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Word / termDefinition

Dalton

Measure of molecular weight or mass; the atomic mass unit; a measure of mass for atoms and subatomic particles. One hydrogen atom has mass of 1 Da. Proteins and other macromolecule molecular weights are usually measured in kDa or kD (kiloDaltons) - 1000 Da.

Daughter cell

A cell that is the offspring of a cell that has undergone mitosis or meiosis.

Darwinian fitness

A measure of the relative contribution of an individual to the gene pool of the next generation. The true measure of evolutionary change of an organism. Darwinian fitness refers to the numerical advantage of having offspring. The individual with the most offspring has the higher fitness. The reasons can be chance or natural selection and are not important to measure fitness. It is often equated with survival of the fittest, which is often meant to be the strongest or best adapted individual. However, this interpretation is wrong, if it does not explain why a certain individual has the most offspring. Overall, the genetic variation of the individuals with the most offspring will dominate the gene pool of a population. The change in genetic variability in a population from generation to generation is the true measure of (micro-)evolution.

DAT

Digital audio tape. A high density storage medium.

Data compression

Large amounts of data can create enormous problems in storage and transmission. Data compression is the term to describe a variety of techniques to compress the volume of data, e.g., to represent large images with as few bits as possible, but to do so with minimum loss of information, according to some fidelity criterion
The widespread, consumer-market use of information in the form of images has contributed much to the development of data compression techniques. All image compression techniques try to get rid of the inherent redundancy, which may be spatial (neighbouring pixels), spectral (pixels in different spectral bands in a colour image) or temporal (correlated images in a sequence, e.g. television).
There are lossless methods, which are reversible, viz. do not sacrifice any information, and lossy methods which may be used if the quality of a compression-decompression sequence is judged by general criteria, like unchanged quality for the human visual system. Note that in image processing jargon, ``lossless'' is sometimes used in the sense of ``no visible loss''.
Examples of lossless methods are run-length codingHuffman coding , or the Lempel-Ziv-Welsh (LZW) method. In run-length coding one replaces runs , sequences of equal greyvalues, by their lengths and the greyvalues. Huffman and LZW coding are approximations to entropy encoding, i.e. frequently used sequences are replaced by short codes, rare sequences by longer codes. In Huffman coding, sequences are single greyvalues, for LZW they are strings of greyvalues.
Of the many lossy coding techniques the simplest may be thresholding, applicable in some situations; the most important ones are predictive and transform coding. In predictive coding, one removes the correlation between neighbouring pixels locally, and quantizes only the difference between the value of a sample and a predicted value (Quantization). Transform coding decorrelates the whole signal, e.g. a block of 8x8 pixels in an image, as a unit, and then quantizes the transform coefficients, viz one sets insignificant coefficients to zero. Only complete sets of unitary transforms are considered, i.e. transforms with the property of equal energy in the spatial domain and in the transform domain. This compression works well if the energy is clustered in a few transform samples. One talks of zonal coding, if certain coefficients are systematically set to zero (e.g. frequencies in the Fourier domain), and of adaptive coding, if coefficients are set to zero according to some threshold criterion of significance ( e.g. rank reduction in principal component analysis)
The following sets of unitary transforms are usually described in the literature:

  • Karhunen-Loeve or principal component analysis,
  • Discrete cosine transform,
  • Fourier transform,
  • Hadamard transform,
  • Slant transform,
  • Haar transform.

They are listed above in order of decreasing energy compaction and computer time used. The popular JPEG algorithm for compression of colour images uses essentially the discrete cosine transform (DCT), followed by quantization and Huffman coding (JPEG, short for the original committee ``Joint Photographic Experts Group'', is a widely used compression standard for still images).

Data normalization

A scaling process for numbers in a data array
In normalization, typically data points in a set are divided by a reference value, to reduce any systematic error in data. Normalization is used where there is such great spread in the data as to render difficult any standard statistical analysis. The transformation of the data results in a new distribution of numbers that are more homogeneously spread within a defined range.
The need for data normalization is determined by the user and depends on the application. Data are often normalized before any application processing and therefore data normalization is usually referred to as data pre-processing.
The purpose of data normalization depends on the proposed data application and methodological approach.  It includes the use of linear scaling to compress a large dynamic range, scaling the values to correct for large variations or for handling obscuring variations, removing systematic errors in data, or efficiently removing redundancy in non-linear models.
Some data normalization methods include:
Mean-corrected data (Covariance matrix): The mean value is subtracted from each data point to produce a new array of normalized data.
Normalization by a maximum value: In this procedure, all un-normalized data are scaled so that they range between zero and one, with the maximum data point scaled to be one and the minimum to be zero and the other data points distributed between these end points.
Normalization by standard deviation: Each data point is normalized by dividing the standard deviation of the data set.
Logarithmic normalization: Each data point is divided by the mean value for that data set. Then the logarithm to base-two (log2) is calculated for that resultant value. The consequence of this logarithmic transformation is that there is a decrease in the variance of values as the transformation reduces the high values faster than the low ones.
Unit total probability mass normalization (Total intensity normalization): Each data point is divided by the sum of all data points, and then multiplied by the mean for that data set.
Standardizing data: Dividing the mean subtracted data (mean-corrected data; see above) by the standard deviation of the data set.

Dataset

The term dataset or "data set" is used in specific ways in different contexts.

  • ICPSR defines a dataset as "a collection of data records" and uses this term to encompass a file or group of files associated with one part of a study. Files associated with a dataset might include a data file, a machine- readable codebook , SPSS control cards, and other files related to the data file. Examples: The files associated with California might be considered one dataset in the 1990 Census of Population and Housing STF 1A study; the files associated with the First Congress, House of Representatives, in the study "Congressional Roll Call Voting Records."

SAS. In the SAS statistical software, a SAS "data set" is the internal representation of data. Raw data when read by SAS command statements is converted into a SAS Data Set before SAS can use the data. SAS Data Sets have specific filename extensions for different operating systems; e.g., a SAS 6.12 Data Set created in Unix has the filename extension ".ssd01" and in Windows, ".sd2".

Dauermodification

The persistence for several generations of an environmentally induced trait.

Daughter cells

The products of cell division of protists.

Death phase

Occurs when cells are being inactivated or killed because conditions no longer support growth or survival. Some environmental factors such as temperature can cause acute inactivation. Others may cause mild inactivation as with growth in the presence of organic acids.

Decay constant

A constant unique to each radioactive nuclide which represents the proportion of the atoms in a sample of that radionuclide undergoing decay in unit time. The decay constant of a nuclide is related to the nuclide's half-life.

Decay series

Most radioisotopes do not decay into a stable daughter element in one single decay but through a series of radioactive intermediaries.

Decerebrate rigidity (also known as Decerebate posture)

Exaggerated posture of extension as a result of a lesion to the prepontine area of the brain stem, and is rarely seen fully developed in humans.  A spasticity resulting from functional disruption of brain stem activities at the mid-collicular level. In reporting, it is preferable to describe the posture seen.

