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.