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Word / termDefinition

G0

Phase of cell cycle designating cells that are quiescent and have not yet entered the growth cycle. Normal cells in this phase have exactly one set of chromosome pairs.

G1 (G1 phase)

The first growth phase of the cell cycle, consisting of the portion of interphase before DNA synthesis begins. In this phase of the cell cycle, cells are committed to division.
These cells have the same number of chromosomes and the same amount of DNA as GO phase cells. They are about to enter S phase where they synthesize DNA.

G2 (G2 phase)

The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs. In this phase of the cell cycle, proliferating cells have duplicated their DNA and formed two sets of chromosome pairs, in preparation for division. G2 follows the S phase and precedes the M (mitosis) phase.

Gag reflex

1. A response to tactile input presented to the back of the tongue or pharyngeal area composed of jaw extension, forward / downward tongue movement, and pharyngeal constriction with eye widening and head and neck extension; very strong at birth, reducing in strength by about 7 months of age; persists throughout adulthood.
2. Gagging and vomiting resulting from irritation of the back part of the tongue.

Gage height

the height of the water surface above the gage datum (zero point). Gage height is often used interchangeably with the more general term, stage, although gage height is more appropriate when used with a gage reading.

Gaging station

a site on a stream, lake, reservoir or other body of water where observations and hydrologic data are obtained. The U.S. Geological Survey measures stream discharge at gaging stations.

Gain (of an amplifier)

The amount by which an amplifier boosts a signal.

Gainless amplifier

Amplifier that works without increasing or decreasing the ratio of output current, voltage, or power to input current, voltage, or power.

Gait

A particular pattern or style in which a person walks.

Gait Training

Instruction in walking, with or without equipment; also called "ambulation training."

Galanin

A neuropeptide neurotransmitter whose receptors are found in brain areas responsible for feeding, as well as for learning and memory.  A small molecule drug which blocks the effects of galanin might be useful in reducing the body's appetite for fatty food, as well as modulating acetylcholine, a neurotransmitter implicated in Alzheimer's disease.

Galactorrhea

Leaking of milk from the breast that can occur in both males and females who are not breastfeeding.

Galactose

Monosaccharide occurring in both levo (L) and dextro (D) forms as a constituent of plant and animal oligosaccharides (lactose and raffinose) and polysaccharides (agar and pectin). Galactose is the sugar derived from digesting lactose.

Galactosemia

A metabolic disorder where an infant has difficulty processing lactose (or galactose, a sugar found in milk).  High levels of galactose build up in the blood and tissues, causing liver and kidney disease, blindness, and mental retardation. Galactosemia is a genetic disease for which both parents must be carriers, and at the present time is unpreventable.

Gallstones

Stones that form when substances in the bile harden: There are two types of gallstones - cholesterol stones and pigment stones.
Cholesterol stones account for about 80 % of gallstones in N. America and Europe.
Pigment stones have a high content of bilirubin and account for over 90 percent of gallstones in Asia.
Gallstones can be as small as a grain of sand or as large as a golf ball. There can be just one large stone, hundreds of tiny stones, or any combination.
Gallstones can block the normal flow of bile if they lodge in any of the ducts that carry bile from the liver to the small intestine. That includes the hepatic ducts, which carry bile out of the liver; the cystic duct, which takes bile to and from the gallbladder; and the common bile duct, which takes bile from the cystic and hepatic ducts to the small intestine. Bile trapped in these ducts can cause inflammation in the gallbladder, the ducts, or, rarely, the liver. Other ducts open into the common bile duct, including the pancreatic duct, which carries digestive enzymes out of the pancreas. If a gallstone blocks the opening to that duct, digestive enzymes can become trapped in the pancreas and cause an extremely painful inflammation called gallstone pancreatitis. If any of these ducts remain blocked for a significant period of time, severe damage or infections can occur, affecting the gallbladder, liver, or pancreas.Gallstone attacks often occur after eating a meal, especially a fatty one. Symptoms can include pain for up to several hours in the upper, back, or under the right shoulder together with nausea, vomiting, abdominal bloating or indigestion. These symptoms can mimic those of other problems, including heart attack, so accurate diagnosis is important. Gallstones are more common among women, Native Americans, Mexican Americans, and people who are overweight.
Laparoscopic surgery to remove the gallbladder is the most common treatment. The surgery is called cholecystectomy. Open surgery is done if there are obstacles to laparoscopy. The gallbladder is a dispensable organ. Without it, the bile flows directly into the small intestine, instead of being stored in the gallbladder.

Gamete

Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans). Gametes unite during sexual reproduction to produce a diploid zygote.

Gametophyte

The multicellular haploid form in organisms undergoing alternation of generations; The haploid stage in the life cycle of an organism undergoing alternations of generations; The phase of the life cycle of sexually reproducing plants and algae in which the gametes (egg and sperm) are produced. The gametophyte arises from the germination of a haploid spore. The gametophyte is multicellular and mitotically produces haploid gametes that unite and grow into the sporophyte generation. In plants, the gametophyte nourishes the zygote and young sporophyte

Gamma radiation

Gamma radiation is a type of high energy electromagnetic radiation which is emitted during the disintegration of certain radioactive isotopes such as 125I and 57Co, which are used as tracers in immunoassays. Gamma emitting samples are measured in solid crystal scintillation counters. These solid crystals, are usually thallium activated sodium iodide and are different sizes to accommodate lower and higher energy radioisotopes.