Decorticate Posture (Decorticate Rigidity)

Exaggerated posture of upper extremity flexion and lower extremity extension as a result of a lesion to the mesencephalon or above. In reporting, it is preferable to describe the posture seen.

Decibel

A decibel (abbreviated dB) is a unit used to record the level of sound. 
A decibel is one-tenth of a bel and, since a bel is a logarithm to base 10, a decibel is a logarithm to base 100.1, i.e. a logarithm to base 1.259 (rounded to 3 decimal places).

An increase in the intensity of sound of 1.259 times represents an addition of one decibel.  A doubling of intensity represents an addition of 3 decibels [because 2 = (1.259)3]; a tripling of intensity represents an addition of about 5 decibels [because 3 = (1.259)5].  Ten times the intensity adds 10 decibels; one thousand times the intensity adds 30 decibels [because 1000 = (1.259)30].  The important thing to realise is that just a few extra decibels makes a substantial difference to the intensity of sound to which a person is exposed.  Because decibels convert a multiplication to an addition, you can’t add decibels in the usual way, e.g. 85dB + 85dB = 88dB (not 170dB).

dB Sound Pressure Level (dB SPL):   Measuring the level of a sound relative to a base level, set to be 20 μPascals (by convention worldwide):

dB Sound Hearing Level (dB HL): Measuring the hearing sensitivity of a subject expressed relative to normal hearing sensitivity in a large normative database.  Here thresholds from a large normal-hearing young population have been measured at each of a number of standard audiometric frequencies; at each frequency the mean value (in dB SPL) is then expressed as 0 dB HL.  Then someone with a hearing level 20 dB worse than normal mean hearing sensitivity at (say 1 kHz) is said to have a hearing threshold of 20 dB HL at 1 kHz, etc.

Decimal numbers

The numbers in the base 10 number system.

Declarative memory (Episodic memory & Semantic memory)

The type of memory used when recalling (or declaring) facts or experiences, as opposed to skills.
Episodic memory: The type of declarative memory used when one talks about events in one’s life (includes time, place and emotions).
Semantic memory: The type of declarative memory used when talking about facts and concepts.
Both semantic and episodic memories are declarative memories and can easily be forgotten.

Decomposition of movement

Lack of fluidity in movement characterized by the breaking down of composite movements into their component parts, a symptom of cerebellar disease

Decreasing function

A function is considered to be decreasing if f(b) < f(a) when a.

Decubitus ulcer  (More commonly called a "pressure sore")

Pressure area, bed sore, skin opening, skin breakdown. A discolored or open area of skin damage caused by pressure. Common areas most prone to breakdown are buttocks or backside, hips, shoulder blades, heels, ankles and elbows. A breakdown of skin tissue caused by pressure on the skin, especially around bony areas (knees, heels, buttocks), when a person remains in one position for a long period of time, especially without padding; friction under a brace; moisture from perspiration; moisture and chemicals from incontinence. A pressure sore, which begins as a red spot on the skin, can progress to an open wound with very serious medical complications. It is necessary to establish a daily routine for prevention which includes periodic pressure relief (e.g. by changing position) and the use of appropriate cushions.

Decussation

The crossing of a neural pathway in the spinal cord or brainstem from one side of that structure to the opposite side.

Deduction

A conclusion arrived at by reasoning.

Deep vein thrombosis (DVT)

A blood clot (thrombus) in a deep vein in the thigh or leg. The clot can break off as an embolus and make its way to the lung, where it can cause respiratory distress and respiratory failure.

Definite integral

The definite integral of f(x) between a and b represents the area under the curve f(x) , above the x - axis, to the right of the line x = a, and to    the left of the line x = b.   
The definite integral of f(x) = F(b) - F(a)  where F is an antiderivative function for f(x).

Degenerate code

A code in which several code words have the same meaning. The genetic code is degenerate because there are many instances in which different codons specify the same amino acid. A genetic code in which some amino acids may each be encoded by more than one codon.

Degree

Unit of measure of an angle. 1/360 of a full rotation.  There are 360 degrees in a circle.

Degrees Brix

Measure of the density of a solution. (see Brix hydrometer scale)

Degrees of freedom

An estimate of the number of independent categories in a particular statistical test or experiment.

Deglutition

Swallowing
Consists of three phases:
The buccal phase occurs voluntarily in the mouth when the tongue forces the food bolus back into the pharynx.
The pharyngeal phase occurs involuntarily when food enters the pharynx, as follows:

  • The soft palate and uvula fold upward and cover the nasopharynx to prevent the passage of food up and into the nasal cavity.
  • The epiglottis, a flexible cartilaginous flap at the top of the larynx, folds down as the larynx rises. As a result, the opening to the larynx is covered, and food can pass only into the esophagus.

The esophageal phase occurs involuntarily in the esophagus. The esophageal sphincter, normally closed, opens to allow food to pass when the larynx rises during swallowing. When food reaches the lower end of the esophagus, the cardiac sphincter opens to allow the food to enter the stomach.

Degrees of freedom

The degrees of freedom of a statistical group are the number of values in the group which are free to vary. This number is usually one less than the sample size, the number of items in the group.A degree of freedom accounts for an independent variable in a system. Independent variables allow for changes within a system. For instance, the movement of two atoms in a gas is independent of each other assigning the distance between two atoms one degree of freedom. Two atoms covalently linked together within a molecule (a chemical bond) are also described by the same degree of freedom, also their movements are usually coupled. While this is true for the movement of the entire molecule, the length of the chemical bond is not static, but vibrates at very short time ranges measured in femto seconds to pico seconds (one trillionth to one billionth of a second). How many degrees of freedom a system has depends on the number of components and their interactions.

Dehiscence

Breaking open at maturity, along a definite line, to release materials (e.g., seeds, spores).

Dehydrated

The state a substance is in when moisture has been removed from it. Too much heat can dehydrate the body.

Dehydration reaction

A chemical reaction in which two molecules covalently bond to one another with the removal of a water molecule.

Deionized water

Water that has had the ions removed. Used in laboratories for making reagents.

Delayed language

A language disorder in which there is a noticeable slowness in the development of the vocabulary and grammar necessary for expressing and understanding thoughts and ideas.

Delayed speech

Failure of speech to develop at the expected age. More specifically: A deficit in speaking proficiency where the person performs like someone much younger.

Deletion (in genetics)

Loss of a DNA (chromosome) segment from a chromosome:
(1) A deficiency in a chromosome resulting from the loss of a fragment through breakage.
(2) A mutational loss of one or more nucleotides from a nucleic acid sequence (a gene).
Deletions are recognised genetically by:
Absence of reverse mutation
Presence of a deletion loop at meiosis that is isualized cytologically
Revealing of recessive lethals
pseudodominance
Hence a deletion chromosome = a chromosome containing a deletion.

Delirium

An acute organic mental disorder characterized by confusion and altered,
possibly fluctuating consciousness due to an alteration of cerebral metabolism which may include delusions, illusions and/or hallucinations. The condition is reversible except when followed by dementia or death. Often emotional changes, typically appearing as anxiety and agitation, are present.

