Amyotrophic lateral sclerosis (ALS)
amixis a reproductive cycle lacking meiosis and fertilization. Asexual reproduction. Contrast with amphimixis, apomixis. AML1 gene See acute myeloid leukemia 1 gene. amniocentesis sampling of amniotic fluid for the prenatal diagnosis of fetal disorders. During the pro- cedure, a hollow needle is inserted through the skin and muscle of the mother’s abdomen, through the uterus, and into the amniotic sac that surrounds the fetus.
Cells that have sloughed from the fetus are suspended in the fluid. Cells in the sample are cul- tured for about three weeks to raise their numbers to the point where chromosomal and biochemical analyses can be made. Amniocentesis cannot be done until about 16 weeks from the last menstrual period, since the sac containing the embryo is not large enough to permit safe withdrawal of the fluid until this time. See Appendix C, 1967, Jacobson and Barter; chorionic villi sampling, informed consent, ge- netic counseling, prenatal genetic testing. amniocytes cells obtained by amniocentesis (q.v.).
amnion A fluid-filled sac within which the em- bryos of reptiles, birds, and mammals develop. The wall of this sac has a two-layered epithelium. The inner epithelium of the wall is the amnion, although the term is sometimes applied to the whole sac. The outer epithelium is usually called the chorion.
Am- niotic fluid within the sac provides a liquid environ- ment for the embryo. amniote a land-living vertebrate (reptile, bird, or mammal) whose embryos have an amnion and allan- tois. Amoeba proteus a common species of rhizopod; a giant protozoan used for microsurgical nuclear trans- plantations. See Appendix C, 1967, Goldstein and Prescott. amoeboid movement cellular motility involving cytoplasmic streaming into cellular extensions called pseudopodia.
amorphic mutation a mutation in which the al- tered gene product fails in its molecular function. Also called a loss of function mutation or a null muta- tion. AMP adenosine monophosphate. See adenosine phosphate. amphidiploid an organism that is diploid for two genomes, each from a different species; synonymous with allotetraploid. See Appendix C, 1925, Good- speed and Clausen.
amphimixis sexual reproduction resulting in an in- dividual having two parents: synonymous with mixis. Contrast with amixis, apomixis. The adjective form is amphimictic. Amphioxus See Branchiostoma. amphipathic descriptive of a molecule that has distinct polar and nonpolar segments (e.g., mem- brane phospholipids). amphoteric compound (also ampholyte) a sub- stance that can act both as an acid and a base.
Thus a protein is amphoteric because it tends to lose pro- tons on the more alkaline side of its isoelectric point and to gain protons on the acid side of its isoelectric point. ampicillin See penicillin. ampR a selectable gene which encodes the enzyme β-lactamase, which inactivates ampicillin Cells containing a plasmid vector (q.v.) which ex- presses ampR can be selected from those that do not by growth in an ampicillin-containing medium.
See penicillin, R (resistance) plasmid. amplicon a segment of the genome that forms multiple linear copies after exposure of the organism to a compound that inhibits the functioning of a gene in the segment. For example, in mammals the enzyme dihydrofolate reductase (q.v.) is inhibited by methotrexate (q.v.).
Exposure to this inhibitor causes amplification of the DHFR gene. More gener- ally, the term amplicons is used for DNA fragments that have been generated in experiments utilizing the polymerase chain reaction (q.v.). amplification See gene amplification, polymerase chain reaction, RNA amplification. amplified RNA See RNA amplification. amylase an enzyme that hydrolyzes glucosidic bonds in polyglucosans such as glycogen. amyloid-beta-precursor protein (A PP), amyloid- beta-peptides (A Ps), amyloid plaques.