Gamma spasticity

A tonic contraction of one muscle or a group of synergistic muscles at a joint caused by excitation of the gamma motoneurons innervating the muscles

Ganglia (singular = ganglion)

Clusters of neurons or nerve cell bodies.

Gap junction

A type of intercellular junction in animal cells that allows the passage of material or current between cells.

Gap phases

In the cell cycle, the phases that precede (G1) and follow (G2) the synthesis (S) phase in which DNA is replicated. In the G1 phase, the cell doubles in size, and its enzymes, ribosomes, and other cytoplasmic molecules and structures increase in number; in the G2 phase, the replicated chromosomes begin to condense and the structures required for mitosis or meiosis are assembled.

Gas Chromatography - Mass Spectroscopy (GC-MS)

A method that combines the features of gas-liquid chromatography (see Chromatography) and mass spectrometry to identify different substances within a test sample. (see Liquid Chromatography - Mass Spectrometry).
GC/MS is used to identify organic materials such as resins, fats and waxes from their chemical composition. The technique requires a sample to be taken but this can be very small, often no larger than a pin-head. The sample is dissolved and injected into the instrument. The first part of the instrument, the gas chromatograph (GC) is an oven containing a very long, narrow glass tube with a polymer coating, called a column. The sample moves along the column in a stream of helium gas. The different chemical constituents of the sample interact with the polymer coating on the column, some more than others. Thus the different constituents take different lengths of time to reach the column’s end. Each constituent emerges from the column and enters the detector where it is recorded as a peak, the size of which is relative to its abundance in the sample. The graph produced is called a chromatogram.
Chromatogram:  The chromatogram can be used as a 'fingerprint' for the material. However, ancient materials do not always have the same composition as modern ones because they are chemically altered by the effects of aging. In GC/MS the detector is a mass spectrometer (MS). As well as detecting the constituents separated in the GC, the MS enables them to be identified. In the MS the chemical compounds are fragmented into ions. Each compound fragments in a different way. The range of fragmented ions and their relative abundance is recorded as a mass spectrum from which the compound can be identified. Spectrum: By using the mass spectra to identify peaks it is possible to identify mixtures of materials (eg. beeswax and pine resin) and to secure identifications for very degraded ancient materials.
(Source: British Museum)

Gastric

Pertaining to the stomach.

Gastric pits

The folds and grooves into which the stomach lining is arranged.

Gastrin

A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric acids.

Gastroesophageal Reflux Disease (GERD)

The return flow of stomach contents into the esophagus and, sometimes, into the pharynx, often due to incompetence of the lower esophageal sphincter
More commonly GERD is known as Severe "heartburn". Weakness of the valve between the oesophagus and stomach may allow stomach acid to reflux (regurgitate, backup) into the oesophagus and irritate and inflame the lining. This results in chest pain which can mimic that of angina.

Gastroesophageal Reflux scan

A nuclear medicine study to identify if Gastroesophageal Reflux Disease (GERD) is present. Generally conducted over a two-hour period. The child is given a typical-sized feeding which contains a specifically calculated amount of radioisotope. Scanning is conducted for the first hour and during the last half hour. Information processed by the computer and interpreted by the radiologist includes that pertaining to the existence of gastroesophageal reflux and aspiration as well as the calculation of gastric emptying time.

Gastroesophageal sphincter  

A ring of muscle at the junction of the oesophagus and the stomach. A sphincter is a ring of muscle that regulates the movement of substances between two spaces or chambers.  The gastroesophageal sphincter regulates the reflux of substances from the stomach to the oesophagus, remaining closed except during swallowing to prevent the stomach contents from entering the oesophagus

Gastrotomy tube

A semi-permanent tube placed into the stomach for feeding purposes.  It is used to introduce liquids, food, or medication into the stomach when the patient is unable to take these substances by mouth. The tubes are inserted surgically and require hospitalization.

Gastroavscular cavity

The central digestive compartment, usually with a single opening that functions as both mouth and anus.

Gastrula

The two-layered, cup-shaped embryonic stage

Gastrulation

The formation of a gastrula from a blastula; The process in which cells proliferate and migrate within the embryo to transform the inner cell mass of the blastocyst stage into an embryo containing all three primary germ layers.
Occurs after the blastocyst stage of embryonic development.  The inner cell mass of the blastocyst goes through gastrulation, which is essentially a shifting or moving of the cell material of the embryo such that the inner cell mass becomes organized into the three distinct cell layers, called germ layers, the ectoderm, the mesoderm, and the endoderm.  Gastrulation and the formation of the three germinal layers is the beginning of the subdivision of the mass of embryonic cells produced by cleavage. The cells then begin to change and diversify under the direction of the genes. The genes brought in by the sperm exert control for the first time; during cleavage all processes seem to be under control of the maternal genes. In cases of hybridization, in which individuals from different species produce offspring, the influence of the sperm is first apparent at gastrulation: paternal characteristics may appear at this stage; or the embryo may stop developing and die if the paternal genes are incompatible with the egg (as is the case in hybridization between species distantly related). The diversification of cells in the embryo progresses rapidly during and after gastrulation. The visible effect is that the germinal layers become further subdivided into aggregations of cells that assume the rudimentary form of various organs and organ systems of the embryo. Thus the period of gastrulation is followed by the period of organ formation, or organogenesis.

