| P (peptidyl) site |
| The site on the ribosome occupied by the
peptidyl-tRNA just before peptide bond formation. |
| P element |
| A Drosophila transposable element that has
been used as a tool for insertion mutagenesis and for germline transformation.
|
| P1 |
| Parental generation. |
| pachynema (pachytene
stage) |
| The stage of prophase 1 of meiosis in which
chromatids are first distinctly visible. |
| palindrome |
| A sequence of letters, words, phrases, or
nucleotides that reads the same regardless of direction, for instance
what Adam said to Eve "Madam I'm Adam". The DNA sequence of type
II restriction endonuclease recognition sites (eg EcoRI 5'GAATTC3')
and the binding sites of many other DNA binding proteins (eg. catabolite
activator protein (CAP) binding site 5'GTGAGTTAGCTCAC3') are not
true palindromes but in double stranded form are similar and have
two-fold rotational symetry. |
| panmictic |
| Referring to unstructured (random-mating)
populations. |
| paracentric inversion
|
| An inversion not involving the centromere.
A chromosomal inversion that does not include the centromere. |
| paralogous genes |
| Two genes or clusters of genes at different
chromosomal locations in the same organism that have structural
similarities indicating that they derived from a common ancestral
gene. |
| paramecin |
| A toxin liberated by a killer Paramecium.
|
| parameters |
| Measurements of attributes of a population.
|
| parapatric speciation
|
| speciation in which the evolution of reproductive
isolating mechanisms occurs when a population enters a new niche
or habitat within the range of the parent species. |
| parental ditype (PD)
|
| A tetrad type containing two different genotypes,
both of which are parental. A spore arrangement in Ascomycetes that
contains only the two non-recombinant-type ascospores. |
| parthenogenesis |
| The production of offspring by a female
with no genetic contribution from a male. The development of an
individual from an unfertilized egg that did not arise by meiotic
chromosome reduction . |
| partial digest |
| A restriction digest that has not been allowed
to go to completion and thus contains pieces of DNA with some restriction
endonuclease sites that have not yet been cleaved. |
| particulate inheritance
|
| The model proposing that genetic information
is transmitted from one generation to the next in discrete units
(particles) so that the character of the offspring is not a smooth
blend of essences from the parents (See blending inheritance). |
| Pascal's triangle |
| A triangular array of numbers made up of
the coefficients of the binomial expansion. |
| path diagram |
| A modified pedigree showing only the direct
line of descent from common ancestors. |
| pathogen |
| An organism that causes disease in another
organism. |
| patroclinous inheritance
|
| Inheritance in which all offspring have
the nucleus-based phenotype of the father. |
| pattern formation |
| The developmental processes by which the
complex shape and structure of higher organisms occurs. |
| pedigree |
| A family tree drawn with standard genetic
symbols, showing inheritance patterns for specific phenotypic characters.
A representation of the ancestry of an individual or family; a family
tree. |
| penetrance |
| The proportion of individuals with a specific
genotype who manifest that genotype at the phenotype level. |
| peptide bond |
| A bond joining two amino acids. |
| peptidyl transferase
|
| The enzymatic centre in the ribosome responsible
for peptide bond formation during translation. |
| pericentric inversion
|
| An inversion that involves the centromere.
A chromosomal inversion that includes the centromere. |
| permissive condition
|
| Environmental condition under which a conditional
mutation shows the wild-type phenotype. |
| permissive temperature
|
| A temperature at which temperature-sensitive
mutants are normal. |
| PEST hypothesis |
| The short life-time of a protein is signaled
by a region rich in the amino acids proline (P); glutamic acid (E);
serine (S); or threonine (T). |
| petite |
| A yeast mutation producing small colonies
and altered mitochondrial functions. In cytoplasmic (neutral and
suppressive) petites the mutation is a deletion in mitochondrial
DNA, while in segregational petites the mutation occurs in nuclear
DNA. |
| petite mutation |
| A mutation of yeast that produces small
(petite) anaerobic-like colonies. |
| Phe |
| Phenylalanine (an amino acid). |
| phenocopy |
| A phenotype that is not genetically controlled
but looks like a genetically controlled phenotype. An environmentally
induced phenotype that resembles the phenotype produced by a mutation.
|
| phenotype |
| (1) The form taken by some character (or
group of characters) in a specific individual. |
| (2) The detectable outward manifestations
of a specific genotype. |
| (3) The observable attributes of an organism.
|
| phenotypic sex determination
|
| Sex determination by non-genetic means.
