| d-loop |
| Configuration found during DNA replication
of chloroplast and mitochondrial chromosomes wherein the origin
of replication is different on the two strands. The first structure
formed is a displacement loop or D-loop. |
| Darwinian fitness |
| The relative probability of survival and
reproduction for a genotype. see fitness. |
| dauermodification |
| The persistence for several generations
of an environmentally induced trait. |
| deficiency |
| The absence of part of the normal genome
or chromosome set. See deletion. |
| 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. |
| degrees of freedom |
| An estimate of the number of independent
categories in a particular statistical test or experiment. |
| deletion |
Loss of a DNA (chromosome) segment from
a chromosome. Deletions are recognised genetically by:
1.absence of reverse mutation
2.presence of a deletion loop at meiosis visualized cytologically
3.revealing of recessive lethals
4.pseudodominance |
| deletion chromosome |
| A chromosome containing a deletion. |
| deme |
| A locally interbreeding population. |
| 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. |
| denaturation map |
| A map of a stretch of DNA showing the locations
of local denaturation loops, which correspond to regions of high
AT content. |
| denatured |
| Loss of natural configuration (of a molecule)
through heat or other treatment. Fully denatured DNA is single-stranded.
|
| denominator element |
| gene on an autosome of Drosophila that helps
regulate the sex switch (sxl) to the off condition (maleness). Refers
to the denominator of the X/A (number of X chromosomes/autosomes)
genic balance equation that determines sex in insects. |
| 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.
|
| depauperate fauna |
| A fauna, especially common on islands, lacking
many species found in similar habitats elsewhere. |
| derepressed |
| The condition of an operon that is transcribing
because repressor control has been lifted. May apply more generally
to any gene being transcribed. |
| determinant |
| A spatially localized molecule that causes
cells to adopt a particular fate or set of related fates. |
| determination |
| The process of commitment of cells to particular
fates. |
| deterministic |
| Referring to events that have no random
or probabilistic aspects but proceed in a fixed predictable fashion.
|
| development |
| The process whereby a single cell becomes
a differentiated organism. The process of orderly change that an
individual goes through in the formation of structure. |
| diakinesis |
| The final stage of prophase 1 of meiosis
when chiasmata terminalize. |
| dicentric chromosome
|
| A chromosome with two centromeres. |
| dictyotene |
| A prolonged diplonema of primary oocytes
that can last many years. |
| dideoxy method |
| A method of DNA sequencing that
uses chain-terminating (dideoxy) nucleotides. |
| differentiation |
| The changes in cell shape and physiology
associated with the production of the final cell types of a particular
organ or tissue. |
| dihybrid |
| An organism heterozygous at two loci. |
| dihybrid cross |
| A cross between two individuals identically
heterozygous at two loci for example, AaBb/AaBb. |
| dimerization |
| The chemical union of two identical molecules.
|
| dimorphism |
| A polymorphism involving only two forms.
|
| dioecious plant |
| A plant species in which male and female
organs appear on separate individuals. |
| diploid |
| 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. |
| diplonema (diplotene
stage) |
| The stage of prophase of meiosis I in which
chromatids appear to repel each other. |
| directed mutagenesis
|
| Altering some specific part of a cloned
gene and reintroducing the modified gene back into the organism.
|
| directional selection
|
| A type of selection that removes individuals
from one end of a phenotypic distribution and thus causes a shift
in the distribution. Selection that changes the frequency of an
allele in a constant direction, either toward or away from fixation
for that allele. |
| disassortative mating
|
| The mating of two individuals with dissimilar
phenotypes. |
| 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. |
| discontinuous variation
|
| Variation that falls into discrete categories
(eg the colour of garden peas). |
| discrete generations
|
| Generations that have no overlapping reproduction.