Gate

A boundary that defines a subset or sub-population of events. Gates are set by drawing boundaries around the subsets on data plots (dot plots or histograms).Gates are used in either data acquisition or analysis. Inclusive gates select only the events that fall within (and on) the boundary. Exclusive gates select only the events that fall outside of the boundary.

Gate control theory

The hypothesis put forward by Ronald Melzack and Patrick Wall in the mid 1960s that postulated that the passage of pain (nociceptive) information across the dorsal horn of the spinal cord could be inhibited (“gated”) by inputs from large nerve fibres.
It forms the rationale for using transcutaneous nerve stimulation as a treatment for pain.

Gated ion channel

A transmembrane protein with a central water-filled pore that allows certain ions to pass through from one side of the membrane to the other, and whose opening or closing is controlled (gated). The three mechanisms that gate ion channels are: (a) binding of a specific chemical to a gating site or release of a chemical from a gating site, (b) changes in the transmembrane voltage, or (c) pressure or stress on the membrane.

Gate set

A collection of gates that limits data acquisition or analysis.  Each parameter can only be used once in a gate set.

Gating

1. The active transition of an ion channel from an open to a closed state or from a closed state to an open state.
2. The process of drawing a gate boundary on a dot plot or histogram with arrow keys or a mouse, or of applying a gate set to data. As implied by the common term gate, the concept of gating when applied to an ion channel implies an active (i.e., controlled) change in state from one state (open or closed) to another state (closed or open).

Gauge bosons

Particles that mediate the transfer of energy between other particles: protons, gravitons, W and Z particles.

Gaussian Distribution

A gaussian, or normal, distribution is a symmetrical frequency distribution having a precise mathematical formula relating the mean and standard deviation of the samples.
Gaussian distributions yield bell shaped frequency curves having a preponderance of values around the mean with progressively fewer observations as the curve extends outward.

Gel electrophoresis

The separation of nucleic acids or proteins, on the basis of their size and electrical charge, by measuring their rate of movement through an electrical field in a gel; A method to analyze the size of DNA (or RNA) fragments.
In the presence of an electric field, larger fragments of DNA move through a gel slower than smaller ones. If a sample contains fragments at four different discrete sizes, those four size classes will, when subjected to electrophoresis, all migrate in groups, producing four migrating "bands". Usually, these are visualized by soaking the gel in a dye (ethidium bromide) which makes the DNA fluoresce under UV light.

Gel shift assay (also called gel mobility shift assay (GMSA), band shift assay (BSA), electrophoretic mobility shift assay (EMSA))

A method by which one can determine whether a particular protein preparation contains factors which bind to a particular DNA fragment.
When a radiolabeled DNA fragment is run on a gel, it shows a characteristic mobility. If it is first incubated with a cellular extract of proteins (or with purified protein), any protein-DNA complexes will migrate slower than the naked DNA - a shifted band.

Gene

The fundamental physical and functional unit of heredity, consisting of an ordered sequence of nucleotides (of DNA) located on a specific locus on a chromosome.
Genes are pieces of DNA containing the code for the synthesis of a protein or functional or structural  polypeptide or RNA molecule (the gene product).
A unit of DNA which performs one function and is the element that determines a trait in an organism. Usually, this is equated with the production of one RNA or one protein. A gene contains coding regions, introns, untranslated regions and control regions. Most genes contain coding regions (exons), non-coding sequences (introns), and transcription-control regions.

Gene amplification

The selective synthesis of DNA, which results in multiple copies of a single gene, thereby enhancing expression. The presence of multiple copies of a gene or segment of DNA; a mechanism by which protooncogenes are activated in malignant cells. A tumor cell amplifies, or copies, DNA segments as a result of cell signals or the effects of environmental insults.

Gene bank

The variant forms of each gene are termed alleles.

Gene cloning

The production of multiple copies of a gene.

Gene cluster

A group of closely linked genes that code for a series of enzymes for successive steps in a metabolic pathway. If the gene cluster is controlled by an operator the whole unit is called operon.

Gene expression

The process by which the encoded information of the genome is converted into cellular components, i.e., the process by which genes coded information is converted into the structures present and operating in the cell; to "express" a gene is to cause it to function. A gene which encodes a protein will, when expressed, be transcribed and translated to produce that protein. A gene which encodes an RNA rather than a protein (for example, a ribosomal RNA, rRNA, gene or a transfer RNA, tRNA, gene) will produce that RNA when expressed. The DNA-coding sequences of expressed genes include those that are transcribed into mRNA and then translated into proteins, and RNA that is transcribed from DNA, yet not translated into protein (i.e., transfer and ribosomal RNAs).

Gene expression markers

Refers to molecules (e.g., synthesized due to a specific gene's expression) or consequences (e.g., cell apoptosis due to a specific gene's expression) that can be measured-as-proof of gene's expression in gene expression analysis.

Gene family

A group of closely related genes that make similar products.

Gene flow

  1. A storage facility where germplasm is stored in the form of seeds, pollen, embryos, semen, pollen, or in vitro culture, or in cryogenic storage, or, in the case of a field gene bank, as plants growing in the field.

(2) The loss or gain of alleles from a population due to the emigration or immigration of fertile individuals, or the transfer of gametes, between populations.

Gene mapping

A linear map determining the relative position of genes along a chromosome or plasmid.  Distances are established by linkage analysis and measured in linkage units.