|
| phenotypic variance |
| Variance of phenotype due to genotypic and
environmental factors combined. |
| phenylketonuria(pku)
|
| A human metabolic disease caused by a mutation
in a gene coding for a phenylalanine processing enzyme (phenylalanine
hydroxylase), which leads to accumulation of phenylalanine and mental
retardation if not treated; inherited as an autosomal recessive
phenotype. |
| pheromone |
| A chemical signal, analogous to a hormone,
that passes information between individuals. |
| Philadelphia chromosome
|
| A translocation between the long arms of
chromosomes 9 and 22, often found in the white blood cells of patients
with chronic myeloid leukemia. |
| phosphodiester bond |
| A bond between a two sugar groups and a
phosphate group; such bonds form the sugar-phosphate-sugar backbone
of DNA and RNA. A diester bond (between phosphoric acid and two
sugar molecules) linking two nucleotides together to form the nucleotide
polymers DNA and RNA. |
| photoreactivation |
| The process whereby dimerized pyrimidines
(usually thymines) in DNA are restored by an enzyme (deoxyribodipyrimidine
photolyase) that requires light energy. |
| phyletic gradualism |
| The process of gradual evolutionary change
over time. |
| phylogenetic tree |
| A diagram showing evolutionary lineages
of organisms. |
| piebald |
| A mammalian phenotype in which patches of
skin are unpigmented because of lack of melanocytes; generally inherited
as an autosomal dominant. |
| pilus (plural pili) |
| A conjugation tube; a hollow hair-like appendage
of a donor Escherichia coli cell that acts as a bridge for transmission
of donor DNA to the recipient cell during conjugation. |
| plant breeding |
| The application of genetic analysis to development
of plant lines better suited for human purposes. |
| plaque |
| A clear area on a bacterial lawn, left by
lysis of the bacteria through progressive infections by a phage
and its descendants. Clear area on a bacterial lawn caused by cell
lysis due to viral attack. |
| plasmid |
| Autonomously replicating extrachromosomal
DNA molecule. An autonomous self-replicating genetic particle usually
of circular double-stranded DNA. |
| plastid |
| A chloroplast prior to the development of
chlorophyll. |
| plate |
| (1) A flat dish used to culture microbes. |
| (2) To spread cells over the surface of
solid medium in a plate. |
| pleiotropic mutation
|
| A mutation that has effects on several different
characters. |
| pleiotropy |
| The phenomenon whereby a single mutation
affects several apparently unrelated aspects of the phenotype. |
| point mutation |
| A mutation that can be mapped to one specific
site within a locus. A small mutation that consists of the replacement
(transition or transversion); addition; or deletion (frameshift)
of one or a few bases. |
| Poisson distribution
|
| A mathematical expression giving the probability
of observing various numbers of a particular event in a sample when
the mean probability of that event on any one trial is very small.
|
| poky mutation |
| A mutation of Neurospora that produces a
petite-like phenotype. |
| polar body |
| A small cell (which eventually disintegrates)
that is the by-product of meiosis in female animals. One functional
ovum and potentially three polar bodies result from meiosis of each
primary oocyte. |
| polar gene conversion
|
| A gradient of conversion frequency along
the length of a gene. |
| polar granules |
| Cytoplasmic granules localized at the posterior
end of a Drosophila oocyte and early embryo. These granules are
associated with determinants of the germ-line and posterior structures.
|
| polar mutation |
| A mutation that affects the transcription
or translation of part of the gene or operon downstream of the mutant
site. For example, nonsense mutations, frameshift mutations, and
insertion sequence(IS)-induced mutations. |
| polarity |
| Meaning directionality and referring either
to an effect seen in only one direction from a point of origin or
to the fact that linear entities (such as a single strand of DNA
or RNA or a protein) have ends (5' and 3'; N and C) that differ
from each other. |
| polarity gene |
| A mitochondrial gene with alleles that are
preferentially found in daughter mitochondria after a recombinational
event between mitochondria. |
| Pollen grain |
| The male gametophyte in higher plants. |
| poly(A) tail |
| A string of adenine nucleotides added to
the 3' end of eukaryotic mRNA after transcription. |
| poly-dA/poly-dT technique
|
| A method of inserting DNA into a vector
by adding poly-dA to the linearised vector and poly-dT to the DNA
of interest. Also feasible with poly-dG and poly-dC. |
| polyacrylamide |
| A material used to make gels for separation
of mixtures of macromolecules by electrophoresis. |
| polycistronic |
| Referring to prokaryotic messenger RNAs
that contain several cistrons within the same mRNA transcript. |
| polycistronic mRNA |
| An mRNA that codes for more than one protein.
|
| polydactyly |
| More than five fingers and/or toes. Inherited
as an autosomal dominant phenotype. |
| polymerase chain reaction
(PCR) |
| A powerful method for amplifying specific
DNA segments which exploits certain features of DNA replication.
