夏 Back to bto home page Jump to Terms beginning with A DEF= LMNO P-RS TUy J Co-Cz G K 2μ(2 micron) plasmid A naturally occurring extragenomic (not part of the genome) circular DNA molecule found in some yeast cells, with a circumference of 2u. Engineered to form the basis for veral types of gene vectors in yeast Adenine or adenosine A(aminoacyl) site The site on the ribosome occupied by an aminoacvl-trna just prior to peptide bond formation A form dna The form of DNA at high humidity; it has tilted base pairs and more base pairs per turn han does b dna abortive transduction The failure of a transducing DNA segment to be incorporated into the recipient acentric chromosome chromosome having no centromere acentric fragment A chromosomal piece without a centromere achondroplasia A type of dwarfism in humans inherited as an autosomal dominant phenoty acrocentric chromosome a chromosome whose centromere lies very near one end. See also metacentric activation energy(delta) Energy needed to initiate a chemical reaction
¤ B a ck t o B TO h ome p age Jump to Terms beginning with: A B Ca-Cn Co-Cz D E F G H I J K L M N O P Q R S T U V W X Y Z 2µ (2 micron) plasmid A naturally occurring extragenomic (not part of the genome) circular DNA molecule found in some yeast cells, with a circumference of 2µ. Engineered to form the basis for several types of gene vectors in yeast. A Adenine or adenosine. A (aminoacyl) site The site on the ribosome occupied by an aminoacyl-tRNA just prior to peptide bond formation. A form DNA The form of DNA at high humidity; it has tilted base pairs and more base pairs per turn than does B DNA. abortive transduction The failure of a transducing DNA segment to be incorporated into the recipient chromosome. acentric chromosome A chromosome having no centromere. acentric fragment A chromosomal piece without a centromere. achondroplasia A type of dwarfism in humans inherited as an autosomal dominant phenotype. acrocentric chromosome A chromosome whose centromere lies very near one end. See also metacentric chromosome activation energy (deltaG) Energy needed to initiate a chemical reaction
active site The part of a protein that must be maintained in a specific shape if the protein is to be functional, for example, the part to which the substrate binds in an enzyme. The part of an enzyme where the actual enzymatic function is performed adaptation In the evolutionary sense, some heritable feature of an individuals phenotype that improves its chances of survival and reproduction in the existing environment adaptive landscape The surface plotted in a three-dimensional graph; with all possible combinations of allele frequencies for different loci plotted in the plane; and mean fitness for each combination plotted in the third dimension adaptive peak a high point(perhaps one of several)on an adaptive landscape; selection tends to drive the genotype composition of the population toward a combination corresponding to an adaptive surface See adaptive landscape adaptive value See fitness additive genetic variance Genetic variance associated with the average effects of substituting one allele for another additive model A mechanism of quantitative inheritance in which alleles at different loci either add a fixed amount to the phenotype or add nothing adenine A purine base that pairs with thymine in the dNa double helix. See purines adenosine The nucleoside containing adenine as its base adenosine triphosphate See atp adjacent segregation In a reciprocal translocation heterozygote during meiosis the segregation of a translocated
active site The part of a protein that must be maintained in a specific shape if the protein is to be functional, for example, the part to which the substrate binds in an enzyme. The part of an enzyme where the actual enzymatic function is performed. adaptation In the evolutionary sense, some heritable feature of an individual's phenotype that improves its chances of survival and reproduction in the existing environment. adaptive landscape The surface plotted in a three-dimensional graph; with all possible combinations of allele frequencies for different loci plotted in the plane; and mean fitness for each combination plotted in the third dimension. adaptive peak A high point (perhaps one of several) on an adaptive landscape; selection tends to drive the genotype composition of the population toward a combination corresponding to an adaptive peak. adaptive surface See adaptive landscape. adaptive value See fitness. additive genetic variance Genetic variance associated with the average effects of substituting one allele for another. additive model A mechanism of quantitative inheritance in which alleles at different loci either add a fixed amount to the phenotype or add nothing. adenine A purine base that pairs with thymine in the DNA double helix. See purines. adenosine The nucleoside containing adenine as its base. adenosine triphosphate See ATP. adjacent segregation In a reciprocal translocation heterozygote during meiosis the segregation of a translocated