Delirium tremens

An acute and sometimes fatal brain disorder (in 10-15% of untreated cases) caused by total or partial withdrawal from excessive alcohol intake.Usually develops in 24-96 hours after cessation of drinking. Symptoms include fever, tremors, ataxia and sometimes convulsions, frightening illusions, delusions and hallucinations. The condition is often accompanied by nutritional deficiencies.

Dementia

An organic mental disorder in which there is a deterioration or previously acquired intellectual abilities of sufficient severity to interfere with social or occupational functioning. Memory disturbance is the most prominent symptom. In addition, there is impairment of abstract thinking, judgment, impulse control and/or personality change.Dementia may be progressive, static or reversible, depending on the pathology with the availability of effective treatment.

Demersal

Living on or near the bottom of a body of water.

Denasality

A voice resonance problem that occurs when too little air passes through the nasal cavity.

Denaturation

1)The separation of the two strands of a DNA double helix, or
2) The severe disruption of the hydrogen bonded structure of any complex molecule without breaking the covalent bonds of its chains; The loss of the native configuration of the macromolecule, such as the unfolding of the tertiary structure of an antibody protein.
Denaturation usually results in the loss of the macromolecule's biological or immunological reactivity or solubility.
For proteins, a process in which a protein unravels and loses its native conformation, thereby becoming biologically inactive. For DNA, the separation of the two strands of the double helix. Denaturation occurs under extreme conditions of pH, salt concentration, and temperature.
See Denatured

Denaturation map

A map of a stretch of DNA showing the locations of local denaturation loops, which correspond to regions of high AT content.

Denaturing gradient gel electrophoresis (DGGE)

A method for separating DNA fragments according to their variable mobilities under increasingly denaturing conditions (usually increasing formamide/urea concentrations).A method for separating species using their DNA. Abbreviated to DGGE.

Denaturing high-performance liquid chromatography (DHPLC)

Large scale chromatographical method to detect sequence polymorphisms.

Denatured

Loss of natural configuration (of a molecule) through heat or other treatment, resulting in a loss of biologically activity of the molecule.
Fully denatured DNA is single-stranded.

Dendrite

One of usually numerous, short, highly branched processes of a neuron that conveys nerve impulses toward the cell body.

Denervation hypersensitivity

Elevated response of a nerve or muscle membrane receptor to a transmitter following resection or removal of its afferent nerve supply.After a nerve or nerves o a target structure has been cut or removed, there is often a hypersensitivity of that target structure to applications of the neurotransmitter that used to be released by the cut nerves.  Thus, the target structure that has been denervated has become hypersensitive.

Denhardt's solution

A solution commonly used during probe hybridizations that involve filters (such as Southern, Northern, or Western blots).

Denitrifcation

The process by which certain bacteria living in poorly aerated soils break down nitrates, using the oxygen for their own respiration and releasing nitrogen back into the atmosphere.

Denominator

The bottom part of a fraction.

Density

Mass per unit volume. Usually expressed as a body’s specific gravity

Density-dependent factor

Any factor influencing population regulation that has a greater impact as population density increases.

Density-dependent inhibition

The phenomenon observed in normal animal cells that causes them to stop dividing when they come into contact with one another.

Density-dependent factors

Any factor influencing population regulation that acts to reduce population by the same percentage, regardless of size.

Density-gradient centrifugation

A method of separating macromolecules by their
(1) differential rate of sedimentation in a centrifugal gradient
(2) differential bouyancy in a density gradient.

Dentatorubro-pallidoluysian atrophy (DRPLA)

A C-A-G trinucleotide repeat disorder that is characterized by abrupt muscle jerking, involuntary movements, and eventual dementia.

Deoxyribonuclease

A class of enzymes which digest DNA. The most common is DNase I, an endonuclease which digests both single and double-stranded DNA.

Deoxynucleotide

Components of DNA, containing the phosphate, sugar and organic base. When in the triphosphate form, they are the precursors required by DNA polymerase for DNA synthesis (i.e., ATP, CTP, GTP, TTP).

Deoxyribonuclease (dnase)

An enzyme that degrades DNA to nucleotides.

Deoxyribonucleic acid (DNA)

The nucleic acid molecule consisting of deoxyribonucleotide building blocks that encode genetic information. An antiparallel double helix of nucleotides (having deoxyribose as their sugars) linked by phosphodiester (sugar-phosphate) bonds to adjacent nucleotides in the same chain and by hydrogen bonds to complementary nucleotides in the opposite chain.
The fundamental substance of which genes are composed. The genome of most organisms is contained in a double-stranded, double-helical form held together with chemical bonds between each strand of complementary nucleotide base pairs.
Deoxyribose = The sugar component of DNA, having one less hydroxyl group than ribose, the sugar component of RNA.
DNA is a double-stranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T). In nature, base pairs form only between A and T and between G and C; thus the base sequence of each single strand can be deduced from that of its partner.

Deoxy-ribonucleotide triphosphate (dNTP)

A generic term referring to the four deoxyribonucleotides: dATP, dCTP, dGTP and dTTP.

Deoxyribose

The sugar component of DNA, having one less hydroxyl group than ribose, the sugar component of RNA.

Dependent variable

The output of a function. In an experiment, the dependent variable is the factor that responds when another factor is manipulated.

Depolarisation

A reduction in the polarity (charge) found across a cell membrane at rest; an electrical state in an excitable cell whereby the inside of the cell is made less negative relative to the outside than at the resting membrane potential.Cell membranes are able to separate charge because of their selective permeability to ions.  This charge separation is found even at rest, is known as the Resting Membrane Potential (RMP or EM), is typically such that the inside of the cell membrane is negatively charged compared to the outside.  When a stimulus or some other event causes inward currents carried by Na+ and Ca++ ions this difference in charge moves towards 0 (i.e., a lesser difference in charge), this is called depolarization.  In many cells, the depolarization can actually overshoot 0 and attain some positive value depending on what ions are permeating the cell and causing the depolarization.

Depression

A type of bipolar disorder characterized by lowered mood, slowed thinking, decreased movement or agitation, loss of interest, guilt, lowered self-esteem, sleep disturbance and decreased appetite.

Derepressed

The condition of an operon that is transcribing because repressor control has been lifted. May apply more generally to any gene being transcribed.

Derivative

The rate of change of a function.  The derivative at x of f(x) is the slope of the tangent line at (x, f(x)).   y' = f ' (x) = [f(x + delta x) - f(x)] / delta x.

Derived

Describes a character state that is present in one or more subclades, but not all, of a clade under consideration. A derived character state is inferred to be a modified version of the primitive condition of that character, and to have arisen later in the evolution of the clade. For example, "presence of hair" is a primitive character state for all mammals, whereas the "hairlessness" of whales is a derived state for one subclade within the Mammalia.

