Homeostasis (also homoeostasis)
holometabolous referring to those insects in which larval and pupal stages are interposed between the embryo and the adult. Contrast with hemimetabo- lous. holophyletic an evolutionary lineage consisting of a species and all of its descendants. holophytic nutrition nutrition requiring only inor- ganic chemicals, as that of photosynthetic plants. holorepressor See aporepressor. holotype the single specimen selected for the de- scription of a species. holozoic nutrition nutrition requiring complex organic foodstuffs, as that of organisms other than photosynthetic plants and protoctists.
homeobox a sequence of about 180 base pairs near the 3′ end of certain homeotic genes. The 60 amino acid segment encoded by the homeobox is a DNA-binding domain with a helix-turn-helix motif (q.v.).
Homeobox proteins can bind to and regulate the transcription of genes that contain homeobox re- sponsive elements (HREs). Homeobox proteins can also regulate translation by binding to mRNAs that also contain HREs. See Appendix C, 1984, McGinnis et al.; 1984, Shepard et al.; 1989, Qian et al.; 1990, Malicki et al.; 1996, Dubnau and Struhl, Rivera-Po- mar et al.; Antennapedia, apetala-2, bicoid, bithorax, engrailed, floral identity mutations, metamerism, pro- boscipedia, spineless-aristapedia, homeotic mutations, Hox genes, RAG-1 and RAG-2. homeodomain synonym for homeobox.
homeologous chromosomes See homoeologous chromosomes. homeoplastic graft (also homoeoplastic) a graft of tissue from one individual to another of the same species. homeosis (also homoeosis) heteromorphosis (q.v.). homeostasis (also homoeostasis) a fluctuation- free state. See developmental homeostasis.
homeotic mutations those in which one develop- mental pattern is replaced by a different, but homol- ogous one. The homeotic mutations of Drosophila cause an organ to differentiate abnormally and to form a homologous organ that is characteristic of an adjacent segment. Three such mutations are illus- trated above. (A) A frontal view of the normal head. (B) The leg-like antenna of an ssa mutant, (C) A pb mutant with its proboscis transformed into legs. (D) A bx male in which halteres are changed into wing- like appendages. Bithorax was the first homeotic mutation to be discovered. Homeotic genes were subsequently shown to control segment identity and were found to contain conserved segments called ho- meoboxes (q.v.). Homeotic mutations in which one floral organ is replaced by another occur in plants.
See Appendix C, 1915, Bridges; 1978, Lewis; 1983, Hafen, Levine, and Gehring; 1984, McGinnis et al.; 1996, Krizek and Meyerowitz; Arabidopsis thaliana, bithorax, floral identity mutations, Hox genes, me- tamerism, proboscipedia, spineless-aristapedia. homing endonuclease a site-specific DNA endo- nuclease, usually encoded by an open reading frame (q.v.) within an intron (q.v.) or an intein (q.v.) se- quence, which mediates the horizontal transfer of the DNA sequence that encodes it to a new genomic location. It does this by introducing double-stranded breaks at or near the insertion site (home) of an in- tron or intein DNA in alleles that lack these inter- vening genetic elements. Homing endonucleases can be distinguished from other site-specific endonucle- ases based on their structural and biochemical prop- erties. hominid a member of the family Hominidae in- cluding humans and related fossil species. Two gen- era are recognized: Australopithecus and Homo.
hominoid a member of the superfamily of pri- mates, the Hominoidea. Its living species include hu- mans, two species of chimpanzee, the gorilla, the orangutan, and nine species of gibbon. See Appendix C, 1967, Sarich and Wilson; Cercopithecoidea. Homo a genus in which humans are placed. It con- tains two fossil species: H. habilis, which lived 2.3 to 1.5 million years ago, and H. erectus, which lived 1.5 to 0.3 million years ago. A third fossil species, Homo neandertalensis, became extinct about 30,000 years ago. It overlapped Homo sapiens (q.v.), which dates from about 300,000 years ago to the present.