For instance replication requires a primer and specificity is determined
by the sequence and size of the primer. The method amplifies specific
DNA segments by cycles of template denaturation; primer addition;
primer annealing and replication using thermostable DNA polymerase.
The degree of amplification achieved is set at a theoretical maximum
of 2^N, where N is the number of cycles, eg 20 cycles gives a theoretical
1048576 fold amplification. |
| polymerize |
| To form a complex compound (polymer) by
linking together many smaller elements (residues). |
| polymorphism |
| The occurrence in a population (or among
populations) of several phenotypic forms associated with alleles
of one gene or homologs of one chromosome. See genetic polymorphism.
|
| polynucleotide phosphorylase
|
| An enzyme that can polymerize nucleotide
diphosphates without the need for a primer. The function of this
enzyme in vivo is probably in its reverse role as an RNA exonuclease.
|
| polypeptide |
| A chain of linked amino acids; a protein.
|
| polyploid |
| A cell or an organism having three or more
chromosome sets. |
| polysaccharide |
| A biological polymer composed of sugar subunits
for example, starch or cellulose. |
| polysome |
| The configuration of several ribosomes simultaneously
translating the same mRNA. Shortened form of the term polyribosome.
|
| polytene chromosome |
| A giant chromosome produced by an endomitotic
process in which, following synapsis of the two homologues, multiple
rounds of replication produce chromatids that remain synapsed together
in a haploid number of chromosomes. Large chromosome consisting
of many chromatids formed by rounds of endomitosis following synapsis
of the two homologues. |
| population |
| A group of organisms of the same species
relatively isolated from other groups of the same species. See deme.
|
| position effect |
| Used to describe a situation in which the
phenotype expressed by a gene is altered by changes in the position
of the gene within the genome. For instance genes translocated to
regions of heterochromatin are often not expressed. |
| position-effect variegation
|
| Variegation caused by the inactivation of
a gene in some cells through its abnormal juxtaposition with heterochromatin.
|
| positive assortative
mating |
| A situation in which like phenotypes mate
more commonly than expected by chance. |
| positive control |
| Regulation mediated by a protein that is
required for the activation of a transcription unit. |
| positive interference
|
| When the occurrence of one crossover reduces
the probability that a second will occur in the same region . |
| postreplicative repair
|
| A DNA repair process initiated when DNA
polymerase bypasses a damaged area. enzymes in the rec system are
used. |
| post-transcriptional
modification |
| Changes in eukaryotic mRNA, tRNA or other
RNAs made after transcription has been completed. The changes to
mRNA include addition of a 5'cap and 3' polyA tail and removal of
introns, and to tRNA include modification of bases and removal of
introns. |
| preemptor stem |
| A configuration of leader transcript mRNA
that allows transcription in attenuator-controlled amino acid operons.
|
| pre-mRNA |
| The first (primary) transcript from a protein
coding gene is often called a pre-mRNA and contains both introns
and exons. Pre-mRNA requires splicing (removal) of introns to produce
the final mRNA molecule containing only exons. |
| Pribnow box |
| Relatively invariant sequence of six nucleotides
(concensus TATAAT) in prokaryotic promoters centred at the position
minus 10 (10 base pairs upstream from the transcription start site).
|
| primary oocyte |
| A cell that undergoes meiosis in female
animals. |
| primary spermatocyte
|
| A cell that undergoes the first meiotic
division in male animals. See secondary spermatocyte. |
| primary structure |
| The sequence of polymerized amino acids
in a protein. |
| primary transcript |
| The product of eukaryotic transcription
before post-transcriptional modifications take place. |
| primase |
| An enzyme that creates an RNA primer for
initiation of DNA replication. |
| primer |
| A short sequence (of RNA or DNA) from which
DNA replication can initiate. May be either a synthetic DNA or RNA
or a length of RNA synthesized in vivo by primase. |
| primosome |
| A complex of two proteins, a primase and
a helicase, that initiates RNA primers on the lagging DNA strand
during DNA replication. |
| prion |
| Protein infectious agent associated with
several neurological diseases (scrapie; kuru; Creutzfeld-Jakob syndrome;
Alzheimer's disease). Each disease has a different prion. |
| Pro |
| Proline (an amino acid). |
| probability |
| The expectation of the occurrence of a particular
event. |
| probability theory |
| The conceptual framework concerned with
quantification of probabilities. See probability. |
| probe |
| Defined nucleic acid (DNA or RNA) that can
be used to identify, usually through autoradiography, specific DNA
or RNA molecules bearing the complementary sequence. A labeled (radioactive;
antigen; enzyme; fluorescent) nucleic acid complementary to the
sequence being searched for in a restriction digest, genome library,
northern blot or in situ hybridization. |
| processivity |
| The ability of an enzyme to repetitively
continue its catalytic function without dissociating from its substrate.