Dermis  

The inner of the two layers of skin. The dermis is a connective tissue layer under the epidermis containing elastic and collagen fibers, capillary networks, and nerve endings that signal somatic information such as nociceptive (painful) and temperature sensations.

Desalination

The removal of salts from saline water to provide freshwater. This method is becoming a more popular way of providing freshwater to populations.

Desaturase

An enzyme (group) "family" that is present within the soybean plant and other oilseed crops (e.g., sunflower, canola, corn/maize).One or more desaturases is involved in the synthesis "pathway" via which oilseed crops produce unsaturated fatty acids (e.g., linoleic acid). A desaturase is also involved in production of beta carotene (in some plants).

Descriptor culling

Reducing the number of descriptive terms used in descriptive analyses to a practical or manageable number for analysis or interpretation.

Desensitisation

See tachyphylaxis.

Designer foods

Foods that are enriched with nutraceuticals, antioxidants, and secondary metabolites to improve the physical performance of the body.

Desmosome

A type of intercellular junction in animal cells that functions as an anchor.

Detector (in assays)

An analytical instrument which is capable of measuring the amount of tracer label in an immunoassay sample, such as scintillation counters, microplate readers, and automated immunoassay analyzers.

Detector data (in assays)

The initial sample measurement(s) recorded by the detector instruments which is converted into the raw data.
The raw data are then transmitted from the detector instrument.

Determinant

The determinant | a  b |  =  ad - bc.
| c  d |

Determinate cleavage

A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.

Determinate growth

A type of growth characteristic of animals, in which the organism stops growing after it reaches a certain size.

Determination (of cells)

The process of commitment of cells to particular fates. The progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo develops.

Deterministic

Events that have no random or probabilistic aspects but proceed in a fixed predictable fashion.

Detritivore

An organism that eats detritus. Fragments of dead plant and animal material before, during and after breakdown by agents of decay. May incorporate inorganic matter (such as mud).

Detritus

Dead organic matter.

Development

The process by which a multicellular organism is produced from a single cell. The progressive production of the phenotypic characteristics of a multicellular organism, beginning with the fertilization of an egg.

Developmental aphasia

A language disorder in children, caused by brain damage, characterized by complete or partial impairment of language comprehension, formulation, and use.

Developmental labeling approach

An approach to labeling that is based on deviations in the course of development from what is considered normal.

Dew point

Temperature to which the air must be cooled to reach saturation (assuming air pressure remains the same). The dew point is a direct measure of the amount of moisture present in the air, and directly affects how you feel or in other words measures the amount of humidity in the air. The temperature never drops below its dew point, but can drop to it.

Diabetes Insipidus

This condition is a result of missing antidiuretic hormone (ADH). This hormone is made by the pituitary gland and helps the kidneys balance the fluid in the body. A person with this condition may have a large amount of urine each day and be very thirsty. If untreated, this condition can lead to dehydration, problems with growth, weight gain and appetite.

Diabetes Mellitus, Types I and II 

A disorder associated with defects in insulin action. Type I diabetes is characterized by inadequate insulin secretion; Type II diabetes is characterized by impaired insulin secretion in response to elevated blood glucose levels or by loss of sensitivity to insulin by target cells.

Diagonal

The line segment connecting two nonadjacent vertices in a polygon.

Diagnostic and Statistical Manual of Mental Disorders

A classification system for mental illnesses developed by the American Psychiatric Association.

Diagnosis

1 The nature of a disease ; the identification of an illness. 2 A conclusion or decision reached by diagnosis. The diagnosis is rabies . 3 The identification of any problem. The diagnosis was a plugged IV.

Dialysis

Removal of small molecules from a solution of a macromolecule, by allowing them to diffuse through a semipermeable membrane into water or buffer.
The process carried out at the kidney.

Diameter

The line segment joining two points on a circle and passing through the center.

Diaphragm

A sheet of muscle that forms the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs.

Diarrhea

Excessive bowel movements of loose, watery stools, which can be acute or chronic.
Chronic diarrhea may lead to nutrient deficiencies because food may be passing through the intestinal tract too quickly for nutrients to be absorbed.
Acute diarrhea may lead to electrolyte imbalances and dehydration which can be life threatening.

Diaschisis

A loss of function due to depression of activity at some distance from a lesion.

Diastole

The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood..

Diastolic pressure

The pressure in an artery during the ventricular relaxation phase of the heart cycle.

Dichogamy

Production of male and female reproductive elements at different times by a hermaphroditic organism in order to ensure allogamy.

Dichotomous tree

A tree where all branching points are dichotomies. That is, a tree is dichotomous if at each branch point there are only two immediate descendents. This is in contrast to a polytomous tree.

Dichotomy (Plural = Dichotomies)

A branch point on a tree that has two immediate descendents.

Dictionary file

A special form of machine-readable codebook that contains information about the structure of a data file and the locations and, often, the names of variables variables in the data file.
Typically, you use a dictionary file and a data file together with statistical software; the statistical software uses the dictionary so that you may specify variables by name, rather than having to specify their locations in the file.

Dideoxy method

A method of DNA sequencing that uses chain-terminating (dideoxy) nucleotides.

Dideoxynucleotides (didN)

Chain-terminating precursors of DNA synthesis that block further polymerization when added to the end of the DNA strand by DNA polymerase.
These nucleotides lack a 3'-OH (hydroxyl) group necessary for continued 5'-to-3' DNA synthesis. Dideoxynucleotides are used in molecular biology for Sanger-type DNA sequencing, and in medicine as anti-retroviral drugs for the treatment of HIV infection (e.g., ddI, ddC, and AZT).

Dielectric constant

Property of a material representing the ability to store electromagnetic energy.

Dielectirc loss

Property of a material representing the ability to dissipate electromagnetic energy as heat.

Difference

The result of subtracting two numbers.

Difference threshold (successive)

The minimum increase in stimulus intensity required to produce a perception of a change in the stimulus.

Differential RNA synthesis

Concept that cells are different from each another because they turn on (transcribe mRNAs from) different sets of genes; thus each cell makes different proteins appropriate to its particular function.

Differentiable

A function is differentiable over an interval if it is continuous over the interval and if the derivative exists everywhere on the interval.

Differential

An infinitesimally small change in a variable, represented by d, as in dx, or dy.

Differentiaton

1. In embryology The process whereby an unspecialized embryonic cell acquires the features of a specialized cell such as a heart, liver, or muscle cell.
2. In mathematics: The process of finding a derivative.
Differentiation is controlled by the interaction of a cell's genes with the physical and chemical conditions outside the cell, usually through signaling pathways involving proteins embedded in the cell surface.

Differentiaton pathways

The chemical/gene expression pathways responsible for causing a single type of cells (e.g., stem cells, embryonic stem cells, etc.) to become multiple, different types of (specialized) cells.Expression of each of the specific genes responsible for (i.e., the genes that code for the particular proteins which cause) differentiation is itself controlled by exquisite methylation and acetylation of the histone proteins adjacent to those genes.