and a normal chromosome together, giving unbalanced gametes with duplications and deficiencies leading to non-viable zygotes. Adjacent segregation is of two kinds depending on whether non-homologous (adjacent-1)or homologous (adjacent-2) centromeres segregate together. Adiacent-l segregation is the usual type of adjacent gregation, adjacent-2 segregation is rare adjacent-l segregation segregation of non-homologous centromeres during meiosis in a reciprocal translocation heterozygote such that unbalanced gametes with duplications and deficiencies are produced. See alternate segregation and adiacent-2 segregation adjacent-2 segregation segregation of homologous centromeres during meiosis in a translocation heterozygote such that unbalanced gametes with duplications and deficiencies are produced. See alternate segregation and adiacent-I segregation ADP Adenosine diphosphate affected Individuals in a pedigree that exhibit the specific phenotype under study Ala Alanine(an amino acid) albino a pigmentless white phenotype, determined by a mutation in a gene coding for a pigment-synthesizing enzyme alkylating agent A chemical agent that can add alkyl groups(for example, ethyl or methyl groups)to another molecule; many mutagens act through alkylation allele Alternative form of a gene. One of the different forms of a gene that can exist at a single allele frequency Often called gene frequency. A measure of how common an allele is in a population; the proportion of all alleles at one gene locus that are of one specific type in a population allelic exclusion a process whereby only one immunoglobulin light chain and one heavy chain gene are transcribed in any one cell; the other genes are repressed
and a normal chromosome together, giving unbalanced gametes with duplications and deficiencies leading to non-viable zygotes. Adjacent segregation is of two kinds depending on whether non-homologous (adjacent-1) or homologous (adjacent-2) centromeres segregate together. Adjacent-1 segregation is the usual type of adjacent segregation, adjacent-2 segregation is rare. adjacent-1 segregation segregation of non-homologous centromeres during meiosis in a reciprocal translocation heterozygote such that unbalanced gametes with duplications and deficiencies are produced. See alternate segregation and adjacent-2 segregation. adjacent-2 segregation segregation of homologous centromeres during meiosis in a translocation heterozygote such that unbalanced gametes with duplications and deficiencies are produced. See alternate segregation and adjacent-1 segregation. ADP Adenosine diphosphate. affected Individuals in a pedigree that exhibit the specific phenotype under study. Ala Alanine (an amino acid). albino A pigmentless white phenotype, determined by a mutation in a gene coding for a pigment-synthesizing enzyme. alkylating agent A chemical agent that can add alkyl groups (for example, ethyl or methyl groups) to another molecule; many mutagens act through alkylation. allele Alternative form of a gene. One of the different forms of a gene that can exist at a single locus. allele frequency Often called gene frequency. A measure of how common an allele is in a population; the proportion of all alleles at one gene locus that are of one specific type in a population. allelic exclusion A process whereby only one immunoglobulin light chain and one heavy chain gene are transcribed in any one cell; the other genes are repressed
allopatric speciation speciation in which the evolution of reproductive isolating mechanisms occurs during physical separation of the populations allopolyploid polyploid produced by the hybridization of two species. See amphidiploid allosteric protein a protein whose shape is changed when it binds a particular molecule. In the new shape the protein's ability to react to a second molecule is altered allosteric transition A change from one conformation of a protein to another allotype The protein product (or the result of its activity of an allele which may be detected as an antigen in another member of the same species (eg histocompatibility antigens, immunoglobulins), obeying the rules of simple Mendelian inheritance allozygosity homozygosity in which the two alleles are alike but unrelated. See autozygosity allozyme Form of an enzvme that differs in amino acid sequence, as shown by electrophoretic mobility or some other property, from other forms of the same enzyme and is encoded by one allele at a single locus. See isozymes alternate segregation At meiosis in a reciprocal translocation heterozygote, the passage(segregation) of both normal chromosomes to one pole and both translocated chromosomes to the other pole giving genetically balanced gametes. Segregation of centromeres during meiosis in a reciprocal translocation heterozygote such that genetically balanced gametes are alternation of generations The alternation of gametophyte and sporophyte stages in the life cycle of a plant alternative splicing Various ways of splicing out introns in eukaryotic pre-mRNAs resulting in one gene producing several different mRNAs and protein products altruism a form of behaviour in which an individual risks lowering its fitness for the benefit of