See Neandertal. homoalleles alternative forms of a gene that differ at the same muton site. Intragenic recombination between homoalleles is not possible. Compare with heteroalleles. homoallelic referring to allelic mutant genes that have mutations at the same site (q.v.). A functional cistron cannot be generated by intragenic recombi- nation between homoalleles.
Contrast with heteroal- lelic. homobrachial inversion paracentric inversion (q.v.) homocaryon homokaryon (q.v.). homocystinuria a hereditary disease in man aris- ing from a deficiency of the enzyme serine dehydra- tase. homodimer a protein made up of paired identical polypeptides. homoeologous chromosomes chromosomes that are only partially homologous. Such chromosomes are derived from ancestral chromosomes that are be- lieved to have been homologous. Evolutionary diver- gence has reduced the synaptic attraction of homoe- ologs. See Appendix C, 1958, Okamoto; differential affinity, isoanisosyndetic alloploid.
homoeosis homeosis. See also homeoplastic graft, homeostasis, homeotic mutations. homoeotic mutations homeotic mutations (q.v.). homogametic sex the sex that produces gametes all of which carry only one kind of sex chromosome; e.g., the eggs of female mammals carry only an X chromosome. Compare with heterogametic sex. homogamy the situation in which the male and female parts of a flower mature simultaneously.
homogenote a partial diploid (merozygote) bacte- rium in which the donor (exogenote) chromosomal segment carries the same alleles as the chromosome of the recipient (endogenote) cell. homogentisic acid a compound derived from the metabolic breakdown of the amino acid tyrosine. See alkaptonuria.
homograft homeoplastic graft (q.v.). homoimmunity the resistance of a lysogenic bac- terium (harboring a prophage, q.v.) to superinfec- tion by phage of the same kind as that carried in the prophage state. The excess repressor molecules generated by the prophage bind to operators on the infecting DNA molecules and prevent their tran- scription. homokaryon a dikaryotic mycelium in which both nuclei are of only one genotype. homokaryotypic referring to an individual carry- ing a chromosomal aberration in the homozygous condition. homolog 1. in classification, a character that de- fines a clade. 2. in evolution, homologs are charac- teristics that are similar in different species because they have been inherited from a common ancestor. 3. in cytology, See homologous chromosomes. homologous referring to structures or processes in different organisms that show a fundamental similar-
ity because of their having descended from a com- mon ancestor. Homologous structures have the same evolutionary origin although their functions may differ widely: e.g., the flipper of a seal and the wing of a bat. See analogous. homologous chromosomes chromosomes that pair during meiosis. Each homolog is a duplicate of one of the chromosomes contributed at syngamy by the mother or father. Homologous chromosomes contain the same linear sequence of genes and as a consequence each gene is present in duplicate. See synaptonemal complex. homologous recombination the exchange or re- placement of genetic material as a result of crossing over (q.v.) or gene conversion (q.v.), respectively. Homologous recombination occurs between two long stretches of DNA with similar sequences, which may be present in two homologous chromo- somes, in non-homologous chromosomes, or within a chromosome.
Meiotic and mitotic recombination (q.v.) are examples of homologous recombination. Compare with site-specific recombination. See Holli- day model. homologue alternative spelling of homolog (q.v.). homology the state of being homologous. In mo- lecular biology, the term is often misused when comparing sequences of nucleotides or amino acids from nucleic acids or proteins obtained from dis- tantly related species.
In such instances it is prefer- able to refer to sequence identities or similarities rather than “homologies.” homomeric protein referring to a protein made up of two or more identical polypeptide chains. An ex- ample would be beta galactosidase (q.v.), which is an aggregate of four identical polypeptides. homomixis referring in fungi to the mating system in which sexual reproduction involves the fusion of genetically similar nuclei derived from one thallus.
homomorphic bivalent a bivalent made up of ho- mologues of similar morphology. See heteromorphic bivalent. homomultimer See homopolymer. homoplasy parallel or convergent evolution; structural similarity in organisms not due directly to inheritance from a common ancestor or develop- ment from a common anlage. homopolar bond covalent bond (q.v.). homopolymer a polymer composed of identical monomeric units (poly U, for example).