Gene pool

The total aggregate of genes in a population at any one time

Gene product

The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease- causing alleles.

Gene silencing

The suppression of gene expression (e.g., of the gene for polygalacturonase which causes fruit to ripen, of the gene for P34 protein in soybeans, etc.) via a variety of methods (e.g., via RNA interference (RNAi), chemical genetics, effect of certain viruses, "zinc finger proteins", sense or antisense genes, etc.).
Also occurs with some genes in an organism as the organism matures (e.g., from an embryo to a seedling/juvenile).
See also Post-transcriptional Gene Silencing (PTGS).

Generalized Procrustes Analysis

Multivariate statistical analysis method used to classify and analyse sensory data from consumers or descriptive panels.

Gene therapy

The insertion of normal or genetically altered genes into cells through the use of recombinant DNA technology, to correct a genetic defect or a mutation.Gene therapy is usually done to replace defective genes as part of the treatment of genetic disorders.

Genetic code

The system of nucleotide triplets in DNA and RNA that carries genetic information; referred to as a code because it determines the amino acid sequence in the enzymes and other protein molecules synthesized by the organism; The sequence of nucleotides, coded in triplets (codons) along the mRNA, that determines the sequence of amino acids in protein synthesis.
The DNA sequence of a gene can be used to predict the mRNA sequence, and the genetic code can in turn be used to predict the amino acid sequence.

Genetic distance

Exchange of genetic traits between populations by movement of individuals, gametes or spores.

Genetic distancing

A measure of the genetic similarity between any pair of populations. Such distance may be based on phenotypic traits, allele frequencies or DNA sequences.
For example, genetic distance between two populations having the same allele frequencies at a particular locus, and based solely on that locus, is zero. The distance for one locus is maximum when the two populations are fixed for different alleles. When allele frequencies are estimated for many loci, the genetic distance is obtained by averaging over these loci..

Genetic diversity

The collection of the data on phenotypic traits, marker allele frequencies or DNA sequences for two or more populations, and estimation of the genetic distances between each pair of populations. From these distances, the best representation of the relationships among all the populations may be obtained.

Genetic drift

Changes in the gene pool of a small population due to chance. Variation in the genetic composition of individuals within or among species; the heritable genetic variation within and among populations.

Genetic engineering

Discipline that deals with transferring DNA sequences from one organism to another.
The technique of removing, modifying or adding genes to a DNA molecule in order to change the information it contains. By changing this information, genetic engineering changes the type or amount of proteins an organism is capable of producing.

Genetic erosion

Random gene frequency changes in a small population due to chance alone.

Genetic isolation  

The absence of genetic exchange between populations or species as a result of geographic separation or of premating or postmating mechanisms (behavioral, anatomical, or physiological) that prevent reproduction

Genetic maps  

An ordered list of genetic loci (genes or other genetic markers) along a chromosome; Diagrams showing the order of and distance between genes. Genetic maps are constructed using crossover information.

Genetic marker

Any genetically derived phenotypic difference used in the analysis of inheritance patterns or to differentiate between types of cells.  An observable site on a chromosome that is heritable and can be either a genetically expressed region or noncoding segment of DNA (intron).

Genetic modification

The modern biotechnology used to alter genetic material of living cells or organisms in order to make them capable of producing new substances or performing new functions. Genetic modification occurs at least through the use of the following techniques: (i) recombinant DNA techniques using vector systems; (ii) techniques involving the direct introduction into an organism of heritable material prepared outside the organism including micro-injection, macro-injection and micro-encapsulation; (iii) cell fusion (including protoplast fusion) or hybridization techniques where live cells with new combinations of heritable genetic material are formed through the fusion of two or more cells by means of methods that do not occur naturally.

Genetically modified organisms

Any organism in which the genetic material has been modified, using modern biotechnology, in a way that does not occur naturally, either through mating or natural recombination. The techniques which may be prescribed for such purposes include (a) any technique for the modification of any genes or other genetic material by recombination, insertion or deletion of, or of any component parts of, that material from its previously occurring state, and (b) any other technique for modifying genes or other genetic material which in the opinion of the Secretary of State would produce organisms which should be treated as having been genetically modified, but not include techniques which involve no more than, or no more than the assistance of, naturally occurring processes of reproduction (including selective breeding techniques or in vitro fertilization).

Genetic mutation

An alteration in the nucleotide sequence of a DNA molecule; often from one allelic form of a gene to another allele alternative.

Genetic recombination

The general term for the production of offspring that combine traits of the two parents.

Genetic screening

A search in a population for persons possessing certain genotypes (genes transmitted from parents to offspring) that are (1) already associated with disease or predisposed to disease, (2) may lead to disease in their descendants, or (3) produce other variations not known to be associated with disease.

Genital herpes  

A sexually transmitted disease caused by the herpes virus; results in sores on the mucus membranes of the mouth or genitals.

Genogram

A visual representation of the life cycle of a family used as a means of understanding family relationships and changes.

Genome

The total complement of genetic material in a cell or person, found physically dispersed amongst the chromosomes.  Since the genome is the full set of genes or genetic information of an organism, it includes both coding and non-coding sequences. See Proteome.
Mammalian genomic DNA (including that of humans) contains 6x109 base pairs of DNA per diploid cell. There are somewhere in the order of a hundred thousand genes, including coding regions, 5' and 3' untranslated regions, introns, 5' and 3' flanking DNA. Also present in the genome are structural segments such as telomeric and centromeric DNAs and replication origins, and intergenic DNA.