allopatric speciation speciation in which the evolution of reproductive isolating mechanisms occurs during physical separation of the populations. allopolyploid polyploid produced by the hybridization of two species. See amphidiploid. allosteric protein A protein whose shape is changed when it binds a particular molecule. In the new shape the protein's ability to react to a second molecule is altered. allosteric transition A change from one conformation of a protein to another. allotype The protein product (or the result of its activity) of an allele which may be detected as an antigen in another member of the same species.(eg histocompatibility antigens, immunoglobulins), obeying the rules of simple Mendelian inheritance. allozygosity homozygosity in which the two alleles are alike but unrelated. See autozygosity. allozyme Form of an enzyme that differs in amino acid sequence, as shown by electrophoretic mobility or some other property, from other forms of the same enzyme and is encoded by one allele at a single locus. See isozymes. alternate segregation At meiosis in a reciprocal translocation heterozygote, the passage (segregation) of both normal chromosomes to one pole and both translocated chromosomes to the other pole, giving genetically balanced gametes. Segregation of centromeres during meiosis in a reciprocal translocation heterozygote such that genetically balanced gametes are produced. alternation of generations The alternation of gametophyte and sporophyte stages in the life cycle of a plant. alternative splicing Various ways of splicing out introns in eukaryotic pre-mRNAs resulting in one gene producing several different mRNAs and protein products. altruism A form of behaviour in which an individual risks lowering its fitness for the benefit of
another Alu famil a dispersed intermediately repetitive DNA sequence found in the human genome in about three hundred thousand copies. The sequence is about 300 bp long. The name Alu comes from the restriction endonuclease alul that cleaves it amber codon The codon UAG, a nonsense codon amber suppressor a mutant allele coding for a tRNa whose anticodon is altered in such a way that the suppressor tRNA inserts an amino acid at an amber codon in translation suppressing (preventing)termination Ames test A widely used test to detect possible chemical carcinogens based on mutagenicity in the bacterium Salmonella amino acid The basic building block of proteins (or polypeptides). Containing a basic amino(NH2) group, an acidic carboxyl( COOH) group and a side chain(R-of a number of different kinds)attached to an alpha carbon atom. Thus NH2-C-CO0H aminoacyl-tRNA Transfer RNA (tRNA) molecule with its cognate amino acid attached aminoacyl-tRNA synthetase enzyme that attaches an amino acid to its cognate tRNA(S) amniocentesis a technique for testing the genotype of an embryo or fetus in utero with minimal risk to the mother or the child amorph A mutant showing the complete lack of some normal substance or structure AMP Adenosine monophosphate
another. Alu family A dispersed intermediately repetitive DNA sequence found in the human genome in about three hundred thousand copies. The sequence is about 300 bp long. The name Alu comes from the restriction endonuclease AluIthat cleaves it. amber codon The codon UAG, a nonsense codon. amber suppressor A mutant allele coding for a tRNA whose anticodon is altered in such a way that the suppressor tRNA inserts an amino acid at an amber codon in translation suppressing (preventing) termination. Ames test A widely used test to detect possible chemical carcinogens; based on mutagenicity in the bacterium Salmonella. amino acid The basic building block of proteins (or polypeptides). Containing a basic amino (NH2) group, an acidic carboxyl (COOH) group and a side chain (R - of a number of different kinds) attached to an alpha carbon atom. Thus: R | NH2-C-COOH | H aminoacyl-tRNA Transfer RNA (tRNA) molecule with its cognate amino acid attached. aminoacyl-tRNA synthetase enzyme that attaches an amino acid to its cognate tRNA(s). amniocentesis A technique for testing the genotype of an embryo or fetus in utero with minimal risk to the mother or the child. amorph A mutant showing the complete lack of some normal substance or structure. AMP Adenosine monophosphate