Diffraction

The deviation in the path of a wave that encounters the edge of an obstacle. e.g., The bending of light as it passes through a small slit or opening

Diffusion feeding

Feeding strategy in which the predator relies on the movements of the prey to make contact - as in heliozoa and suctoria.

Diffuse Axonal Injury (DAI)

A shearing injury of large nerve fibers (axons covered with myelin) in many areas of the brain. It appears to be one of the two primary lesions of brain injury, the other being stretching or shearing of blood vessels from the same forces, producing hemorrhage

Diffuse Brain Injury

Injury to cells in many areas of the brain rather than in one specific location.

Diffusion

The spontaneous movement of substances from a region of high concentration (high chemical potential) to a region of low concentration (low chemical potential)Molecules undergo spontaneous Brownian motion, which is dependent on the temperature (the higher the temperature, the greater the Brownian motion).  In regions of high concentration of a molecule, the chances of colliding with other molecules are therefore higher than in regions of low concentration of that molecule.  In turn this means that there is greater chance of “bouncing off” after collisions with each other from a region of high molecule concentration to a region of low concentration.Diffusion governs the movement of many substances in the body, especially across the cell membrane. The structure of the cell membrane confers selective permeability whereby the cell can regulate what substances can move across from one side of the cell membrane to the other side.

Diffusion coefficient (D)

The diffusion coefficient D describes the relationship between a concentration gradient DC/Dx and the flow of matter per unit area (flux rate J).

DiGeorge syndrome (Also known as Shprintzen, velo-cardio-facial, and 22q11.2 deletion syndrome.)

A genetic disease caused by a missing piece of chromosome material on chromosome #22 that results in many different health problems, and affects the normal fetal development of the heart, thymus, and parathyroid glands.

Digit

The ten symbols, 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 are digits.  Example:  the number 365 has three digits:  3, 6, and 5.

Digital mammography

A technique for recording x-ray images in computer code instead of on x-ray film, as with conventional mammography.
The images are displayed on a computer monitor and can be enhanced (lightened or darkened) before they are printed on film. From the patient’s perspective, the procedure for a mammogram with a digital system is the same as for conventional mammography. Digital mammography may have some advantages over conventional mammography. The images can be stored and retrieved electronically, which makes long-distance consultations with other mammography specialists easier. Because the images can be adjusted by the radiologist, subtle differences between tissues may be noted. The improved accuracy of digital mammography may reduce the number of follow-up procedures. Despite these benefits, studies have not yet shown that digital mammography is more effective in finding cancer than conventional mammography.

Digestion

The process of breaking down food into molecules small enough for the body to absorb.

Digoxin

A drug commonly used to treat congestive heart failure and certain heartbeat irregularities that is obtained from leaves of the Foxglove (digitalis) plant.

Dihybrid

A hybrid individual that is heterozygous for two genes or two characters.

Dihybrid cross

A breeding experiment in which parental varieties differing in two traits are mated.

Dikaryotic

Having two different and distinct nuclei per cell; found in the fungi. A dikaryotic individual is called a dikaryon.

Dilution

A dilution is the ratio of the volume of pure specimen to the total volume of specimen plus a diluent such as buffer.Dilutions are expressed as the number of parts of pure specimen, a colon, and the total number of parts in the solution. The number of parts of diluent are determined by subtracting the number of pure specimen parts from the total. A dilution of 1:10 contains one part pure specimen and nine parts diluent. A dilution factor is the total number of parts with the volume of pure specimen being one part

Dimension

The dimension of a space is the number of coordinates needed to identify a location in that space.

Dimerization

The chemical union of two identical molecules.

Dimorphism

Displaying two separate growth forms.

Dipeptide

Compound consisting of two amino acid units joined together, linking the amino (-NH2) group of one with the carboxylic acid group (-COOH) of the other.

Diplepia

Paralysis that affects both sides of the body, due to injury of both hemispheres of the brain. More commonly affects the legs more than the arms.

Diphtheria

A serious bacterial infection which can cause pneumonia, heart failure, nerve damage, or death by suffocation. Immunization with the Diphtheria-Tetanus-Pertussis (DTP) vaccine protects against this disease.

Diploid

Having two different sets of chromosomes in the same nucleus of each cell; A full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. The state of having each chromosome in two copies per nucleus or cell.
A cell having two chromosome sets, or an individual having two chromosome sets in each of its cells. Diploid organisms have diploid cells. Diploid cells have two copies of each chromosome.
The amount of DNA in diploid cells defines the normal DNA content for the species Most metazoans and plants are diploid. Most animal cells except the gametes have a diploid set of chromosomes. The diploid human genome has 46 chromosomes.

Diploid life cycle

Occurs when the only multicellular stage in an organism's life cycle is diploid.

Diplopia

Double vision, or the simultaneous awareness of two images of the same object.
Results from a failure of the two eyes to work in a coordinated fashion Covering one eye will erase one of the images.

Dipole

1. A molecule having both partial positive (d+) and partial negative (d+) charges.
2. For oscillating magnetic fields, a magnetic particle that contains a *north* and *south* magnetic pole.

Dipole-dipole interaction

A weak intermolecular force of attraction between a partial positive (d+) charge on one particle and a partial negative (d+) charge on a second particle.

Direct fit

Computer-fitting of binding data (either saturation or competition data) to the logistic equation:

          Bmax [L]n
    b = -------------                              
          [L]n + Kn

where b = concentration of ligand bound to the receptor, [L] = conc. of free ligand, K is the dissociation constant for the ligand and n is a constant which should equal 1 if the assumptions in kinetics of binding are valid (it can represent the number of ligand molecules which bind to each receptor). When saturation or competition data are directly fitted to a logistic equation in this way, the Hill plot becomes unnecessary, since the calculated value of n is equivalent to the slope of the Hill plot.

Directed differentiation

The manipulation of stem cell culture conditions to induce differentiation into a particular cell type.

Directed molecular evolution

A laboratory version of evolution at the molecular level that can produce "designer molecules."A large starting population of molecules (typically nucleic acids) that varies randomly in base sequence and shape is subjected to replication with variation, followed by selection. After several cycles of replication and selection, the population of molecules will evolve toward one containing a high proportion of molecules well adapted to the selection criterion applied.

Directed mutagenesis

Altering some specific part of a cloned gene and reintroducing the modified gene back into the organism.

Direct fluorescent antibody test (DFA)

A test that uses a fluorescent antibody to directly detect an antigen.

Directional cloning

DNA insert and vector molecules are digested with two different restriction enzymes to create non-complementary sticky ends at either end of each restriction fragment.
This allows the insert to be ligated to the vector in a specific orientation and prevent the vector from self-ligation.

Directional selection

Natural selection that favors individuals on one end of the phenotypic range.
Selection leading to a consistent directional change in any character of a population through time, for example selection for larger eggs.

Directly proportional

y is directly proportional to x if y = kx

Disaccharide

A class of sugars composed of two monosaccharide units joined together (by dehydration synthesis). The best known example is sucrose, which is composed of a glucose molecule joined with a fructose molecule. Likewise, lactose is composed of a glucose unit with a galactose unit, and maltose is a disaccharide composed of two joined glucose units.

