GLOSSARY Abundance of an mRNA is the average number of molecules per cell. Abundant mRNAs consist of a small number of individual species, each present in a large number of copies per cell. Acceptor splicing site—see right splicing junction. Acentric fragment of a chromosome (generated by breakage) lacks a centromere and is lost cell division. Acrocentric chromosome has the centromere located nearer one end than the other. Active site is the restricted part of a protein to which a substrate binds. Allele is one of several alternative forms of a gene occupying a given locus on a chromosome. Allelic exclusion describes the expression in any particular lymphocyte of only one allele coding for the expressed immunoglobulin. Allosteric control refers to the ability of an interaction at one site of a protein to influence the activity of another site. Alu family is a set of dispersed, related sequences, each~300 bp long, in the human genome. The individual members have Alu cleavage sites at each end (hence the name). Alu-equivalent family is the set of sequences in a mammalian genome that is related to the human Alu family. α-Amanitin is a bicyclic octapeptide derived from the poisonous mushroom Amanita phalloides; it inhibhits transcription by certain eukaryotic RNA polymerases, especially RNA polymerase II. Amber codon is the nucleotide triplet UAG, one of three codons that cause termination of protein synthesis. Amber mutaion describes any change in DNA that creates an amber codon at a site previously occupied by a codon representing an amino acid in a protein. Amber suppressors are mutant genes that code for tRNAs whose anticodons have been altered so that they can respond to UAG codons as well as or instead of to their previous codons. Aminoacyl-tRNA is transfer RNA carrying an amino acid; the covalent linkage is between the NH2 group of the amino acid and either the 3’-or-2’-OH group of the terminal base of the tRNA. Aminoacyl-tRNA synthetases are enzymes responsible for covalently linking amino acids to the 2’ or 3’-OH position of tRNA. Amphipathic structures have two surfaces, one hydrophilic and one hydrophobic. Lipids are amphipathic; and some protein regions may form amphipathic; and some protein regions may form amphipathic helices, with one charged face and one neutral face. Amplification refers to the production of additional copies of a chromosomal sequence, found as intrachromosomal or extrachromoxomal DNA. Anchorage dependence describes the need of normal eukaryotic cells for a surface to attach to in order to grow in culture. Aneuploid chromosome constitution differes from the usal diploid constitution by loss or duplication of chromosomes or chromosomal segments. Annealing is the pairing of complementary single strands of DNA to form a double helix. Antibody is a protein (immunoglobulin) produced by B lymphocyte cells that recognizes a particular foreign ‘antigen,’and thusw triggers the immune response. Anticoding strand of duples DNA is used as a template to direct the synthesis of RNA that is complementary to it. Antigen is any molecule whose entry into an organism provokes synthesis of an antibody (immunoglobulin). Antiparallel strands of the double helix are organized in opposite orientation, so that the 5’ end of one strand is aligened with the 3’ end of the other strand. Antitermination proteins allow RNA polymerase to transcribe through certain terminator sites. Ap endonucleases make incisions in DNA on the 5’ side of either apurinic or apyrimidinc sites. Apoinducer is a protein that binds to DNA to switch on transcription by RNA polymerase. Archebacteria comprise a minor line of prokaryotes, and may have introns in the genome. Ascus of a fungus contains a tetrad or octad of the (haploid) spores, representhing the products of a sihngle meiosis. att sties are the loci on a phage and the bacterial chromosome at which recombination integrates the phage into. or excises it from , the bacterial chromosome. Attenuation describes the regulation of termination of transcription that is involved in controlling the expression of some bacterial operons. Attenuator is the terminator sequence at which attenuatioj occurs. Autogenous conhtrol describes the action of a gene product that either inhibits (negative autogenous control) or activates (positive autogenous control) expression of the gene coding for it. Autonomous controlling element in maize is an active transposon with the ability to transpose (cf nonautonomous controlling element). Autoradiography detects radioactively labeled molecules by their effect in creating an image on pholtographic film. Autosomes are all the chromosomes except the sex chromosomes; a diploid cell has two copies of each autosome. Blymphocytes (or B cells) are the cells responsible for synthesizing antibodies. Backcross is another (earlier) term for a testcross. Back mutation reverses the effect of a mutation that hand inactivated a gene; thus it restores wild type. Bacteriophages are viruses that infect bacteria; often abbreviated as phages. Balbaini ring is an extremely large puff at a band of a polytene chromosome. Bands of polytene chromosomes are visible as dense regions that contain the majority of DNA; bands of normal chromosomes are relatively much larger and are generated in the form of regions that retain a stain on certain chemical treatments. Base pair (bp) is a