Genomic blot

A type of Southern blot specifically used to analyze a mixture of DNA fragments derived from total genomic DNA.  Because genomic DNA is very complicated, when it has been digested with restriction enzymes, it produces a complex set of fragments ranging from tens of bp to tens of thousands of bp. However, any specific gene will be reproducibly found on only one or a few specific fragments. A million identical cells will produce a million identical restriction fragments for any given gene, so probing a genomic Southern with a gene-specific probe will produce a pattern of perhaps one or just a few bands.

Genomic clone

A piece of DNA taken from the genome of a cell or animal, and spliced into a bacteriophage or other cloning vector. A genomic clone may contain coding regions, exons, introns, 5' flanking regions, 5' untranslated regions, 3' flanking regions, 3' untranslated regions, or it may contain none of these...it may only contain intergenic DNA (usually not a desired outcome of a cloning experiment!).

Genomic imprinting

The parental effect on gene expression. Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.

Genomic library

A set of thousands of DNA segments from a genome, each carried by a plasmid, phage, or other cloning vector. A collection of clones made from a set of randomly generated overlapping DNA fragments representing the entire genome of an organism.

Genomics

The study and understanding of genes and their implementation in the development of healthcare products and services.

Genotype  

The genetic (alleleic) makeup of an organism with regard to an observed trait. Genes transmitted from parents to offspring. The entire genetic constitution of an organism, or the genetic composition at a specific gene locus or set of loci
Contrast to phenotype. Two uses: one is a verb, the other a noun. To 'genotype' (verb) is to example polymorphisms (e.g. RFLPs, microsatellites, SNPs) present in a sample of DNA. You might be looking for linkage between a microsatellite marker and an unknown disease gene. With such information, you can infer the chromosomal location of the unknown gene, and can sometimes identify the gene.
As a noun, a 'genotype' is the result of a genotyping experiment, e.g., a SNP or microsat.

Genus (plural genera)

A grouping of one or several species that possess common characteristics. A classification above species and below family

Geometric mean

The geometric mean of two numbers is the square root of the product of the numbers.  The geometric mean of n numbers is the nth root of the product of the numbers.

Geometric sequence

A sequence of numbers of the form   a, ar, ar2, ar3,....., arn-1.

Geometric series

The sum of a geometric sequence.

Gerdy’s tubercle

A bony projection on the anterolateral tibial plateau between the patellar tendon and fibular head that is used as a landmark for examining or operating on the knee.

Germ cells

Gametes or the cells that give rise to gametes

Germ layers

The three embryonic cell layers or germinal layers: the outer layer is the ectoderm, the middle layer is the mesoderm, and the innermost layer is the endoderm (entoderm).
After the blastocyst stage of embryonic development, the inner cell mass of the blastocyst goes through gastrulation, which is essentially a shifting or moving of the cell material of the embryo such that the inner cell mass becomes organized into three distinct cell layers, called germ layers. The three layers are the ectoderm, the mesoderm, and the endoderm. The ectoderm gives rise to the skin covering, to the nervous system, and to the sense organs. The mesoderm produces the muscles, excretory organs, circulatory organs, sex organs (gonads), and internal skeleton. The endoderm lines the alimentary canal and gives rise to the organs associated with digestion and, in chordates, with breathing.

Germplasm

Genetic material, especially its specific molecular and chemical constitution, that comprises the physical basis of the inherited qualities of an organism.

Gestation

Period of time between fertilization and birth of an animal.

Gestural systems

Ways of communicating by using natural gestures; behavior showing likes, dislikes, choices, and touch cues or tactile prompts.

Geyser

a geothermal feature of the Earth where there is an opening in the surface that contains superheated water that periodically erupts in a shower of water and steam.

Giardiasis

A disease that results from an infection by the protozoan parasite Giardia Intestinalis, caused by drinking water that is either not filtered or not chlorinated.  The disorder is more prevalent in children than in adults and is characterized by abdominal discomfort, nausea, and alternating constipation and diarrhea. a disease that results from an infection by the protozoan parasite Giardia Intestinalis, caused by drinking water that is either not filtered or not chlorinated. The disorder is more prevalent in children than in adults and is characterized by abdominal discomfort, nausea, and alternating constipation and diarrhea.

Gingival hyperplasia

Excessive proliferation of gum tissue.

Glacier

a huge mass of ice, formed on land by the compaction and recrystallization of snow, that moves very slowly downslope or outward due to its own weight.

Gland

A structure composed of modified epithelial cells specialized to produce one or more secretions that are discharged to the outside of the gland.

Glasgow Coma Scale

A standardized system used to assess the degree of brain impairment and to identify the seriousness of injury in relation to outcome. The system involves three determinants: eye opening, verbal responses and motor response all of which are evaluated independently according to a numerical value that indicates the level of consciousness and degree of dysfunction. Scores run from a high of 15 to a low of 3. Persons are considered to have experienced a `mild' brain injury when their score is 13 to 15. A score of 9 to 12 is considered to reflect a `moderate' brain injury and a score of 8 or less reflects a 'severe' brain injury.