Discharge

The volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.

Discontinuous replication

Replication of DNA in short 5' to 3' segments using the 5' to 3' strand as a template while going backward away from the replication fork.

Discriminability index (d’)

A measure of the separation between two distributions, and thus the ability to discriminate between the two distributions. The difference in means of two distributions divided by their standard deviations. It is a poor measure of performance when the amount of data in the distributions is low or when the two distributions are widely separated.

Discriminance analysis

Multivariate statistical analysis method
A dependent variable Y and several independent X. Can the combination of several X terms explain product differences Y?

Discriminant

The discriminant tells how many roots there are for the equation and the nature of the roots.
The discriminant of a quadratic equation, ax2 + bx + c = 0 is b2 - 4ac.

Disinhibition

Inability to suppress (inhibit) impulsive behavior and emotions. Generally occurs after frontal lobe damage.

Disintegrations Per Minute (DPM)

The number of radioactive disintegrations which actually occurred during one minute.The DPM of a sample is usually determined by dividing the number of disintegrations which were recorded by the efficiency of the detector instrument under the same conditions, particularly the amount of quenching, as were present in the measured sample.

Disjoint

Having no elements in common.

Disjunction

An OR statement.

Disorientation

Not knowing where you are, who you are, or the current date. Health professionals often speak of a normal person as being oriented "times three" which refers to person, place and time.

Disorganized schizophrenia (Also known as"hebephrenic")

Characterized by disorganized thinking, shallow and inappropriate affect, inappropriate giggling, silly and regressive behavior and mannerisms and frequent hypochondriacal complaints.  Delusions and hallucinations are usually bizarre and disorganized.

Dispersal

The scattering of organisms of a species, often following a major reproductive event.
Spores and larvae are commonly dispersed into the environment. Pollen or gametes may also be dispersed, but in this case the intent is to target another individual so that reproduction may occur. Organisms may disperse as spores, seeds, eggs, larvae, or adults.

Dispersive replication (of DNA)

A proposed model of DNA replication. This model involves the breaking of the parental DNA strands during replication, and somehow, a reassembly of molecules that were a mix of old and new fragments on each strand of DNA. (see Conservative replication and Semiconservative replication)

Disruptive selection

Selection favouring individuals that deviate in either direction from the population average. Selection favours individuals that are larger or smaller than average.

Dissociation constant

(1) An equilibrium constant (Kd) for the dissociation of a complex of two or more biomolecules into its components; for example, dissociation of a substrate from an enzyme.
(2) The dissociation constant of an acid (Ka); or base (Kb), describing its dissociation into its conjugate base and a proton; or conjugate acid and a hydroxide ion.

Dissociation disorder

A mental condition in which ideas or desires are separated from the mainstream of consciousness or from one's personality to a degree that they are no long accessible to memory or consciousness. The person has difficulty or is unable to perceive things or situations as a whole, but instead tends to respond to stimuli in terms of parts or segments.

Distal  

Away from. Contrast with Proximal.

Distal tubule  

The section of the renal tubule where tubular secretion occurs. Tubular secretion is the process in which ions and other waste products are transported into the distal tubules of the nephron.

Distortion (in speech)

An articulation error in which there are inaccurate productions of phonemes that resemble the target form. Some examples of these distortions are the lisps (lateral and dental), palatal distortions (where the wrong part of the tongue is used to form s, z, sh, and zh sounds), and the retroflex distortions (too much curling of the tip of the tongue).

Distributive property

a(b + c) = ab + ac

Diuretic

A drug used to increase urine formation and output. Diuretics are prescribed for the treatment of edema (the accumulation of excess fluids in the tissues of the body), which often occurs as the result of disease of the kidneys, liver, lungs, or heart. Diuretics are also used to treat hypertension (high blood pressure).

Diurnal

Having a period or cycle that is a full tidal day; Being active during the daytime rather than at night. Involving a 24-hour period that includes a day and the adjoining night.

Divergent series

A series whose sum is infinite.

Diverticulosis/diverticulitis

Small pouches may form along the walls of the large intestine called diverticuli which if symptomatic, causing discomfort to the patient, is called diverticulosis. These abnormal outpocketings may collect and not be able to empty fecal material which can lead to inflammation, diverticulitis

Diversity

Term used to describe numbers of taxa, or variation in morphology.

Diversifying selection

Natural selection that favors extreme over intermediate phenotypes.

Dividend

The number being divided. In a / b = c,   a is the dividend.

Divisor

The number doing the dividing. In a / b = c, b is the divisor.

DNA

Long molecule in the nucleus of cells, shaped like a double-helix, contains the genetic information that determines the development and functioning of an organism's cells.  See Deoxyribonucleic acid

DNA bank

Storage of DNA, which may or may not be the complete genome, but should always be accompanied by inventory information.

DNA-DNA hybridization

When DNA is heated to denaturation temperatures to form single strands and then cooled double helices will re-form (renaturation) at regions of sequence complementarity. This technique is useful for determining sequence similarity among DNAs of different origin and the amount of sequence repetition within one DNA.

DNA fingerprint

A technique for identifying individual organisms based upon the uniqueness of their DNA pattern.The largely individual-specific autoradiographic banding pattern produced when DNA is digested with a restriction endonuclease that cuts outside a family of VNTRs, and a Southern blot of the electrophoretic gel is probed with a VNTR-specific probe.The technique has applications in forensics, paternity testing, anthropology, conservation biology and ecological research.

DNA Index

The ratio of GO/G1 peak channel in a DNA histogram of an experimental sample to the GO/G1 peak channel of a reference sample, when normal human diploid cells or nuclei are the reference. This is a measure of DNA aneuploidy, or abnormal DNA content.

DNA ligase

An enzyme that rejoins cut pieces of DNA. A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3' end of a new DNA fragment to the 5' end of a growing chain.

DNA marker

Small DNA fragment used to locate a specific base sequence within a larger fragment
Any unique DNA sequence which can be used in DNA hybridization, PCR or restriction mapping experiments to identify that sequence.

DNA methylation

The addition of methyl groups (–CH3) to bases of DNA after DNA synthesis; may serve as a long-term control of gene expression.

DNA polymerase

An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.

DNA polymorphism

One of two or more alternate forms (alleles) of a chromosomal locus that differ in nucleotide sequence or have variable numbers of repeated nucleotide units.

DNA probe

A single-stranded piece of DNA that binds specifically to a complementary DNA sequence. A chemically synthesized, radioactively labeled segment of nucleic acid used to find a gene of interest by hydrogen-bonding to a complementary sequence.
The probe is labeled (e.g., with a fluorescent or radioactive tag or enzyme) in order to detect its incorporation through hybridization with DNA in a sample.

DNA replication

The process of making an identical copy of a section of duplex (double-stranded) DNA, using existing DNA as a template for the synthesis of new DNA strands. In humans and other eukaryotes, replication occurs in the cell nucleus.