partnership of A with T or of C with G in a DNA double helix; other pairs can be formed in RNA under certain circumstances. Bidirectional replication is accomplished when two replication forks move away from the same origin in different directions. Bivalent is the structure containing all four chromatids (two representing each homologue) at the start of meiosis. Blastoderm is a stage of insect embryogenesis in which a layer of nuclei or cells around the embryo surround an internal mass of yolk. Blocked reading frame cannot be translated into protein because it is interrupted by termination codons. Blunt-end ligation is a reaction that joins two DNA duplex molecules directly at their ends. bp is an abbreviation for base pairs; distance along DNA is measured in bp. Branch migration describes the ability of a DNA strand partially paired with its complement in a duplex to extend its pairing by displacing the resident strand with which it is homologous. Breackage and reunion describes the mode of genetic recombination, in which two DNA duplex molecules are broken at corresponding points and then rejoined crosswise (involving formation of a length of heteroduplex DNA around the site of joining). Buoyant desity measures the ability of a substance to float in some standard fluid, for example, CsCl. C banding is a technique for generating stained regions around centromeres. C genes code for the constant regions of immunoglobulin protein chains. C value is the total amount of DNA in a haploid genome. CAAT box is part of a conserved sequence located upstream of the startpoints of eukaryotic transcription units; it is recognized by a large group of transcription factors. Cap is the structure at the 5’ end of eukaryotic mRNA, introduced after transcripton by linking the terminal phosphate of 5’ GTP to the terminal base of the mRNA. The added G (and sometimes some other bases) are methylated, giving a structure of the form 7MeG5’ppp 5’Np… CAP(CRP) is a positive regulator protein activated by cyclic AMP. It is needed for RNA polymerase to initiate transcription of certain (catabolitesensitive) operons of E. coli. Capsid is the external protein coat of a virus particle. Catabolite repression describes the decreased expression of many bacterial operons that results from addition of glucose. It is caused by a decrease in the level of cyclic AMP, which in turn inactivates the CAP regulator. cDNA is a single-stranded DNA complementary to an RNA, synthesized from it by reverse transcription in vitro. cDNA clone is a duplex DNA sequence representing an RNA, carried in a cloning vector. Cell cycle is the period from one division to the next. Cell hybrid is a somatic cell containing chromosomes derived from parental cells of different species (e.g. a man-mouse somatic cell hybrid), generating by fusing the cells to form a heterokaryon in which the nuclei subsequently fused. Centrioles are small hollow cylinders consisting of microtubules that become located near the poles during mitosis. They reside within the centrosomes. Centromere is a constricted region of a chromosome that includes the site of attachment to the mitotic or meiotic spindle (see also kinetochore). Centrosomes are the regions from which microtubules are organized at the poles of a mitotic cell. In animal cells, each centrosome contains a pair of centrioles surrounded by a dense amorphous region to which the microtubules attach. See also MTOC. Molecular chaperone is a protein that is needed for the assembly or proper folding of some other protein, but which is not itself a component of the target complex. Chemical complexity is the amount of a DNA component measured by chemical assay. Chi sequence is an octamer that provides a hotspot for RecA-mediated genetic recombination in E. coli. Chi structure is a joint between two duplex molecules of DNA revealed by cleaving an intermediate of two joined circles to generate linear ends in each circle. It resembles a Greek chi in outline, hence the name. Chiasma (pl. chiasmata) is a site at which two homologous chromosomes appear to have exchanged material during meiosis. Chromatids are the copies of a chromosome produced by replication. The name is usually used to describe them in the period before they separate at the subsequent cell division. Chromatin is the complex of DNA and protein in the nucleus of the interphase cell. Individual chromosomes cannot be distinguished in it .It was originally recognized by its reaction with stains specific for DNA. Chromocenter is an aggregate of heterochromatin from different chromosomes. Chromomeres are densely staining granules visible in chromosomes under certain conditions, especially early in meiosis, when a chromosome may appear to consist of a series of chromomeres. Chromosome is a discrete unit of the genome carrying many genes. Each chromosome counsists of a very long molecule of duplex DNA and an approximately equal mass of proteins. It is visible as a morphological entity only during cell division. Chromosome walking describes the sequential isolation of clones carrying overlapping sequences of DNA, allowing large regions of the chromosome to be spanned. Walking is often performed in order to reach a particular locus of interest. cis-acting locus affects the activity only of DNA sequences on its own molecule of DNA; this property usually implies that the locus does not code