amphidiploid An allopolyploid; a polyploid formed from the union of two separate chromosome sets and their subsequent doubling. An organism produced by hybridization of two species followed by chromosome doubling. An allotetraploid that appears to be a normal diploid amplification of DNa The production of many DNA copies from one or a few copies anagenesis The evolutionary process whereby one species evolves into another without any splitting of the phylogenetic tree. See cladogenesis anaphase The stage of meiosis and mitosis in which sister chromatids(meiosisll) or homologous chromosomes(mitosis and meiosis) are separated by spindle fibers. An intermediate stage of nuclear division during which chromosomes or chromatids are pulled to the poles of the spindle aneuploid cell A cell having a chromosome number that differs from the normal chromosome number for the species by a small number of chromosomes aneuploid Individuals or cells exhibiting aneuploidy aneuploidy The condition of a cell or of an organism that has additions or deletions of a small number of whole chromosomes from the expected balanced diploid number of chromosomes angiosperm Plant whose seeds are enclosed within an ovary. Flowering plants animal breeding The practical application of genetic analysis for development of lines of domestic animals suited to human purposes annealing Spontaneous alignment of two complementary polynucleotide(RNA, or DNA,or RNA and dna)strands to form a double helix antI-oncogene a gene that prevents malignant ( cancerous) growth and whose absence, by mutation
amphidiploid An allopolyploid; a polyploid formed from the union of two separate chromosome sets and their subsequent doubling. An organism produced by hybridization of two species followed by chromosome doubling. An allotetraploid that appears to be a normal diploid. amplification of DNA The production of many DNA copies from one or a few copies. anagenesis The evolutionary process whereby one species evolves into another without any splitting of the phylogenetic tree. See cladogenesis. anaphase The stage of meiosis and mitosis in which sister chromatids (meiosisII) or homologous chromosomes (mitosis and meiosisI) are separated by spindle fibers. An intermediate stage of nuclear division during which chromosomes or chromatids are pulled to the poles of the spindle. aneuploid cell A cell having a chromosome number that differs from the normal chromosome number for the species by a small number of chromosomes. aneuploid Individuals or cells exhibiting aneuploidy. aneuploidy The condition of a cell or of an organism that has additions or deletions of a small number of whole chromosomes from the expected balanced diploid number of chromosomes. angiosperm Plant whose seeds are enclosed within an ovary. Flowering plants. animal breeding The practical application of genetic analysis for development of lines of domestic animals suited to human purposes. annealing Spontaneous alignment of two complementary single polynucleotide (RNA, or DNA, or RNA and DNA) strands to form a double helix. anti-oncogene A gene that prevents malignant (cancerous) growth and whose absence, by mutation, results in malignancy (eg retinoblastoma)
antibody a protein produced by a B lymphocyte that protects the organism against an antigen. A protein(immunoglobulin) molecule, produced by the immune system, that recognizes a particular foreign antigen and binds to it, if the antigen is on the surface of a cell, this binding leads to cell aggregation and subsequent destruction anticoding strand The DNA strand that forms the template for both the transcribed mRNa and the coding DNA Strand antinode The three-base sequence in tRNA complementary to a codon on mRNA. A nucleotide in a trna mole with a RNA nfluence of the ribosome, so that the amino acid carried by the trNa is added to a antigen A molecule whose shape triggers the production of antibodies (immunoglobulins) that will bind to the antigen. A foreign substance capable of triggering an immune response in antimorpl A mutant expressing some agent that antagonizes a normal gene product antimutator mutation mutation of dNa polymerase that decreases the overall mutation rate antiparallel a term used to describe the opposite orientations of the two strands of a DNA double helix; the 5 end of one strand aligns with the 3'end of the other strand antisense RNA RNA product of mic(mRNA-interfering complementary RNA) genes that regulates antisense strand antiterI Itor protein A protein that, when bound at its normal attachment sites in DNA, lets RNa polymera AP endonuclease