DNA-RNA hybridization

When a mixture of DNA and RNA is heated to denaturation temperatures to form single strands and then cooled RNA can hybridize (form a double helix) with DNA that has a complementary nucleotide sequence. Useful for determining relationships between DNAs and RNAs.

DNA sequencing

Procedures for determining the nucleotidesequence of a DNA fragment.

DNA Synthesizer

A machine which automatically makes short, artificial polynucleotides or oligonucleotides with any desired sequence of nucleotide bases.

Docking protein

Responsible for attaching (docking) a membrane-bound vesicle (e.g., a ribosome or transmitter-containing vesicle) to a membrane by interacting with a signal particle attached to a the structure destined to be membrane bound.

Domain

1. In molecular biology: A discrete portion of a protein with its own function.
2. In mathematics and logic: The set of all possible values of the argument of a function.
The combination of domains in a single protein determines its overall function.

Domestic water use

water used for household purposes, such as drinking, food preparation, bathing, washing clothes, dishes, and dogs, flushing toilets, and watering lawns and gardens. About 85% of domestic water is delivered to homes by a public-supply facility, such as a county water department. About 15% of the Nation's population supply their own water, mainly from wells.

Dominance hierarchy

A linear "pecking order" of animals, where position dictates characteristic social behaviors.

Dominant allele

In a heterozygote, the allele that is fully expressed in the phenotype.

Dormancy

A period of suspended growth and metabolic activity. Many plants, seeds, spores, and some invertebrates become dormant during unfavorable conditions. Many animals hibernate during winter, a form of dormancy. A period during which growth ceases and metabolic activity is greatly reduced; dormancy is broken when certain requirements, for example, of temperature, moisture, or day length, are met.

Dorsal

Pertaining to or situated near the back; opposite of ventral.

Dorsiflexion

Backward flexion of a foot or hand, or of their digits. That is, bending fingers or toes backwards towards the upper surface of the foot or hand.When applied to the ankle, the ability to bend at the ankle, moving the front of the foot upward.                       (see Plantar flexion)

Dose-Response Relationship

The relationship between (1) the dose, actually based on "administration dose" ( i.e. exposure) rather than actual absorbed dose, and (2) the extent of therapeutic or toxic effect produced by the xenobiotic.

Dot blotting

A technique for measuring the amount of one specific DNA or RNA in a complex mixture. A method for detecting proteins by the specific binding of an antibody or binding molecule to a sample spot on nitrocellulose paper.
The samples are spotted onto a hybridization membrane (such as nitrocellulose or activated nylon, etc.), fixed and hybridized with a radioactive probe. The bound sample is visualized using an enzymatic or fluorimetric reporter conjugated to the probe. The extent of labeling (as determined by autoradiography and densitometry) is proportional to the concentration of the target molecule in the sample. Standards provide a means of calibrating the results.
Used for blotting cloned DNA without prior restriction digestion and electrophoresis. Autoradiography reveals dots indicating probe hybridization.

Dot plot

A two parameter data graph used for acquisition and analysis. Each dot on the display represents one event that the flow cytometer analyzed.

Double bond

The sharing of four electrons between two atoms.

Double blind study

A clinical trial or experiment in which neither the investigators nor the patients know which treatment has been administered.

Double circulation

A circulation scheme with separate pulmonary and systemic circuits, which ensures vigorous blood flow to all organs.

Double digest

The product formed when two different restriction endonucleases act on the same sample of DNA.

Double fertilization

A mechanism of fertilization in angiosperms, in which two sperm cells unite with two cells in the embryo sac to form the zygote and endosperm.

Double helix

The normal structural configuration of DNA consisting of two adjacent helices (of polynucleotide strands) winding about the same axis.
The interlocking helices are joined by hydrogen bonds between paired bases.

Double hemiplegia

Paralysis that involves both sides of the body, with one side being more greatly affected.

Double infection

Infection of a bacterium with two genetically different phages.

Double membrane

In mitochondria and plastids, there is a two-layered membrane which surrounds the organelle. This is believed to be the result of endosymbiosis, with the outer membrane coming from the eukaryotic cell, and the inner membrane belonging to the original prokaryote which was "swallowed".

Doublet

Two particles stuck together, which a flow cytometer records as one larger event. Particles may also occur in triplets and higher associations.

Down syndrome

A condition resulting from a chromosomal abnormality, primarily the presence of an extra (or part of) a chromosome (specifically chromosome 21) , characterized by mental retardation and heart and respiratory defects.  Characteristic features include mental retardation of varying degrees, epicanthal folds, oval-shaped eyes, thicker tongue, short neck. microcephaly, looseness of the joints, flat bridge of nose, etc.

Downstream (in genetics)

A convention used to describe features of a DNA sequence, gene or mRNA related to the position and direction (5' to 3') of transcription by RNA polymerase or translation by the ribosome. Downstream (or 3' to) is in the direction of transcription (or translation) whereas upstream (5' to) is in the direction from which the polymerase (or ribosome) has come. Conventionally DNA sequences, gene maps and RNA sequences are drawn with transcription (or translation) from left to right and so downstream is towards the right.

Drainage basin

land area where precipitation runs off into streams, rivers, lakes, and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large drainage basins, like the area that drains into the Mississippi River contain thousands of smaller drainage basins. Also called a "watershed."

Drawdown

lowering of the ground-water surface caused by pumping.

Drip irrigation

a common irrigation method where pipes or tubes filled with water slowly drip onto crops. Drip irrigation is a low-pressure method of irrigation and less water is lost to evaporation than high-pressure spray irrigation

Driving force

A terminology used in thermodynamics expressing the availability of energy to 'drive' a process such as mechanical work or chemical synthesis. Driving forces exist where a potential gradient exist. A potential gradient can be in form of a temperature gradient causing heat to flow, an electrical gradient causing electrons or ions to flow, or a concentration gradient causing diffusion.

Dropping point

Temperature at which a sample of fat becomes sufficiently fluid to flow under the conditions of the test. A portion of molten fats introduced into a sample cup, crystallized and then heated at a constant rate. The temperature at which the sample is able to flow through the orifice in the bottom of the cup is the end point.

Dual energy X-ray absorptometry (DEXA or DXA)

A technique for scanning bone and measuring bone mineral density (BMD).
A DXA scanner produces 2 X-ray beams, each with different energy levels. One beam is high energy while the other is low energy. The amount of x-rays that pass through the bone is measured for each beam. This will vary depending on the thickness of the bone. Based on the difference between the 2 beams, the bone density can be measured. DXA is relatively easy to perform and the amount of radiation exposure is considered low.