for protein. cis-acting protein has the exceptional property of acting only on the molecule of DNA from which it was expressed. cis configuration describes two sites on the same molecule of DNA. cis/trans test assays the effect of relative configuration on expression of two mutations. In a double heterozygote, two mutations in the same gene show mutant phenotype in trans configuration, wild-type in cis configuration. Cistron is the geneti unit defined by the cis/trans test; equivalent to gene ib comprising a unit of DNA representing a protein. Class switching is a change in the expression of the c cregion of an immunoglobulin heavy chain during lymphocyte differentiation. Clone describes a large number of cells or molecules identical with a single ancestral cell or molecule. Cloning vector is a plasmid or phage that is used to ‘carry’ inserted foreign DNA for the purposes of producing more material or a protein product. Closed reading frame contains termination codons that prevent its translation into protein. Coated vesicles are vesicles whose membrane has on its surface a layer of the protein clathrin. Coconversion is the simultaneous correction of two sites during gene conversion. Coding strand of DNA has the same sequence as mRNA. Codominant alleles both contribute to the phenotype; neither is dominant over the other. Coevolution —see concerted evolution. Cognate tRNAs are those recognized by a particular aminoacy-tRNA synthetase. Coointegrate structure is produced by fusion of two replicons, one originally possessing a transposon, the other lacking it; the cointegrate has copies of the transposon present at both junctions of the replicons, oriented as directrepeats. Cold-sensitive mutant is defective at low temperature but functional at normal temlperature. Colony hybridization is a technique for using in situ hybridization to identify bacteria carrying chimeric vectors whose inserted DNA is homologous with some particular sequence. Compatibility group of plasmids contains members unable to coexist in the same bacterial cell. Complementation refers to the ability of independent (nonallelic)genes to provide diffusible products that produce wild phenotype when two mutants are tested in trans configuration in a heterozygote. In vitro complementation assay consists of identifying a component of a wid-type cell that can confer activity on an extract prepared from a mutant cell. The assay identifies the component rendered inactive by the mutation. Complementation group is a series of mutations unable to complement when tested in pairwise combinations in trans; defines a genetic unit (the cistron) that might better be called a noncomplex mentation group. Complex locus (of D. melanogaster) has genetic properties inconsistent with the function of a gene representing a single protein. Complex loci are usually very large (>100kb) at the molecular level. Complexity is the total length of different sequences of DNA present in a given preparation. Compostie transposons have a central region flanked on each side by insertion sequences, either or both of which may enable the entire element to transpose. Concatemer of DNA consists of a series of unit genomes repeated in tandem. Concatenated circles of DNA are interlocked like rings one a chain. Concerted evolution describes the ability of two related genes to evolve together as though constituting a single locus. Condensation reaction is one in which a covalent bond is formed with loss of a water molecule, as in the addition of an amino acid to a polypeptide chain. Conditional lethal mlutations kill a cell or virus under certain (nonpermissive) conditions, but allow it to survive under other (permissive) conditions. Conjugation describes ‘mathing’ between two bacterial cells, when (part of ) the chromosome is transferred from one to the other. Consensus sequence is an idealized sequence in which each position represents the base most often found when many actual sequences are compared. Conservative recombination involves breakage and reunion of preexisting strands of DNA without any synthesis of new stretches of DNA. Conservative transposition refers to the movement of large elements, originally classified as transposons, but now considered to be episomes. The mechanism of movement resembles that of phage lambda. Constant regions of immunoglobulins are coded by C genes and are the parts of the chain that vary least. Those of heavy chains identify the type of immunoglobulin. Constitutive genes are expressed as a function of the interaction of RNA polymerase with the promoter, without additional regulation; sometimes also called household genes in the context of describing functions expressed in all cells at a low level. Constitutive heterochromatin describes the inert state of permanently nonexpressed sequences, usually satellite DNA. Constitutive mutations cause genes that usually are regulated to be expressed without regulation. Contractile ring is a ring of actin filaments that forms around the equator at the end of mitosis and is responsible for pinching the daughter cells apart. Controlling elements of maize are transposable units originally identified solely by their genetic properties. They may be autonomous (able to transpose independently) or nonautonomous (able to element). Coordinate regulation refers to the common control of a group of genes. Cordycepin is 3’ deoxyadenosine, an inhibitor of polyadenylation of RNA. Core DNA is the 14.6 bp of DNA contained on a core particle. Core particle is a digestion product of the nucleosome that retains the histone octamer and has 14.6 bp of DNA; its structure appears similar to that of the nucleosome itself. Corepressor is a small molecule that triggers repression of transcription by binding to a regulator protein. Cosmids are plasmids into which phage lambda cos sites have been inserted; as a result, the plasmid DNA can be packaged in vitro in the phage coat. Cot is the product of DNA concentration and time of incubation in a reassociation reaction. Cot is the Cot required to proceed to half completion of the reaction; it is directly proportional to the unique length of reassociating DNA. Cotransfection is the simultaneous transfection of two markers. Crossing-over describes the reciprocal exchange of material between chromosomes that occurs during meiosis and is responsible for genetic recombination. Crossover fixation refers to a possible consequence of unequal crossing-over that allows a mutation in one member of a tandem cluster to spread through the whole cluster (or to be eliminated). Cruciform is the structure produced at inverted repeats of DNA if the repeated sequence pairs with its complement on the same strand (instead of with its regular partner in the other strand of the duplex). Cryptic satellite is a satellite DNA sequence not identified as such by a separate peak on a density gradient; that is, it remains present in main-band DNA. ctDNA is chloroplast DNA. Cyclic AMP (cAMP) is a molecule of AMP in which the phosphate group is joined to both the 3’ and 5’ positions of the ribose; its binding activates the CAP, a postive regulator of prokaryotic transcription. Cyclins are proteins that accumulate continuously throughout the cell cycle and are then destroyed by proteolysis during mitosis. (see also MPF). Cytokinesis is the final process involved in separation and movement apart of daughter cells at the end of mitosis. Cytological hybridization—see in situ hybridization. Cytoplasm describes the material between the plasma membrane and the nucleus. Cytoplasmic inheritance is a property of genes located in mitochondria or chloroplasts (or possibly other extranuclear organelles). Cytoplasmic protein synthesis is the ranslation of mRNAs representing nuclear genes; it occurs via ribosomes attached to the cytoskeleton. Cytoskeleton consists of networks of fibers in the cytoplasm of the eukaryotic cell. Cytosol describes the general volume of cytoplasm in which organelles ( such as the mitochondria ) are located. D loop is a region within mitochondrial DNA in which a short stretch of RNA is paired with one strand of DNA, displacing the orignal partner DNA strand in this region. The same term is used also to describe the displacement of a region of one strand of duplex DNA byu a single-stranded invader in the reaction catalyzed by RecA protein. Degeneracy in the genetic code refers to the lack of an effect of many changes in the third base of the codon on the amino acid that is represented. Deletions are generated by removal of a sequence of DNA, the regions on either side being joined together. Denaturation of DNA or RNA describes its conversion from the double-stranded to the singlestranded state; separation of the strands is most often accomplished by heating. Denaturation of protein describes its conversion from the physiological conformation to some other (inactive) conformation. Derepressed state describes a gene that is turned on. It is synonymous with induced when describing the normal state of a gene; it has the same meaning as constitutive in describing the effect of mutation. Dicentric chromosome is the product of fusing two chromosome fragments, each of which has a centromere. It is unstable and may be broken when the two centromeres are pulled to opposite poles in mitosis. Diploid set of chromosomes contains two copies of each autosome and two sex chromosome. Direct repeats are identical (or related) sequences present in two or more copies in the same orientation in the same molecule of DNA; they aer not necessarily adjacent. Discontinuous replication refers to the synthesis of DNA in short (Okazaki) fragments that are later joined into a continuous strand. Disjunction describes the movement of members of a chromosome pair to opposite poles during cell division. At mitosis and the second meiotic division, disjunction applies to sister chromatids; at first meiotic division it applies to sister chromatid pairs. Divergence is the percent difference in nucleotide sequence between two related DNA sequences or in amino acid sequences between two proteins. Divergent transcription refers to the intitiation of transcription at two promoters facing in the opposite direction, so that transcription proceeds away in both directions from a central region. dna mutants of bacteria are temperature-sensitive; they cannot synthesize DNA at 42℃, but can do so at 37℃. DNAase is an enzyme that attacks bonds in DNA. DNA-driven bybridization involves the reaction of an excess of DNA with RNA. DNA polymerase is an enzyme that synthesizes a daughter strand(s) of DNA (under direction from a DNA template). May be involved in repair or replication. DNA replicase is a DNA-synthesizing enzyme required specifically for replication. Domain of