Glaucoma

A disorder in the eye characterized by high pressure inside the eyeball. Glaucoma is characterized by the slow, progressive degeneration of retinal ganglion cells, resulting in the loss of axons that make up the optic disc (producing optic disc cupping) and a concomitant pattern of visual field loss. Although the mechanisms underlying retinal degeneration in glaucoma are complex, elevated intraocular pressure (IOP) resulting in vascular insufficiency and HI (particularly at the optic nerve head) is considered a primary cause. Human risk factors for the development of glaucoma include chronic RGC hypoxia/ischemia (HI) due to elevated IOP and homocysteinaemia, which can be caused by a number of lifestyle, genetic or disease conditions. These factors can increase reactive oxidative and nitrosative stress (RONS) in the retina, leading to inhibition of methionine synthase. Methionine synthase is essential for the remethylation of homocysteine to methionine and the regeneration of tetrahydrofolate (which is required for DNA synthesis). This key perturbation in one carbon metabolism could result in methylation stress caused by decreased concentrations of SAMe and increased levels of the potent methyltransferase inhibitor, S-adenosylhomocysteine (SAH). Deleterious downstream sequelae from methionine synthase inhibition include decreased creatine and glutathione synthesis in Müller cells, down-regulation of creatine transporter (CRT-1) activity and expression in RGCs, and escalation of RONS. Mitochondrial DNA damage caused by RONS would be exacerbated by a reduced capacity for DNA synthesis and repair, and may ultimately decrease the mitochondrial density in ganglion cells. A subsequent reduction in mitochondrial energy generation coupled with impaired energy buffering by creatine kinase in ganglion cells may be a primary cause of ganglion cell dysfunction and degeneration in glaucoma.

Glial cell (or Neuroglial cell)

A cell type of the nervous system. There are about 5 different types of glial cells and their primary role (as indicated by the fact that glial comes from the Latin for “glue”) is to create an ideal operating environment for neurons. These cells do not conduct information in the form of Action Potentials. Instead they create an ideal operating environment for neurons by carrying out different functions including providing support, insulation, and protection for the neurons (each function by a different type of glial cell).

Global optimum

The best possible solution to a problem.
See Local optimum

Global Positioning System

A system used to determine latitude, longitude, and elevation anywhere on or above the Earth's surface. Location is obtained by simultaneously determining position relative to a number of specialized satellites using radio signals. Abbreviated to GPS

Globular protein

A polypeptide chain folded into a roughly spherical shape.

Glomerular filtration rate

The amount of plasma filtered by the kidney each minute

Glomerulus

A tangle of capillaries that makes up part of the nephron. The glomerulus is the site of filtration of the plasma. It is made up of a ball of capillaries surrounded by Bowman's capsule in the nephron and serves as the site of filtration in the vertebrate kidney

Glucagon

A peptide hormone secreted by pancreatic endocrine cells that raises blood glucose levels. Acts antagonistically (in opposition) to insulin.

Glucogenesis

Synthesis of glucose from non-carbohydrate sources such as fats or proteins. This occurs when the  glycogen supply in the liver is exhausted.

Glucocorticoid

A corticosteroid hormone secreted by the adrenal cortex that influences glucose metabolism and immune function.

Glucocorticoid Response Element (GRE)

A binding site in a promoter to which the activated glucocorticoid receptor can bind.
The glucocorticoid receptor is essentially a transcription factor which is activated only in the presence of glucocorticoids. The activated receptor will bind to a GRE, and transcription of the adjacent gene will be altered. See Response element.

Glucose

A six-carbon sugar (C6H12O6); the most common monosaccharide in animals.
Monosaccharide sugar originating in plants and used in animals to carry energy throughout the body, just as sucrose is the main carrier of energy throughout the plant.

Glycemic index (GI)

A measure of the rate at which glucose from different forms of carbohydrates enter the bloodstream (i.e., ranks foods on how they affect blood glucose levels in the two or three hours after eating.)Carbohydrates that break down rapidly during digestion releasing glucose rapidly into the bloodstream have a high GI and increase blood sugar rapidly. Carbohydrates that break down slowly, releasing glucose gradually into the bloodstream, have a low GI and produce a lower more constant insulin response; they tend to possess GI numbers under 60.

Glycerin

Syrupy type of alcohol derived from sugar which is used in food flavourings to maintain desired food consistency.

Glycerol

Colourless, odourless, syrupy liquid chemically, an alcohol, that is obtained from fats and oils and used to retain moisture and add sweetness to foods.
A three-carbon molecule with three hydroxyl () groups attached; a glycerol molecule can combine with three fatty acid molecules to form a fat or an oil.

Glycocalyx

A fuzzy coat on the outside of animal cells, made of sticky oligosaccharides.

Glycogen

An extensively branched glucose storage polysaccharide; the animal equivalent of starch. Like starch, this is a polysaccharide of glucose, but found in animals instead of plants. This is the form in which glucose is stored in muscles and in the liver.

Glycolipids

Organic molecules similar in structure to fats, but in which a short carbohydrate chain rather than a fatty acid is attached to the third carbon of the glycerol molecule.
As a result of its structure, the molecule has a hydrophilic "head" and a hydrophobic "tail." Glycolipids are important constituents of the plasma membrane and of organelle membranes.

Glycolysis

The cell’s metabolic process in which glucose is split into two pyruvate molecules.
Glycolysis is the one metabolic pathway that occurs in all living cells, serving as the starting point for fermentation or aerobic respiration. It is the cellular metabolic process used universally to produce ATP and NADH (which are the cell’s energy stores).

