gestation period in a viviparous animal, the time from conception to birth. The average gestation pe- riods in days for representative mammals are mouse 19, rat 21, rabbit 31, dog 61, cat 63, chimpanzee 238, woman 267, cow 284, mare 340, and elephant 624. GFP green fluorescent protein (q.v.). GH, GHIF, GHRH growth hormone, growth hor- mone inhibiting factor, growth hormone releasing hormone. See human growth hormone.
Giardia lamblia a species of protoctists times called G. intestinalis) that is the most common flagellated protozoan found in the human digestive tract and is often a cause of diarrhea in hikers. Giar- dia was first described about 1681 by van Leeuwen- hoek. The parasite is tear-drop shaped, about 12 µm long, and is binucleate. The stationary phase tropho- zoites arrest in the G2 phase with a ploidy of 8N (2 nuclei, each with 4N ploidy). The haploid number of chromosomes is 5 and the C value is 10-12 Mb. Each cell has 4 pairs of flagella, but lacks conven- tional mitochondria. However, the cells each con- tain several dozen mitosomes (q.v.).
These provide clusters of iron and sulfur atoms that the Giardia need to synthesize ATP (q.v.). Giardia trophozoites form a Golgi apparatus as they differentiate into cysts, but no Golgi can be identified within non- cysting cells. Comparative analyses of the 16 S-like RNAs of Giardia and several other eukaryotes show that Giardia represents the earliest diverging lineage in the eukaryotic line of descent yet encountered. See Appendix A, Protoctista, Archaeprotista; Appen- dix C, 1677, van Leeuwenhoek; Archaeprotista, ribo- somes. gibberellin one of a family of phytohormones of widespread distribution in plants. Many single-gene, dwarf mutants in Pisum, Vicia, and Phaseolus are cured by the application of gibberellins. Gibberellins also promote seed germination, the breaking of dor- mancy, and flowering.
In maize, hybrids contain higher concentrations of gibberellins than their ho- mozygous parents. This suggests that heterosis (q.v.) has a phytohormonal basis. gibbon See Hylobates. gigabase a unit of length for DNA molecules, consisting of one billion nucleotides; abbreviated gb, or gbp for gigabase pairs. The genomes for Homo sa- piens and Pisum sativum are 3.2 and 4.1 gigabases, respectively. See base pair, megabase. gland an organ that synthesizes specific chemical compounds (secretions) that are passed to the out- side of the gland. See endocrine system, exocrine. glaucocystophytes a group of protoctists that are able to live photoautotrophically with the aid of cya- nelles (q.v.). Glaucocystophytes are all freshwater organisms that are rarely seen in nature.
The most frequently encountered species belong to the genera Glaucocytis (see illustration on page 183) and Cya- nospora. C. paradoxa has provided most of the data on cyanelle DNA. These species are of interest be- cause of the support they give to the theory of the evolution of eukaryotic cells by symbiogenesis. See Appendix A, Protoctista, Glaucocystophyta; serial symbiosis theory. glaucophytes synonym for glaucocystophytes (q.v.). gln glutamine. See amino acid. globins a widespread group of respiratory proteins including the tetrameric hemoglobins of protochor- dates and various invertebrates, the monomeric my- oglobins, and the leghemoglobins. In the case of myoglobins and the vertebrate hemoglobins, phylo- genetic trees have been constructed showing that the ʦ
Glaucocystis nostochinearum. C, cyanelle; G, Golgi; M, mitochondrion with tubular cristae; N, nucleus; R, endoplasmic reticulum; S, starch granule; U, paired, short undulipodia; V, vacuole; Z, cell wall.
globin sequences have diverged during evolution at a constant rate. From these data, the timing of the gene duplications that gave rise to the family of he- moglobins has been deduced. See hemoglobin genes, myoglobin gene. globulin any of certain proteins that are insoluble in distilled water, but soluble in dilute aqueous salt solution. See albumin, immunoglobulin. Glossina a genus of viviparous dipterans that serve as vectors of trypanosomes. G. morsitans is the Afri- can tsetse fly. See Trypanosoma. glu glutamate; glutamic acid. See amino acid. glucagon a polypeptide hormone from the alpha cells of the pancreas that promotes the breakdown of liver glycogen and the consequent elevation of blood glucose. glucocerebroside any of certain compounds re- lated to sphingomyelins.
They differ in that the phosphorylcholine seen in sphingomyelin (q.v.) is replaced by glucose. See cerebroside, Gaucher dis- ease. glucocorticoids steroid hormones produced by the adrenal cortex having effects on intermediary metabolism, such as stimulating glycogen deposition by the liver. Some glucocorticoids, such as cortisone, produce anti-inflammatory effects. gluconeogenesis the formulation of glucose or other carbohydrates such as glycogen (glyconeogen- esis) from noncarbohydrate precursors such as glyco- genic amino acids, lactate, and Krebs TCA cycle in- termediates. Gluconeogenesis occurs in mammalian liver under conditions such as starvation or low car- bohydrate intake. glucose a six-carbon sugar widely distributed in plants, animals, and microorganisms.
In aqueous so- lution, less than 1% of the glucose molecules are in an open chain form, the majority being in a cyclic form. The C1 aldehyde in the linear glucose mole- cule reacts with the C5 hydroxyl group, resulting in the formation of the glucose ring. The structure on page 184 represents the chair conformation of β-D- glucose, the predominant form of glucose. The heavy lines in the ring are closest to the reader. This structure shows that the glucose ring is not planar; rather, C1 and C4 atoms lie either above or below the average plane defined by the C2, C3, C5, and O5 atoms of the ring.
The labels α and β specify that the hydroxyl group attached to C1 is below or above the plane of the ring, respectively. The α and β forms of glucose interconvert rapidly by way of the open-chain. The D designation indicates that glu- cose is dextrorotary, i.e., it rotates polarized light to the right. For this reason, glucose is also known as dextrose. Glucose is a basic monosaccharide unit in some disaccharides (e.g., maltose, lactose, and su- À