|
| product of meiosis |
| One of the (usually four) cells formed by
the two meiotic divisions. |
| product rule |
| The probability of two independent events
occurring simultaneously is the product of the individual probabilities.
The rule stating that the probability of the occurrence of independent
events is the product of their separate probabilities. |
| proflavin |
| A mutagen that tends to produce frameshift
mutations. |
| progeny testing |
| Breeding of offspring to determine their
genotypes and that of their parents. |
| prokaryote |
| An organism lacking a true nucleus, such
as a bacterium or a blue-green alga. |
| prokaryotic cell |
| A cell having no nuclear membrane and hence
no separate nucleus. |
| promoter |
| A regulatory region a short distance upstream
from the 5' end of a transcription start site that acts as the binding
site for RNA polymerase. A region of DNA to which RNA polymerase
binds in order to initiate transcription. |
| proofread |
| Literally to read for the purpose of detecting
errors for later correction. DNA polymerase has 3' to 5' exonuclease
activity which is used during polymerization to remove nucleotides
it has recently added if they are incorrectly paired, and is a correcting
ability to remove errors in replication. |
| prophage |
| A phage chromosome inserted as part of the
linear structure of the DNA chromosome of a bacterium. A temperate
phage integrated into the host chromosome. |
| prophase |
| The early stage of nuclear division during
which chromosomes condense and become visible. The initial stage
of mitosis or meiosis in which chromosomes become visible and the
spindle apparatus forms. |
| proplastid |
| Mutant plastids that do not grow and develop
into chloroplasts. |
| propositus |
| The person through whom a pedigree was discovered.
|
| proto-oncogene |
| The non-activated form of a cellular oncogene
in an untransformed cell. A gene that, when mutated or otherwise
affected, becomes an oncogene. |
| protoplast |
| A plant cell whose wall has been removed.
|
| prototroph |
| A strain of organisms that will proliferate
on minimal medium (compare auxotroph). |
| provirus |
| A virus chromosome integrated into the DNA
of the host cell. |
| pseudoallele |
| allele that is functionally but not structurally
allelic, that is wild-type recombinants can be recovered by intragenic
recombination from heterozygotes containing two different pseudoalleles.
|
| pseudoautosomal gene
|
| A gene that occurs on both sex-determining
heteromorphic chromosomes. |
| pseudodominance |
| The sudden appearance of a recessive phenotype
in a pedigree, due to deletion of a masking dominant gene. The phenomenon
in which a recessive allele shows itself in the phenotype when only
one copy of the allele is present, as in hemizygous alleles or in
deletion heterozygotes . |
| pseudogene |
| An inactive gene derived from an ancestral
active gene. |
| pulse-chase experiment
|
| An experiment in which cells are grown in
radioactive medium for a brief period (the pulse) and then transferred
to nonradioactive medium for a longer period (the chase). |
| pulsed-field gel electrophoresis
|
| An electrophoretic technique in which the
gel is subjected to electrical fields alternating between different
angles, allowing very large DNA fragments to snake through the gel,
and hence permitting efficient separation of mixtures of such large
fragments. |
| punctuated equilibrium
|
| The evolutionary process involving long
periods without change (stasis) punctuated by short periods of rapid
speciation. |
| Punnett square |
| A diagrammatic representation of a particular
cross used to predict the progeny of the cross. A grid used as a
graphic representation of the progeny zygotes resulting from different
gamete fusions in a specific cross. |
| pure-breeding line or
strain |
| A group of identical individuals that always
produce offspring of the same phenotype when intercrossed. |
| purines |
| A type of nitrogen base; the purine bases
in DNA and RNA are adenine and guanine. |
| pyrimidine |
| Nitrogenous bases of which thymine is found
in DNA; uridine in RNA; and cytosine in both. |
