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 th