antibody A protein produced by a B lymphocyte that protects the organism against an antigen. A protein (immunoglobulin) molecule, produced by the immune system, that recognizes a particular foreign antigen and binds to it; if the antigen is on the surface of a cell, this binding leads to cell aggregation and subsequent destruction. anticoding strand The DNA strand that forms the template for both the transcribed mRNA and the coding DNA strand. anticodon The three-base sequence in tRNA complementary to a codon on mRNA. A nucleotide triplet in a tRNA molecule that aligns with a particular codon in mRNA under the influence of the ribosome, so that the amino acid carried by the tRNA is added to a growing protein chain. antigen A molecule whose shape triggers the production of antibodies (immunoglobulins) that will bind to the antigen. A foreign substance capable of triggering an immune response in an organism. antimorph A mutant expressing some agent that antagonizes a normal gene product. antimutator mutation mutation of DNA polymerase that decreases the overall mutation rate. antiparallel A term used to describe the opposite orientations of the two strands of a DNA double helix; the 5' end of one strand aligns with the 3' end of the other strand. antisense RNA RNA product of mic (mRNA-interfering complementary RNA) genes that regulates another gene by base pairing with and thus preventing translation from its mRNA. antisense strand See anticoding strand. antiterminator protein A protein that, when bound at its normal attachment sites in DNA, lets RNA polymerase read through normal terminator sequences (eg the N- and Q-gene products of phage lambda). AP endonuclease
endonuclease that initiates excision repair at apurinic and apyrimidinic sites on dnA AP site Apurinic or apyrimidinic site resulting from the loss of a purine or pyrimidine residue from the dna Arg nine(al ascospore A sexual spore from certain fungus species in which spores are found in a sac called an ascus. Haploid spores found in the asci of Ascomycete fungi ascus In fungi, a sac that encloses a tetrad or an octad of ascospores. The sac in Ascomycete fungi that holds the ascospores asexual spore Sn Asparagine(an amino acid) Aspartate(an amino acid) assignment test a test that determines whether a locus is on a specific human chromosome by observation of the concordance of the locus and the specific chromosome in a panel of human-mouse hybrid cell lines containing only one or a few of the normal set(22 autosomes, X and Y of human chromosomes assortative mating The mating of individuals with similar phenotypes ataxia-telangiectasia a disease(loss of muscle control, and reddening of the skin) in human beings caused by a defect in DNA repair mechanisms induced by ionising radiation(X-rays, beta and alpha particles, gamma rays) ATP (adenosine triphosphate) The energy molecule of cells, synthesized mainly in mitochondria and chloroplasts; energy from the breakdown of ATP drives many important
endonuclease that initiates excision repair at apurinic and apyrimidinic sites on DNA. AP site Apurinic or apyrimidinic site resulting from the loss of a purine or pyrimidine residue from the DNA. Arg Arginine (an amino acid). ascospore A sexual spore from certain fungus species in which spores are found in a sac called an ascus. Haploid spores found in the asci of Ascomycete fungi. ascus In fungi, a sac that encloses a tetrad or an octad of ascospores. The sac in Ascomycete fungi that holds the ascospores. asexual spore See spore. Asn Asparagine (an amino acid). Asp Aspartate (an amino acid). assignment test A test that determines whether a locus is on a specific human chromosome by observation of the concordance of the locus and the specific chromosome in a panel of human-mouse hybrid cell lines containing only one or a few of the normal set (22 autosomes, X and Y) of human chromosomes. assortative mating The mating of individuals with similar phenotypes. ataxia-telangiectasia A disease (loss of muscle control, and reddening of the skin) in human beings caused by a defect in DNA repair mechanisms induced by ionising radiation (X-rays, beta and alpha particles, gamma rays). ATP (adenosine triphosphate) The energy molecule of cells, synthesized mainly in mitochondria and chloroplasts; energy from the breakdown of ATP drives many important reactions in the cell
attached X A pair of Drosophila X chromosomes joined at one end and inherited as a single unit attenuator A control region at the promoter end of repressible amino acid operons that exerts transcriptional control based on the translation of a small leader peptide gene. A region djacent to the structural genes of the trp (tryptophan) operon; in the presence tryptophan this region acts to reduce the rate of transcription from the structural genes attenuator stem A configuration of the leader transcript that signals transcription termination in attenuator-controlled amino acid operons autogamy nuclear reorganization in a single Paramecium cell similar to the changes that occur during conjugation autonomous replication sequence (ARs) A segment of a DNA molecule necessary for the initiation of its replication; generally a site recognized and bound by the proteins of the replication system autopolypI a polyploid formed from the doubling of a single genome, Polyploidy in which all the hromosomes come from the same species autoradiography a process by which radioactive materials, often though not exclusively incorporated into cell structures, are located by exposure to a photographic emulsion forming a pattern on the film corresponding to the location of the radioactive compounds within the cell. A technique in which radioactive molecules make their location known by exposing photographic films or emulsions autoregulation The control of the transcription of a gene by its own gene product autosomal set The non-sex chromosomes consisting of one from each homologous pair in a diploid autosome Any chromosome that is not a sex chromosome autotroph