All reproduction takes place between individuals in the same generation.
|
| dispersive replication
|
| Disproved model of DNA synthesis suggesting
more-or-less random interspersion of parental and new segments in
daughter DNA molecules. A postulated mode of DNA replication combining
aspects of conservative and semiconservative replication; known
to be incorrect. |
| disruptive selection
|
| A type of selection that removes individuals
from the centre of a phenotypic distribution and thus causes the
distribution to become bimodal. |
| distribution function
|
| A graph of some precise quantitative measure
of a character against its frequency of occurrence. |
| DNA (deoxyribonucleic
acid) |
| 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. |
| DNA clone |
| A section of DNA that has been inserted
into a vector molecule, such as a plasmid or a phage chromosome,
and then replicated to form many identical copies. See also clone
|
| DNA fingerprint |
| 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.
|
| DNA glycosylase |
| endonuclease initiating excision repair
at various damaged or improper bases in DNA. |
| DNA gyrase |
| A topoisomerase that relieves supercoiling
in DNA by creating a transient break in the double helix. |
| DNA ligase |
| An enzyme that closes nicks or discontinuities
in one strand of double-stranded DNA by creating an ester bond between
adjacent 3' OH and 5' PO4 ends on the same strand. |
| DNA polymerase |
| An enzyme that can synthesize new DNA strands
using a DNA template; several such enzymes exist. One of several
classes of enzymes that polymerize DNA nucleotides using single
or double-stranded DNA as a template. |
| 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-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 marker |
| Any unique DNA sequence which can be used
in hybridization, PCR or restriction mapping experiments to identify
that sequence. See expressed sequence tag and sequence tagged site.
|
| dnase(deoxyribonuclease)
|
| An enzyme that degrades DNA to nucleotides.
|
| docking protein |
| Responsible for attaching (docking) a ribosome
to a membrane by interacting with a signal particle attached to
a ribosome destined to be membrane bound. |
| dominance variance |
| Genetic variance at a single locus attributable
to dominance of one allele over another. |
| dominant |
| An allele that determines phenotype even
when heterozygous. Also the trait controlled by that allele. |
| dominant allele |
| An allele that expresses its phenotypic
effect even when heterozygous with a recessive allele; thus if A
is dominant over a, then AA and Aa have the same phenotype. |
| dominant phenotype |
| The phenotype of a genotype containing the
dominant allele; the parental phenotype that is expressed in a heterozygote.
|
| dosage compensation |
| A mechanism by which species with sex chromosomes
ensure that the homogametic sex does not have too much or the heterogametic
sex too little activity of loci on the homogametic sex chromosome.
The process, in organisms using a chromosomal sex determination
mechanism (such as XX versus XY), that allows standard structural
genes on the homogametic sex chromosome to be expressed at the same
levels in females and males, regardless of the number of homogametic
sex chromosomes. In mammals, dosage compensation operates by maintaining
only a single active X chromosome in each cell (see Lyon hypothesis),
while in Drosophila it operates by hyperactivating the single male
X chromosome. |
| dot blotting |
| A technique of blotting cloned DNA without
prior restriction digestion and electrophoresis (See Southern blotting).
Autoradiography reveals dots indicating probe hybridization. |
| double crossover |
| Two crossovers occurring in a chromosomal
region under study. |
| double digest |
| The product formed when two different restriction
endonucleases act on the same sample of DNA. |
| double helix |
| The normal structural configuration of DNA
consisting of two helices winding about the same axis. The structure
of DNA first proposed by Watson and Crick with two interlocking
helices joined by hydrogen bonds between paired bases. |
| double infection |
| Infection of a bacterium with two genetically
different phages. |
| double reduction |
| The condition in polyploids in which a heterozygous
individual produces homozygous gametes. |
| doublesex |
| Locus (dsx) in Drosophila in which some
mutations convert both males and females into intersexes. Encodes
male and female specific proteins produced by differential splicing
of the primary transcript in each sex. |
| Down syndrome |
| An abnormal human phenotype, including mental
retardation, due to a trisomy of chromosome 2l; more common in babies
born to older mothers. |
| downstream |
| 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. |
| 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. |
| duplicate gene |
| An identical locus mapping to a second site
in a genome. |
| duplication |
| More than one copy of a particular chromosomal
segment in a chromosome set. Duplications supply genetic material
capable of evolving new functions. |
| dyad |
| Two sister chromatids attached to the same
centromere. A pair of sister chromatids joined at the centromere,
as in mitosis and meiosis. |
