| N segment |
| Sequence of nucleotides added in a template-independant
fashion at the joining junctions of heavy-chain immunoglobulin genes.
|
| N-end rule |
| The life span of a protein is determined
by its amino-terminal (N-terminal) amino acid. |
| natural selection |
| The process in nature whereby one genotype
leaves more offspring than another genotype because of superior
life history attributes (fitness)such as survival or fecundity.
|
| nearest-neighbour analysis
|
| A technique of transferring radioactive
atoms between adjacent nucleotides in DNA used to demonstrate that
the two strands of DNA run in opposite directions. |
| negative assortative
mating |
| Preferential mating between phenotypically
different partners. |
| negative control |
| Transcription regulation mediated by factors
that block, turn down or turn off transcription. |
| negative interference
|
| The phenomenon whereby a crossover in a
particular region enhances the occurrence of other apparent crossovers
in the same region of the chromosome. |
| neo-darwinism |
| The merger of classical Darwinian evolution
with population genetics. |
| neomorph |
| Mutant showing a novel substance or structure
not found in wild-types. |
| neoplasm |
| New growth of abnormal tissue. |
| neurofibromatosis |
| A human disease with tumours of nerve cells
all over the body and pigmented spots in the skin. The allele generally
arises from germinal mutation, but it is inherited as an autosomal
dominant. |
| Neurospora |
| A pink mold, commonly found growing on old
food. |
| neutral gene hypothesis
|
| The hypothesis that most genetic variation
in natural populations is not maintained by selection because most
alleles have equal fitness. |
| neutral mutation |
| (1) A mutation that has no effect on the
Darwinian fitness of its carriers. |
| (2) A mutation that has no phenotypic effect.
|
| neutral petite |
| A petite yeast that produces all wild-type
progeny when crossed with wild-type. |
| nicking |
| Nuclease action to sever the sugar-phosphate
backbone in one DNA strand but not the other at one specific site.
|
| nitrocellulose filter
|
| A type of filter used to bind DNA for hybridization.
|
| nitrogen base |
| Type of molecule that forms an important
part of nucleic acid, composed of a nitrogen-containing ring structure.
Hydrogen bonds between bases in opposing complementary strands link
the two strands of a DNA double helix. |
| non-Mendelian ratio |
| An unusual ratio of progeny phenotypes that
does not reflect the simple operation of Mendel's laws; for example,
mutant:wild-type ratios of 3:5, 5:3, 6:2, or 2:6 in tetrads indicate
that gene conversion has occurred. |
| non-disjunction |
| The failure of a pair of homologous chromosomes
to separate properly during meiosis. The failure of homologues (at
meiosis) or sister chromatids (at mitosis) to separate properly
to opposite poles, that is two chromosomes or chromatids go to one
pole and none to the other. |
| non-histone protein |
| The protein remaining in chromatin after
the histones are removed. The scaffold structure is made of nonhistone
proteins. |
| non-linear tetrad |
| A tetrad in which the meiotic products are
in no particular order. |
| non-parental ditype (NPD)
|
| A spore arrangement in Ascomycetes that
contains only the two recombinant-type ascospores (assuming two
segregating loci). A tetrad type containing two different genotypes,
both of which are recombinant. |
| non-recombinant |
| In mapping studies the offspring that have
alleles arranged as in the original parents are non-recombinants.
|
| nonsense codon |
| One of the mRNA sequences (UAA, UAG, UGA)
that signals the termination of translation. A codon for which no
normal tRNA molecule exists. The presence of a nonsense codon causes
termination of translation (ending polypeptide chain synthesis).
There are three nonsense codons are called amber(UAG) ochre(UAA)
and opal(UGA). |
| nonsense mutation |
| A mutation that alters a gene so as to produce
a . |
| nonsense suppressor |
| A mutation in the anticodon of tRNA that
alters the anticodon so it is now complementary to a nonsense codon
allowing the tRNA to insert its cognate amino acid at this nonsense
codon during translation. |
| norm of reaction |
| The pattern of phenotypes produced by a
given genotype under different environmental conditions. |
| normal distribution |
| Any of a family of bell-shaped frequency
curves whose relative position and shape are defined on the basis
of the mean and standard deviation. |
| Northern blot |
| Transfer of electrophoretically separated
RNA molecules from a gel onto an absorbent sheet, which is then
immersed in a labeled probe that will hybridize to an RNA of interest
to reveal its presence. See Southern blotting. |
| nuclear transplantation
|
| The technique of placing a nucleus from
another source into an enucleated cell. |
| nuclease |
| One of the several classes of enzymes that
degrade nucleic acid. An enzyme that can degrade DNA or RNA by breaking
phosphodiester bonds. See endonucleases; exonucleases. |
| nuclease hypersensitive
site |
| Region of eukaryotic chromosome DNA that
is specifically vulnerable to nuclease attack perhaps because it
is not wrapped in histone as nucleosomes. |
| nucleoid |
| A DNA mass within a chloroplast or mitochondrion.
|
| nucleolar organizer |
| The chromosomal region around which the
nucleolus forms, a site of tandem repeats of the rRNA gene. A region
(or regions) of the chromosome set physically associated with the
nucleolus and containing rRNA genes. |
| nucleolus |
| The globular nuclear organelle formed at
the nucleolar organizer. Site of ribosome construction. An organelle
found in the nucleus, containing rRNA and multiple copies of the
genes coding for rRNA. |
| nucleoprotein |
| The substance of eukaryotic chromosomes
consisting of proteins and nucleic acids. |
| nucleoside |
| A sugar-base compound that is a nucleotide
precursor. Nucleotides are nucleoside phosphates. A nitrogen base
linked to a sugar molecule. |
| nucleosome |
| A nu body; the basic unit of eukaryotic
chromosome structure; a ball of eight histone molecules wrapped
about by two coils of about 220 base pairs of DNA. Arrangement of
DNA and histones forming regular spherical structures in eukaryotic
chromatin. |
| nucleotide |
| Subunit that polymerizes into nucleic acids
(DNA or RNA). Each nucleotide consists of a nitrogenous base; a
sugar; and one to three phosphate groups. |
| nucleotide pair |
| A pair of complementary nucleotides (one
in each strand of DNA) that are joined by hydrogen bonds. |
| nucleotide-pair substitution
|
| The replacement of a specific nucleotide
pair by a different pair; often mutagenic. |
| nucleus |
| The membrane bound organelle containing
the genome of eukaryotes organized into chromosomes. |
| null allele |
| An allele whose effect is either an absence
of normal gene product at the molecular level or an absence of normal
function at the phenotypic level. |
| null hypothesis |
| The statistical hypothesis that states that
there are no differences between observed and expected data. |
| nullisomic |
| A diploid cell or individual missing both
copies of the same chromosome. A cell or individual with one chromosomal
type missing, with a chromosome number such as n-1 or 2n-2. |
| numerator element |
| Genes on the X chromosome in Drosophila
that regulate the sex switch (sxl) to the on condition (femaleness).
Refers to the numerator of the X/A (number of X chromosomes/autosomes)
genic balance equation. |
| nurse cell |
| A sister cell of an oocyte in insects. Nurse
cells produce the bulk of the cytoplasmic contents of the mature
oocyte. |
