For example, a mutant tyrosine-tRNA anticodon 3′AUC would recognize 5′UAG, tyrosine would be inserted, and chain growth would continue. Ambystoma mexicanum the Mexican axolotl, a widely used laboratory species. The urodele for which the most genetic information is available. It was in the nuclei of oocytes of this species that the giant lampbrush chromosomes (q.v.) were first observed. Ambystoma has 14 pairs of chromosomes, and a ge- nome size of about 35 gbp of DNA. It is one of the few vertebrates able to regenerate entire body struc- tures. See Appendix A, Animalia, Chordata, Verte- brata, Amphibia, Urodela; Appendix C, 1882, Flem-ming; neoteny, regeneration.
amelogenins highly conserved proteins that are secreted by ameloblasts and constitute 90% of the organic matrix in the enamel of teeth. The amelo- genins of humans come in a number of isoforms; the most common one contains 192 amino acids. The genes that encode amelogenins reside on the X and Y chromosomes. AMELX is at Xp22.22 and AMELY is at Yp11.2, and both are transcribed in the tooth buds of males. The X-linked gene contains 7,348 bases and the Y-linked gene 8,109. During forensic analyses, amelogenin primers from human X-Y DNA are often used in gender determination.
amelanogenesis imperfecta defects in the miner- alization of teeth that affect the enamel layer. The teeth are small, pitted, and show yellow to brown discolorations. The condition often results from mu- tations in the genes that encode amelanogenins (q.v.).
amensalism a species interaction in which one is adversely affected and the other is unaffected. Ames test a bioassay for detecting mutagenic and possibly carcinogenic compounds, developed by Bruce N. Ames in 1974. Reverse mutants to histidine in- dependence are scored by growing his− Salmonella typhimurium on plates deficient in histidine in the presence of the chemical (test) and in its absence (control).
amethopterin methotrexate (q.v.). amino acid activation a coupled reaction cata- lyzed by a specific aminoacyl synthetase that at- taches a specific amino acid (AA) to a specific trans- fer RNA (tRNA) in preparation for translation (q.v.). AA + ATP → AA-AMP + 2P AA-AMP + tRNA → AA-tRNA + AMP
amino acid attachment site the 3′ end of a tRNA molecule to which an amino acid is covalently attached by an aminoacyl bond. See amino acid acti- vation, aminoacyl-tRNA synthetases, transfer RNA. amino acids aminocarboxylic acids that are com- ponents of proteins and peptides. They also occur in their free form or attached to transfer RNAs (q.v.).
There are 20 different amino acids for which at least one specific codon exists in the DNA genetic code. These universal amino acids are illustrated on page 21. Their abbreviations and messenger RNA code designations are on page 22. Amino acids are joined together to form polypeptides.
Polymers containing 50 or more amino acids are called proteins. All amino acids contain a central carbon atom (desig- nated alpha) to which an amino group, a carboxyl group, and a hydrogen atom are attached. There is also a side chain or residue (R), and this gives each amino acid its characteristic properties.
Note that proline is unique in that the alpha C and its amino group are incorporated into the side chain, which is in the form of a five-atom ring. At pH 7 the side chains of lysine, arginine, and histidine are positively charged and the side chains of aspartic acid and glu- tamic acid are negatively charged.
Therefore, the net charge born by a protein is determined by the rela- tive proportions of these five amino acids in it. Other amino acids control the shape of proteins. Amino acids like isoleucine, leucine, phenylalanine, and valine are repelled by water molecules and therefore tend to be found buried within the interior of the protein structure. See genetic code, peptide bond, translation. amino acid sequence the linear order of the amino acids in a peptide or protein. See protein structure.
amino acid side chain a group attached to an amino acid, represented by R in the general formula for an amino acid: aminoaciduria the presence of one or more amino acids in the urine in abnormal quantities because of a metabolic defect. aminoacyl adenylate the activated compound that is an intermediate in the formation of a covalent bond between an amino acid and its specific transfer RNA; abbreviated AA-AMP. See AMP, transfer RNA.
Structural formulas of the universal amino acids
aminoacyl site one of two binding sites for tRNA molecules on a ribosome; commonly called the A site. See translation. aminoacyl-tRNA an aminoacyl ester of a transfer RNA molecule.
aminoacyl-tRNA binding site See translation.
aminoacyl-tRNA synthetases enzymes that acti- vate amino acids and attach each activated amino acid to its own species of tRNA. These enzymes cat- alyze: (1) the reaction of a specific amino acid (AA) with adenosine triphosphate (ATP) to form AA- AMP, and (2) the transfer of the AA-AMP complex to a specific transfer RNA, forming AA-tRNA and
free AMP (adenosine monophosphate). See adeno- sine phosphate. amino group a chemical group (−NH2) which with the addition of a proton can form −NH+. p amino benzoic acid a component of folic acid (q.v.).
aminopeptidase an enzyme (in both prokaryotes and eukaryotes) that removes the formylated methi- onine (fMet) or methionine from the NH2 terminus of growing or completed polypeptide chains. aminopterin See folic acid.
aminopurine 2-aminopurine (2-AP) is a fluores- cent analog of adenine (6-aminopurine). The fluo- rescence of 2-AP ia quenched once it is incorporated into the base-stacked structure of dsDNA. However, if the base stacking or base pairing of DNA is locally perturbed, the fluorescence of 2-AP is enhanced.
Therefore the intensity of fluorescence emissions by 2-AP molecules can be used to monitor pertur-bations in DNA structure caused by interactions between DNA and proteins, such as DNA polymer- ases, helicases, repair enzymes, and methyl transfer- ases (all of which see). amino terminal end the end of a polypeptide chain that has a free amino group.
Amish a human population descended from a lim- ited number of founders who emigrated from south- western Germany to the United States during the eighteenth century. The population is highly inbred, since marriage is allowed only within the commu- nity.
Beneficial collaboration between geneticists and religious leaders has led to discoveries concerning certain genetic diseases that occur at unprecedent- edly high frequencies among the Amish. See carti- lage-hair hypoplasia (CHH), consanguinity, Ellis-van Creveld syndrome, inbreeding.
Amitochondriates a subkingdom of protoctists that includes the Archaeprotista and the Microspora (see Appendix A). These phyla contain anaerobic mi- croorganisms that lack mitochondria and presum- ably were without them from the outset of their evolution. amitosis the division of a nucleus into two parts by constriction without the participation of a mi- totic apparatus. Accessory nuclei (q.v.) grow by ami- tosis.