Glycoprotein

A protein with covalently attached carbohydrate.

Glycosuria

An increased concentration of glucose in the urine.

Goiter

Enlargement of the thyroid gland which is located in the front of the neck, behind the trachea.

Goldman equation (or Goldman-Hodgkin-Katz (GHK) equation or “constant field” equation)

An equation for calculating the electrical potential difference across a cell membrane.
This equation relates the trans-membrane potential difference to the concentrations of the permeating ions (i.e., those ions that can diffuse across the cell membrane) on the two sides of the membrane and to the permeability constants of each of those ions:

where V = voltage difference, R = Universal gas constant, P = permeability coefficient of the ion, F = Faraday’s constant, Co = concentration on one side of the membrane, Ci = concentration on other side of the membrane, o = outside, I = inside..

Typical values for P are:
PK+ = 1 x 10 -7 cm / sec
PCl- = 1 x 10 -8 cm / sec
PNa+ = 1 x 10 -8 cm / sec

(see also Resting Membrane Potential, Nernst equation, and Equilibrium potential)

Golgi apparatus

Eukaryotic cell organelle which packages cell products, such as enzymes and hormones, and coordinate their transport to the outside of the cell. An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum, for transport to other parts of the cell or export from the cell

Gonadotropes

Cells in the anterior pituitary that synthesize Lutenizing Hormone (LH) and Follicle Stimulating Hormone (FSH).  The release of LH and FSH by the gonadotropes is controlled by Gonadotropin releasing hormone (GnRH).

Gonadotropins

Hormones that stimulate the activities of the testes and ovaries; a collective term for the peptide hormones Lutenizing Hormone (LH) and Follicle Stimulating Hormone (FSH). These hormones are released by gonadotropes in the anterior pituitary, and act on the gonads to stimulate them, thereby affecting reproduction

Gonadotropin release inhibiting hormone (GnIH)

A peptide hormone responsible for blocking the release of gonadotropins from the anterior pituitary. GnIH is synthesized and released by the hypothalamus.

Gonadotropin releasing hormone (GnRH)

A peptide hormone responsible for the release of gonadotropins from the anterior pituitary, affecting reproduction. GnRH is also known as Luteinizing-hormone releasing hormone (LHRH). It is synthesized and released by the hypothalamus.

Gonads

The male and female sex organs. The gamete-producing organs in most animals.

Gonimoblast

Tissue derived from the the fertilized nucleus of some red algae, equivalent to the carposporophyte. It may develop from the carpogonium or an auxiliary cell, and it ultimately produces carposporangia by mitosis

Gonorrhea  

A sexually transmitted disease. It is caused by a bacterium that inflames and damages epithelial cells of the reproductive system

Gout

Condition characterized by abnormally elevated levels of uric acid in the blood, recurring attacks of joint inflammation (arthritis ), deposits of hard lumps of uric acid in and around the joints, and decreased kidney function and kidney stones.
Uric acid is a breakdown product of purines, that are part of many foods we eat. The tendency to develop gout and elevated blood uric acid level (hyperuricemia) is often inherited and can be promoted by obesity , weight gain, alcohol intake, high blood pressure , abnormal kidney function, and drugs. The most reliable diagnostic test for gout is the identification of crystals in joints, body fluids and tissues.

G-proteins

A heterotrimeric GTP-binding protein that relays signals from a plasma-membrane signal receptor, known as a G-protein linked/coupled receptor, to other signal-transduction proteins inside the cell.When a G-protein linked/coupled receptor is activated, it in turn activates the G protein, causing it to bind a molecule of GTP in place of GDP. This causes the Gα subunit to separate from the Gβ/γ complex. 18 Gα subunits exist as 4 main families based on structural and functional homologies: Gαi, Gαs, Gαq, Gα12. Each family signals through one of 3 main effectors, adenylyl cyclase (AC, Gαi and Gαs), phospholipase Cβ (PLC, Gαq) and rho kinase (Gα12).
The precise actions of each of the G subunits (which have multiple members) may be tissue dependent and is further regulated by a range of other proteins including ‘regulators of G-protein signalling’ (RGS) or ‘activators of G-protein signalling’ (AGS). Gβ/γ complexes mediate a wide range of effects; they lack specific inhibitors. Some Gβ/γ effectors include PLC and AC are current targets. ERK1/2 is commonly acvitated by Gα protein signalling causing its phosphorylation (G34). pERK1/2 has many targets including tyrosine hydroxylase (TH) or phosopholipase A2. The name is short for guanine nucleotide binding protein. They are a class of membrane proteins which link the activation of a 7-helical receptor to the subsequent activation of its second messenger. They are heterotrimeric proteins, i.e. they have three different subunits (an alpha- subunit, and the beta- and gamma- subunits which are tightly bound to each other). 
More than 100 examples of at least 20 different types of G-protein have so far been cloned, of which many can be grouped into categories depending on which second messenger they predominantly control. Thus, Gs frequently activates adenylyl cyclase, whereas Gi inhibits it; Gq mediates the stimulation of phospholipase C and hence phosphoinositide turnover. However, these distinctions are not absolute: many G proteins do not necessarily couple to the same second messenger in different cells, and they may even couple to more than one in a single cell. 
Gs protein is irreversibly activated by cholera toxin. Subtypes of Gi protein have been identified, some of which are irreversibly inactivated by pertussis toxin. In both instances by ADP-ribosylation of the alpha-subunit.