a chromosome may refer either to a discrete structural entity defined as a region within which supercoiling is independent of other domains; or to an extensive region including an expressed gene that has heightened sensitivity to degradation by the enzyme DNAase I. Domain of a protein is a discrete continuous part of the amino acid sequence that can be equated with a particular function. Dominant allele determines the phenotype displayed in a heterozygote with another (recessive) allele. Donor splicing site—see left splicing junction. Down promoter mutations decrease the frequency of initiaton of transcription. Downstream identifies sequences proceeding farther in the direction of expression, for example, the conding region is downstream of the initiation condon. Early development refers to the period of a phage infection before the start of DNA replication. Extopic expression describes the expression of a gene in a tissue in which it is not usually expressed; for example, in a transgenic animal. Elongation factors (EF in prokaryotes, eEF in eukaryotes) are proteins that associate with ribosomes cyclically, during addition of each amino acid to the polypeptide chain. End labeling describes the addition of a radioactively labeled group to one end (5’ or 3’) of a DNA strand. End-product inhibition describes the ability of a product of a metabolic pathway to inhibit the activity of an enzyme that catalyzes an early step in the pathway. Endocytosis is a process by which proteins at the surface of the cell are internalized, being transported into the cell within membranous vesicles. Endocytic vesicles are membranous particles that transport proteins through endocytosis; also known as clathrin-coated vesicles. Endonucleases cleave bonds within a nucleic acid chain; they may be specific for RNA or for singlestranded of double-stranded DNA. Endoplasmic reticulum is a highly convoluted sheet of membranes, extending from the outer layer of the nuclear envelope into the cytoplasm. Enhancer element is a cis-acting sequence that increases the utilization of (some) eukaryotic promoters, and can function in either orientation and in any location (upstream or downstream) relative to the promoter. Envelopes surround some organelles (for example , nucleus or mitochondrion) and consist of concentric membranes, each membrane consisting of the usual lipid bilayer. Epigenetic changes influrence the phenotype without altering the genotype. They consist of changes in the properties of a cell that are inherited but that do not represent a change in genetic information. Episome is a plasmid able to integrate into bacterial DNA. Epistasis describes a situation in which expression of one gene wipes out the phenotypic effects of another gene. Essential gene is one whose deletions is lethal to the organism (see also lethal locus). Established cell lines consist of eukaryotic cells that have been adapted to indefinite growth in culture (they are said to be immortalized). Eubacteria comprise the major line of prokaryotes. Euchromatin comprises all of the genome in the interphase nucleus except for the heterochromatin. Evolutionary clock is defined by the rate at which mutations accumulate in a given gene. Excision-repair systems remove a single-stranded sequence of DNA containing damaged or mispaired bases and replace it in the duplex by synthesizing a sequence complementary to the remaining strand. Exocytosis is the process of secreting proteins from a cell into the medium, by transport in membranous vesicles from the endoplasmic reticulum, through the Golgi, to storage vesicles, and finally (upon a regulatory signal) through the plasma membrane. Exocytic vesicles (also secretory vesicles) are membranous particles that transport and store proteins during excytosis. Exon is any segment of an interrupted gene that is represented in the mature RNA product. Exonucleases cleave nucleotides one at a time from the end of a polynucleotide chain; they may be specific for either the 5’ or 3’ end of DNA or RNA. Expression vector is a cloning vector desined so that a coding sequence inserted at a particular site will be transcribed and translated into protein. Extranuclear genes reside outside the nucleus in organelles such as mitochondria and chloroplasts. F factor is a bacterial sex or fertility plasmid. F1 generation is the first generation produced by crossing two parental (homozygous) lines. Facultative heterochromatin describes the inert state of sequences that also exist in active copies—for example, one mammalian X chromosome in females. Fast component of a reassociation reaction is the first to reature and contains highly repetitive DNA. Fate map is a map of an embryo showing the adult tissues that will develop from the descendants of cells that occupy particular regions of the embryo. Figure eight describes two circles of DNA linked together by a recombination event that has not yet been completed. Filter hybridization is performed by incubating a deatured DNA preparation immobilized on a nitrocellulose filter with a solution of radioactively labeled RNA or DNA. Fingerprint of DNA is a pattern of polymorphic restriction fragments that differ between individual genomes. Fingerprint of a protein is the pattern of fragments (usually resolved on a two dimensional electrophoretic gel) generated by cleavage with an enzyme such as trypsin. Fluidity