attached X A pair of Drosophila X chromosomes joined at one end and inherited as a single unit. attenuator A control region at the promoter end of repressible amino acid operons that exerts transcriptional control based on the translation of a small leader peptide gene. A region adjacent to the structural genes of the trp (tryptophan) operon; in the presence of tryptophan this region acts to reduce the rate of transcription from the structural genes. attenuator stem A configuration of the leader transcript that signals transcription termination in attenuator-controlled amino acid operons. autogamy nuclear reorganization in a single Paramecium cell similar to the changes that occur during conjugation. autonomous replication sequence (ARS) A segment of a DNA molecule necessary for the initiation of its replication; generally a site recognized and bound by the proteins of the replication system. autopolyploid A polyploid formed from the doubling of a single genome. Polyploidy in which all the chromosomes come from the same species. autoradiography A process by which radioactive materials, often though not exclusively incorporated into cell structures, are located by exposure to a photographic emulsion forming a pattern on the film corresponding to the location of the radioactive compounds within the cell. A technique in which radioactive molecules make their location known by exposing photographic films or emulsions. autoregulation The control of the transcription of a gene by its own gene product. autosomal set The non-sex chromosomes consisting of one from each homologous pair in a diploid species. autosome Any chromosome that is not a sex chromosome. autotroph
Organism that is able to utilize carbon dioxide as a carbon source autozygosity homozygosity in which the two alleles are identical by descent(ie they are copies of an ancestral gene) auxotroph An organism with specific nutritional requirements auxotrophic mutant A mutant strain of microorganism that will proliferate only when the medium is supplemented with some specific substance not required by wild-tvpe organisms axoneme a bundle of microtubules and other proteins forming the core of each cilium or flagellum B form dnA A right-handed double-helical conformation of DNA normally seen in solution described by Watson and Crick. A second DNA conformation(A form) is seen in unhydrated DNA fibres or crystals of oligonucleotide bacillus A rod-shaped bacterium back mutation The process that causes reversion. A change in a nucleotide pair in a mutant gene that restores the original sequence and hence the original phenotype backcross The cross of an individual with one of its parents or an organism with the same genotype as a parent. bacterial lawn A continuous cover of bacteria on the surface of a growth medium bacteriophage(phage) a virus that infects bacteria balanced lethal system An arrangement of recessive lethal alleles that maintains a heterozygous chromosome combination. homozygotes for any lethal-bearing chromosome perish balanced polymorphism Stable genetic polymorphism maintained by natural selection
Organism that is able to utilize carbon dioxide as a carbon source. autozygosity homozygosity in which the two alleles are identical by descent (ie they are copies of an ancestral gene) . auxotroph An organism with specific nutritional requirements. auxotrophic mutant A mutant strain of microorganism that will proliferate only when the medium is supplemented with some specific substance not required by wild-type organisms. axoneme A bundle of microtubules and other proteins forming the core of each cilium or flagellum. B form DNA A right-handed double-helical conformation of DNA normally seen in solution described by Watson and Crick. A second DNA conformation (A form) is seen in unhydrated DNA (fibres or crystals of oligonucleotides). bacillus A rod-shaped bacterium. back mutation The process that causes reversion. A change in a nucleotide pair in a mutant gene that restores the original sequence and hence the original phenotype. backcross The cross of an individual with one of its parents or an organism with the same genotype as a parent. bacterial lawn A continuous cover of bacteria on the surface of a growth medium. bacteriophage(phage) A virusthat infects bacteria. balanced lethal system An arrangement of recessive lethal alleles that maintains a heterozygous chromosome combination. homozygotes for any lethal-bearing chromosome perish. balanced polymorphism Stable genetic polymorphism maintained by natural selection