G-protein coupled receptors

A signal receptor protein in the plasma membrane that responds to the binding of a signal molecule by activating a G protein. These are seven transmembrane domain receptors that upon ligand binding interact with heterotrimeric GTP binding proteins (G-proteins).
Binding of ligand to these receptors causes the receptors to interact with heterotrimeric GTP binding proteins (G-proteins).  As decribed above, hydrolysis of the bound GTP to GDP inactivates the G protein.

Graded potentials

A local voltage change in a neuron membrane induced by stimulation of a neuron, with strength proportional to the strength of the stimulus and lasting about a millisecond.

Gradualism

A view of Earth's history that attributes profound change to the cumulative product of slow but continuous processes.
A model of evolution that assumes slow, steady rates of change. Charles Darwin's original concept of evolution by natural selection assumed gradualism. Contrast with punctuated equilibrium.

Gram stain

A staining method that distinguishes between two different kinds of bacterial cell walls.

Gram-negative or Gram-positive

The classification given to bacteria according to their staining properties as defined by the Gram stain procedure.

Grand mal seizure / attacks

Epileptic seizures characterized by myoclonus, progressive myoclonus, loss of erect posture, and unconsciousness.
The seizures involve a sudden loss of consciousness followed immediately by a generalized convulsion.

Granule

Solid inclusions in cells or items adhering to the surface of cells. Usually refractile, in that they may look bright when viewed with the microscope.

Granumloma

A tumor that occurs secondary to trauma such as that which may be caused by intubation. If a tube is forced through the glottis, the surrounding membranes of the vocal cords can become torn. The tissue can then become a large tumor which may cause aphonia, a voice disorder, and possible obstruction of breathing. These tumors must be surgically removed.

Graph

The graph of an equation is the set of points that make the equation true.

Graves' disease

The most common form of hyperthyroidism.  Occurs when the immune system attacks thyroid gland and causes it to overproduce the hormone thyroxine.

Gravida

The number of times a woman has been pregnant.

Gravity

The attractive central gravitational force exerted by a celestial body such as earth.

Gray units

Unit that measures the radiation dose (Gy). International health and safety authorities have endorsed the safety of irradiation for all foods up to a dose level of 10,000 Gy (10 kGy). One gray equals one joule of energy absorbed per kilogram of food being irradiated.

Great circle

The circle formed by the intersection of a plane passing through the center of a  sphere

Greatest common factor

The greatest common factor of two numbers, and b, is the largest  number that divides both and b evenly.

Grey water

wastewater from clothes washing machines, showers, bathtubs, hand washing, lavatories and sinks.

ground water

1) water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper surface of the saturate zone is called the water table. (2) Water stored underground in rock crevices and in the pores of geologic materials that make up the Earth's crust.

ground water confined

ground water under pressure significantly greater than atmospheric, with its upper limit the bottom of a bed with hydraulic conductivity distinctly lower than that of the material in which the confined water occurs.

ground water recharge

inflow of water to a ground-water reservoir from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Also, the volume of water added by this process.

ground water unconfined

water in an aquifer that has a water table that is exposed to the atmosphere.

Group action

The action produced at a joint by a group of muscles working together.
A muscle in the group may play the role of agonist, antagonist, synergist, or fixator in order to perform the action.
Example:
group action: flex forearm at elbow;
agonist: biceps brachii;
antagonist: triceps brachii;
synergist: brachialis, brachioradialis;
fixator: deltoid, pectoralis major, etc

Group translocation systems

A sequence of molecules used to move a molecule and transform it in the process.
It is also one of the three forms of Primary (active) transport systems. It involves 5 proteins that are irreversible & saturable. No ATP is involved but a high energy Phosphate group is supplied by PEP. It is shuttled from enzyme I to the transporter, and then to glucose. The substrate (glucose here) gets modified and is trapped inside cell. eg., Glucose transport system of E. coli

Growth factor

A protein that must be present in the extracellular environment (culture medium or animal body) for the growth and normal development of certain types of cells.

Growth phase (of cells)

During the growth phase, cells grow exponentially and at a constant rate.
The maximum slope of the curve is the specific growth rate of the organism. Cell growth is dependent upon the current environment (nutrients, temperature, pH, etc.), but is not dependent upon the previous physiological state. In the field of predictive microbiology, growth rate is commonly expressed as the change in cell number per time interval.

Guaiac Test (Hemoccult, Fecult)

A chemical test to identify blood in the stool (feces).

Guanine

A purine  nitrogenous base, one member of the base pair G- C (guanine and cytosine).

Guar gum

Substance made from the endosperm of seeds of the guar plant which acts as a stabilizer in food systems. Is found as a food additive in cheese, including processed cheese, ice cream and dressings. Provides products with high viscosities.

Guillain-Barré syndrome (GBS)

A reversible condition that affects the nerves in the body. GBS can result in muscle weakness, pain, and even temporary paralysis of the facial, chest, and leg muscles. Paralysis of the chest muscles can lead to breathing problems.

Gynecomastia

The development of abnormally large mammary glands (breast tissue) in males resulting in breast enlargement.

Gyrencephalic

When the cerebral cortex is highly folded and convuluted (due to gyri and sulci)

Gyrus (plural gyri)

The convolutions (creases) of the cortex of the brain.