See Alz- heimer disease (AD). amyloplast a starch-rich plastid. amyotrophic lateral sclerosis (ALS) a disease in humans resulting from the degeneration of motor neurons in the lateral columns of the spinal cord. The disease begins with an asymmetric weakness in the limbs and progresses to complete paralysis and death. ALS is sometimes called Lou Gehrig disease after the famous American baseball player who suf-
fered from ALS. In familial cases of ALS, the condi-
Anabolismtion was first shown to be due to mutations in a gene (ALS1) located at 21q22.1 which encoded the en- zyme superoxide dismutase ALS2 was mapped to 2q33, and it encoded a protein (alsin) thought to be a GTPase regulator protein. Fa- milial ALS due to ALS1 is an adult-onset disease, and it shows dominant inheritance; whereas FALS due to ALS2 causes a disease with juvenile onset which shows recessive inheritance.
Two other forms of FALS occur: one caused by mutations in ALS3 at 18q21 and the other by mutations in ALS4 at 9q34. However, the cause of 90 percent of ALS cases (spo- radic or non-familial) is unknown. See Appendix C, 1993, Rosen, Siddique et al. anabolism the metabolic synthesis of complex molecules from simpler precursors, usually requiring the expenditure of energy and specific anabolic en- zymes. Contrast with catabolism. anaerobe a cell that can live without molecular oxygen. A strict anaerobe cannot live in the presence of oxygen.
See Appendix C, 1861, Pasteur. anagenesis phyletic evolution within a single lin- eage without subdivision or splitting; the opposite of cladogenesis. analog a compound related to, but slightly differ- ent structurally from a biologically significant mole- cule, such as an amino acid (see azaserine), a pyrimi- dine or purine (see base analogs), or a hormone (see ZR515). analogous referring to structures or processes that have evolved convergently, as opposed to the term homologous (q.v.). Analogous structures have similar functions but are different in evolutionary origin: e.g., the wing of a butterfly and of a bat. See homoplasy.
analysis of variance a statistical technique that allows the partitioning of the total variation ob- served in an experiment among several statistically independent possible causes of the variation. Among such causes are treatment effects, grouping effects, and experimental errors. Checking the absence of an effect due to the treatment is often the purpose of the inquiry. The statistical test of the hypothesis that the treatment had no effect is the F test, or variance- ratio test. If the ratio of the mean square for treat- ments to the mean square for error exceeds a certain constant that depends on the respective degrees of freedom of the two mean squares at a chosen sig- nificance level, then the treatments are inferred to have been effective. Analysis of variance is particu- larly useful in judging which sources of uncontrolled variation in an experiment need to be allowed for in testing treatment effects.
anamnestic response See immune response, im-munological memory. anaphase See mitosis. anaphase lag delay in the movement of one or more chromosomes from the metaphase plate dur- ing anaphase, often resulting in chromosome loss (q.v.). anaphylaxis a systemic allergic or hypersensitivity response leading to immediate respiratory and/or vascular difficulties. Anas platyrhyncha the mallard duck, ancestor to the domestic or Pekin duck, A.p. domestica. anastomosis the joining of two or more cell pro- cesses or tubular vessels to form a branching system. anastral mitosis the type of mitosis characteristi- cally found in plants.
A spindle forms, but no centri- oles or asters are observed. anautogenous insect an adult female insect that must feed for egg maturation. See autogenous insect. anchorage-dependent cells cells (or in vitro cell cultures) that will grow, survive, or maintain func- tion only when attached to an inert surface such as glass or plastic; also known as substrate-dependent cells.
The only normal animal cells that are designed to survive without attachment and spreading are cells that circulate in the blood. Some tumor cells acquire this ability to be anchorage-independent and leave their original tissue sites to form metastases.
See microcarriers, suspension culture. Anderson disease See Fabry disease. androdioecy a sexual dimorphism in plants having bisexual and separate male individuals. androecious referring to plants having only male flowers. androecium the aggregate of the stamens in a flower. androgen any compound with male sex hormone activity.
In mammals, the most active androgens are synthesized by the interstitial cells of the testis. See testosterone. androgenesis 1. development from a fertilized egg followed by disintegration of the maternal nu- cleus prior to syngamy. The resulting individual pos- sesses only paternal chromosomes and is haploid. 2. production of an embryo having a diploid set of pa- ternal chromosomes by nuclear transfer (q.v.). Com- pare with gynogenesis.