Duchenne muscular dystrophy

A lethal muscle disease in humans caused by mutation in a huge gene coding for the muscle protein dystrophin; inherited as an X-linked recessive phenotype. The most common and severe form of muscular dystrophy. Dystrophic muscles are fragile, prone to injury, regenerate poorly after damage, and susceptible to Ca2+-overload and degeneration. Defective Ca2+ regulation has been implicated in these processes and it is widely accepted that dystrophic muscles have an elevated cytosolic [Ca2+] even at rest. DMD is an X-linked neuromuscular disease affecting ~1 in 3500 live born males from early childhood. It is caused by a variety of mutations and deletions in the dystrophin gene on chromosome Xp21. The disease is progressive and eventually affects all muscles such thatpatients become dependent on a wheelchair, usually before their teens, and have only 25% of the muscle mass of healthy children. The relentless muscle wasting leads to premature death from respiratory or cardiac failure. The primary abnormality is the absence of the dystrophin protein, which normally links the actin cytoskeleton to laminin in the extracellular matrix through the dystrophin associated protein complex (DAPC). The DAPC stabilises the membrane against shear stresses and plays a protective role against muscle damage caused by “eccentric” (lengthening) actions. In addition to its mechanical functions, dystrophin has been suggested to play a role in intracellular processes such as Ca2+ regulation and signal transduction. There is no cure and existing therapies are ineffective.

Ductal lavage

An investigational technique for collecting samples of cells from breast ducts for analysis under a microscope. A saline solution is introduced into a milk duct through a catheter inserted into the opening of the duct on the surface of the nipple. Fluid, which contains cells from the duct, is withdrawn through the catheter. The cells are checked microscopically to identify changes that may indicate cancer that may increase the risk for breast cancer. The usefulness of ductal lavage is still under study.

Duodenum

The first section of the small intestine, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall.

Duplication

An aberration in chromosome structure resulting from an error in meiosis or mutagens; duplication of a portion of a chromosome resulting from fusion with a fragment from a homologous chromosome.

Dura mater (Latin for tough mother)

One of the three layers of meninges that cover and protect the brain and spinal cord. The dura mater is the thick and tough outer membrane lying just inside the skull and vertebrae, and close to the cranium and vertebrae.Separated from the arachnoid mater by the subdural space. In the brain, there are channels within the dura mater, the dural sinuses, which contain venous blood returning from the brain to the jugular veins.In the spinal cord, the dura mater is often referred to as the dural sheath. Here the inner and outer layers of the dura mater are separated by the extradural space. This fat-filled space between the dura mater and the vertebrae acts as a protective cushion to the spinal cord.     (see Arachnoid mater, Meninges, Pia Mater)

D-value, decimal reduction time

Time required for a one-log cycle reduction in the microbial population, at a specific temperature, pressure, or electric field intensity.  For the D-value to be meaningful, the semi logarithmic survivor curve must be a straight line.

Dynamo

A device that creates electricity by turning around a magnet near a coil of wire.

Dynamic compression

The compression of a fracture by weight bearing and/or muscle contraction. Fracture fixations that employ dynamic compression include dynamic hip screws, anti-glide plates, dynamized tibial and femoral nails, and tension band wiring.

Dynein

A microtubule-dependent motor protein; A large contractile protein forming the sidearms of microtubule doublets in cilia and flagella. Cytoplasmic dyneins work in organelle transport and mitosis, whereas the closely related ciliary dyneins are attatched to outer doublet microtubules in a cilium or flagellum, providing sliding/bending force used in (for example) sperm propulsion

Dysarthria

A group of speech problems where sounds may be slurred, and speech may be slow or effortful; Difficulty in forming words or speaking them.
Changes in pitch, loudness, rhythm, and quality of speech may also be noticed. Such problems are due to paralysis, weakness, or incoordination of muscles used in speaking. Arises because of weakness of muscles used in speaking or because of disruption in the neuromotor stimulus patterns required for accuracy and velocity of speech Dysarthria occurs in both children and adults, and is associated with neuromuscular diseases such as cerebral palsy, parkinsonism, Lou Gehrig's disease, or later stages of multiple sclerosis. It can also occur from stroke, brain injury, and tumors.

Dyscalculia

Lack of ability to perform mathematical functions, usually associated with neurological dysfunction or brain damage.

Dys-diadochokinesia.

Impairment of ability to perform rapid alternating movements, a symptom of cerebellar disease

Dysesthesia.

A persistent, painful sensation, produced by gentle stimulation, that often occurs after destruction of CNS pathways.

Dysgraphia

Extremely poor handwriting or the inability to perform the motor movements required for handwriting. The condition is often associated with neurological dysfunction.

Dyskinesia

Impairment of the ability to move resulting in fragmentary or incomplete movements. Caused by partial impairment of the coordination of voluntary muscles, which results in obvious clumsy movements and poor physical control.

Dyslexia

A type of learning disability where, despite conventional classroom experience, a person may have problems remembering and recognizing written letters, numbers, and words, might read backwards, and have poor handwriting. The term is frequently used when neurological dysfunction is suspected as the cause of the reading disability.

Dyslipidemia

A disorder of the amount of lipids in the blood.A primary risk factor for the development of atherosclerosis and subsequent cardiovascular diseases.

Dysmanthia

A difficulty in learning, especially in learning language

Dyspepsia (indigestion)

A functional disorder of the gastrointestinal tract. Symptoms of dyspepsia as originating from the upper gastrointestinal tract, primarily the stomach and first part of the small intestine. These symptoms include: upper abdominal pain (above the navel), belchingnausea (with or without vomiting), abdominal bloating (the sensation of abdominal fullness without objective distention), early satiety (sensation of fullness after a very small amount of food), and, possibly, abdominal distention (swelling as opposed to bloating).
Most dyspepsia (not due to non-gastrointestinal diseases or drugs) is believed to be due to abnormal function (dysfunction) of the muscles of the organs of the gastrointestinal tract or the nerves controlling the organs.
Many gastrointestinal diseases have been associated with dyspepsia. However, many non-gastrointestinal diseases also have been associated with dyspepsia, e.g., diabetes, thyroid disease, hyperparathyroidism (overactive parathyroid glands), and severe kidney disease. It is unclear, however, how these non-gastrointestinal diseases might cause dyspepsia.
A second important cause of dyspepsia is drugs, e.g., nonsteroidal anti-inflammatory drugs (NSAIDs such as ibuprofen), antibiotics, and estrogens). In fact, most drugs are reported to cause dyspepsia in at least some patients.

Dysphagia

Difficulty in swallowing. A swallowing disorder characterized by difficulty in oral preparation for the swallow, or in moving material from the mouth to the stomach. This also includes problems in positioning food in the mouth.

Dysphoria

An unpleasant mood, such as depression, anxiety, or irritability.

Dyspraxia

Inability to perform coordinated movements, especially speech, with no apparent problem in the muscles or nerves.

Dyssynergia

Disturbance of coordination in closely related muscles.

Dystonia

Acute tonic muscular spasms, often of the tongue, jaw, eyes, and neck, but sometimes of the whole body. Sometimes occurs during the first few days of antipsychotic drug administration.

Dystrophin

Protein in skeletal muscle. It makes up only 0.002% of all protein in skeletal muscle but appears to be vital for proper functioning of the muscle. Sufferers of muscular dystrophy appear to lack dystrophin.