DNA fragment is radioactively labelled at the 5′ end, partially digested with DNAase (q.v.) in the presence and absence of the binding protein, and the resulting fragments compared by electrophoresis (q.v.) and autoradiography (q.v.) on a gel that also runs in parallel the reaction products of a sequencing reaction performed on the unprotected sample of DNA. This produces an autoradiograph with ladders of oligonucleotides of varying lengths, increasing in single-nucleotide increments.
The DNA region cov- ered by the binding protein is protected from DNAase degradation and appears as a gap, or a foot- print, that is missing from the sample lacking the protective protein. The footprint-containing ladder aligned with DNA sequencing ladders then identifies the exact sequence of bases in the footprint. See di- methyl sulfate protection, DNAase protection, DNA sequencing techniques.
DNAase protection the shielding of DNA se- quences bound by a protein from degradation by an endonuclease. DNAase protection is used to charac- terize a DNA segment that binds a specific protein. In this approach, the protein in question is bound to the DNA, DNAase added to degrade the surround- ing unprotected sequences, and the remaining bound DNA isolated and analyzed.
See dimethyl sul- fate protection, DNAase footprinting. DNA-binding motifs sites on proteins which facil- itate their binding to DNA. See DNA Grooves, DNA methylation, helix-turn-helix motif, homeobox, leu- cine zipper, POU genes, T box genes, zinc finger pro- teins. DNA chip See DNA microarray technology.
DNA clock hypothesis the postulation that, when averaged across the entire genome of a species, the rate of nucleotide substitutions in DNA remains constant. Hence the degree of divergence in nucleo- tide sequences between two species can be used to estimate their divergence node (q.v.).
See Appendix C, 1983, Kimura and Ohta. DNA clone a DNA segment that has heen inserted via a viral or plasmid vector into a host cell with the following consequences: the segment has replicated along with the vector to form many copies per cell, the cells have mutiplied into a clone, and the insert has been magnified accordingly. DNA complexity a measure of the amount of non- repetitive DNA characteristic of a given DNA sam- ple. In an experiment involving reassociation kinet- ics (q.v.), DNA complexity represents the combined length in nucleotide pairs of all unique DNA frag- ments.
The DNA of evolutionarily advanced species is more complex than that of primitive species. DNA damage checkpoint a system that checks for regions where DNA has single-stranded or mis- matched regions or stalled replication forks. Further progress through mitosis is then halted until the damage is corrected. If it cannot be rectified, the cell is diverted to apoptosis (q.v.).
See Appendix C, 1989, Hartwell and Weinert; Adriamycin, ATM kinase, RAD. DNA-dependent RNA polymerase RNA poly- merase (q.v.). Contrast with RNA-dependent DNA polymerase. DNA-driven hybridization reaction a reaction in- volving the reassociation kinetics of complementary DNA strands when DNA is in great excess of a ra- dioactive RNA tracer; employed in cot analysis to determine the repetition frequencies of the corre- sponding genome sequences. See reassociation ki- netics. DNA duplex a DNA double helix. See deoxyribo- nucleic acid.
DNA fiber autoradiography light microscopic au- toradiography of tritiated thymidine-labeled DNA molecules attached to millipore filters. The tech- nique was devised by Cairns (1963) for studying DNA replication in E. coli and later adapted by Hub- erman and Riggs (1968) for visualizing the multiple replicons of mammalian chromosomes. DNA fingerprint technique a technique properly termed DNA typing) that relies on the presence of simple tandem-repetitive sequences that are scattered throughout the human genome. Al- though these regions show considerable differences in lengths, they share a common 10-15 base pair core sequence.
DNAs from different individual hu- mans are enzymatically cleaved and separated by size on a gel. A hybridization probe containing the core sequence is then used to label those DNA frag- ments that contain complementary sequences. The pattern displayed on each gel is specific for a given individual. The technique has been used to establish family relationships in cases of disputed parentage.
In violent crimes, blood, hair, semen, and other tis- sues from the assailant are often left at the scene. The DNA fingerprinting technique provides the fo- rensic scientist with a means of identifying the assail- ant from a group of suspects. See Appendix C, 1985, Jeffries, Wilson, and Thien; alphoid sequences, DNA forensics, fingerprinting technique, oligonucleotide fingerprinting, restriction fragment length polymor- phisms, VNTR locus.
DNA forensics the use of DNA technology during the evidence-gathering phases of criminal investiga- tions, as well as any use of DNA evidence in the legal system. The first case in which a person was convicted of a crime on the basis of DNA evidence occurred in 1987. In 1989, the first conviction was overturned on the basis of DNA evidence. DNA profiling has also been successful in identifying vic- tims of catastrophes, in establishing paternity, and in determining the bacteria or viruses responsible for outbreaks of infectious diseases. See CODIS, DNA fingerprint technique, Romonov, STR analysis.
DNA glycosylases a family of enzymes, each of which recognizes a single type of altered base in DNA and catalyzes its hydrolytic removal from the sugar-phosphodiester backbone. See AP endonucle- ases.