teins. In humans, α-galactosidase exists in two forms, A and B. The A form is encoded by a gene on the X chromosome. Fabry disease (q.v.) is caused by mutations at this locus. The B form is encoded by a gene on chromosome 22.
alpha helix one of two common, regularly repeat- ing structures seen in proteins (compare with beta pleated sheet). The alpha helix is a compact spiral with the side chains of the amino acids in the poly- peptide extending outward from the helix.
The helix is stabilized by hydrogen bonds that form between the CO group of each amino acid and the NH group of the amino acid, which lies four residues ahead in the sequence. All main-chain CO and NH groups are hydrogen-bonded according to this pattern. One turn of the helix occurs for each 3.6 amino acid resi- dues. Alpha helices are built from a continuous se- quence that contains as few as 4 to as many as 50 amino acids.
See Appendix C, 1951, Pauling and Corey; 1958, Kendrew et al.; protein structure. alpha particle a helium nucleus consisting of two protons and two neutrons, and having a double posi- tive charge. alpha tocopherol vitamin E (q.v.). alphoid sequences a complex family of repetitive DNA sequences found in the centromeric hetero- chromatin of human chromosomes. The alphoid family is composed of tandem arrays of 170 base pair segments.
The segments isolated from different chromosomes show a consensus sequence, but alsodifferences with respect to individual bases, so that the 170 base pair units may vary in sequence by as much as 40%. The repeats are organized in turn into groups containing several units in tandem, and these groups are further organized into larger sequences 1 to 6 kilobases in length. These large segments are then repeated to generate segments 0.5 to 10 mega- base pairs in size.
Such larger, or “macro,” DNA re- peats are chromosome-specific. Since alphoid se- quences are not transcribed, they play an as yet undefined structural role in the chromosome cycle. The variation in the sequences within the alphoid DNA results in a high frequency of RFLPs.
These are inherited and can be used to characterize the DNAs of specific individuals and their relatives. See DNA fingerprint technique, restriction fragment length polymorphisms.
alteration enzyme a protein of phage T4 that is injected into a host bacterium along with the phage DNA; this protein modifies host RNA polymerase by linking it to ADP-ribose.
RNA polymerase modi- fied in this way renders it incapable of binding to sigma factor and thus unable to initiate transcription at host promoters. See RNA polymerase. alternate disjunction, alternate segregation See translocation heterozygote.
alternation of generations reproductive cycles in which a haploid phase alternates with a diploid phase. In mosses and vascular plants, the haploid phase is the gametophyte, the diploid the sporo- phyte.
alternative splicing a mechanism for generating multiple protein isoforms from a single gene that in- volves the splicing together of nonconsecutive exonsduring the processing of some, but not all, tran-scripts of the gene.
This is illustrated in the diagram, where a gene is made up of five exons joined by in-trons i1-i4. The exons may be spliced by the upper pathway shown by the dotted lines to generate a ma- ture transcript containing all five exons. This type of splicing is termed constitutive. The alternative mode of splicing shown generates a mature transcript that lacks exon 4.
If each exon encodes 20 amino acids, the constitutive splicing path would result in a poly- peptide made up of 100 amino acids. The alternative path would produce a polypeptide only 80 amino acids long. If the amino acid sequences of the two proteins were determined, the first 60 and the last 20 would be identical. The premessenger RNAs (q.v.) of at least 40% of all human genes undergo alternative splicing.
This removes the intron RNAs and joins the adjacent exon RNAs by phosphodies- ter linkages. The splicing takes place in spliceosomes (q.v.) that reside within the nucleus. Therefore the number of proteins encoded by the human genome is many times larger than the number of structural genes it contains.
See Appendix C, 1977, Weber et al.; adenovirus, DSCAM, fibronectin, Human Genome Project, isoforms, posttranscriptional processing, myo- sin genes, RNA splicing, tropomyosin.
altricial referring to the type of ontogeny seen in vertebrate species characterized by large litters, short gestations, and the birth of relatively undevel- oped, helpless young. Compare with precocial.
altruism behavior of an individual that benefits others. To the extent that the “others” are related to the altruist (the one exhibiting altruistic behavior), such actions may actually be an expression of fitness. See inclusive fitness.