is a property of membranes; it indicates the ability of lipids to move laterally within their particular monolayer. Focus formation describes the ability of transformed eukaryotic cells to grow in dense clusters, piled up on one another. Focus forming unit (ffu) is a quantitative measure of forcus formation. Foldback DNA consists of inverted repeats that have renatured by intrastrand reassociation of denatured DNA. Foot printing is a technique for identifying the site on DNA bound by some protein by virtue of the protection of bonds in this region against attack by nucleases. Forward mutations inatctivate a wild-type gene. Founder effect refers to the presence in a population of many individuals all with the same chromosome (or region of a chromosome) derived from a single ancestor. Frameshift mutations arise by deletions or insertions that are not a multiple of 3bp; they change the frame in which triplets are translated into protein. G banding is a technique that generates a striated pattern in metaphase chromosomes that distinguishes the members of a haploid set. G1 is the period of the eukaryotic cell cycle between the last mitosis and the start of DNA replication. G2 is the period of the eukaryotic cell cycle between the end of DNA replication and the start of the next mitosis. Gamete is either type of reproductive (germ) cell—sperm or egg—with haploid chromosome content. Gap in DNA is the absence of one or more nucleotides in one strand of the duplex. Gap (cistron) is the segment of DNA involved in producing a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons). Gene conversion is the alteration of one strand of a heteroduplex DNA to make it complementary with the other strand at any position(s) where there were mispaired bases. Gene dosage gives the number of copies of a particular gene in the genome. Gene family consists of a set of genes whose exons are related; the members were derived by duplication and variation from some ancestral gene. Gene cluster is a group of adjacent genes that are identical or related. Genetic code is the correspondence between triplets in DNA (or RNA) and amino acids in protein. Genetic marker—see marker. Genomic (chromosomal) DNA clones are sequences of the genome carried by a cloning vector. Genotype is the genetic constitution of an organism. Golgi apparatus consists of indivivual stacks of membranes near the endoplasmic reticulum; involed in glycosylating proteins and sorting them for transport to different cellular locations. G proteins are guanine nucleotide-binding trimeric proteins that reside in the plasma membrane. When bound by GDP the trimer remains intact and is inert. When the GDP bound to the αsubunit is replaced by GTP, the αsubunit is released from the βγ dimmer. One of the separated units (either the α monomer or the βγ dimmer) then activates or represses a target protein. Gratuitous inducers resemble authentic inducers of transcription but are not substrates for the induced enzymes. GT-AG rule describes the presence of these constant dinucleotides at the first two and last two positions of introns of nuclear genes. Gyrase is a type II topoisomerase of E. coli with the ability to introduce negative supercoils into DNA. Hairpin describes a double-helical region formed by base pairing between adjacent (inverted) complementary sequences in a single strand of RNA or DNA. Haploid set of chromosomes contains one copy of each autosome and one sex chromosome; the haploid number n is characteristic of gametes of diploid organisms. Haplotype is the particular combination of alleles in a defined region of some chromosome, in effect the genotype in miniature. Originally used to describe combinations of MHC alleles, it now may be used to describe particular combinations of RFLPs. Hapten is a small molecule that acts as an antigen when conjugated to a protein. Helper virus provides functions absent from a defective virus, enabling the latter to complete the infective cycle during a mixed infection. Hemizygote is a diploid individual that has lost its copy of a particular gene (for example, because a chromosome has been lost) and which therefore has only a single copy. Heterochromatin describes regions of the genome that are permanently in a highly condensed condition and are not genetically expressed. May be constitutive or facultative. Heterorduplex (hybrid) DNA is generated by hase pairing between complementary single strands derived from the different parental duplex molecules; it occurs during genetic recombination. Helerogametic sex has the diploid chromosome constitution 2A+XY. Heterogeneous nuclear (hn) RNA comprises transcripts of nuclear genes made by RNA polymerase II; it has a wide size distribution and low stability. Heteromultimeric proteins consist of nonidentical subunits (coded by different genes). Heterokaryon is a cell containing two (or more nuclei in a common cytoplasm, generate by fusing somatic cells. Heterzygote is an idividual with different alleles at some particular locus. Highly repetivive DNA is the first component to reassociate and is equated with satellite DNA. Histones are conserved DNA-binding proteins of eukaryotes that form the nucleosome, the basic subunit of chromatin. Homeobox describes the conserved sequence that is part of the coding region of D. melanogaster homeotic genes; it is also found in amphibian and mammalian genes