noncyclic electron flow Thefirst stage of photo-
synthesis; begins when light energy enters a cluster of
pigment molecules (called the photosystem) located in
the thylakoid. The light-induced flow of electrons from
water to NADP in oxygen-evolving photosynthesis
involving both photosystems I and II. Photosystems
are large complexes of proteins and chlorophyll that
capture energy from sunlight. Both systems I and II
include special forms of chlorophyll A. Photosystem I,
or P-700, includes chlorophyll A pigment with a spe-
cific absorbance of 700 nm (red light). Photosystem II,
or P-680, contains the reaction center responsible for
oxygen evolution and contains a special chlorophyll A
that absorbs light at 680 nm (red light). If the photo-
chemical reactions in photosystem II are inhibited,
photosystem I is inhibited as well.
noncyclic photophosphorylation The formation of
ATPby NONCYCLIC ELECTRON FLOW.
nondisjunction The failure of paired chromosomes
to separate normally and migrate to opposite poles
after cell division, thereby giving rise to cells that have
too many or too few chromosomes. This results in
monosomy, whereone member of a chromosome pair
is missing, or trisomy, wherethere is an extra chromo-
some in any of the chromosome pairs.
nonpolar covalent bond Acovalent bond formed
by the equal sharing of electrons between two atoms
with the same electronegativity. Electronegativity is the
tendency of an atom to attract electrons to itself in a
covalent bond.
nonsense mutation A mutation where one of the
mRNA sequences (UAA, UAG, UGA) signals the termi-
nation of translation, the process whereby the genetic
code carried by mRNA directs the synthesis of proteins
from amino acids; a codon is changed to a stop codon,
prematurely stopping polypeptide chain synthesis.
These three nonsense codons are amber (UAG), ocher
(UAA), and opal (UGA).
See alsoMESSENGER RNA.
norm of reaction An expression or variation in
pattern among phenotypes produced by a single
genotype caused by a variation in environmental con-
ditions. While phenotypes are an expression of genes
inan organism that give it certain traits and charac-
ters (anatomical structure, physiology, and behavior)
that can be measured, sometimes the traits are actual-
ly norms of reaction, i.e., traits expressed in different
ways in different environments. For example, human
body weight or height is different in identical envi-
ronments or on different diets. Take a set of genes
that produce a particular trait together, list all the
possible environmental conditions that they can sur-
vive in, and the norm of reaction is the total variation
in that trait in all of the survivable environmental
conditions.notochord Arodlike cord, a rudimentary skeleton,
composed of stiff cartilage in chordates that runs
lengthwise under the dorsal (top) surface of the body.
It forms the support structure of the body, supporting
the nerve cord. It is replaced by the vertebral column
later in the development of most chordates; it persists
in primitive fishes such as hagfish, coelacanth, and chi-
maeras. The phylum Chordata comprises animals
characterized by bodies that have elongated bilateral
symmetry and that, in some part of their development,
have a notochord and gill slits (or pouches), and usual-
ly have a head, tail, and digestive system with open-
ings at both ends. Chordates include the fish,
amphibians, reptiles, birds, and mammals, but not all
chordates are vertebrates, e.g., invertebrate tunicates
and lancelets.N-terminal amino acid residue SeeAMINO ACID
RESIDUE.nuclear envelope(nuclear membrane) A two-
membrane structure that surrounds the nucleus. The
space between the two membranes is referred to as
the perinuclear space. The outermost membrane meets
the rough endoplasmic reticulum and has ribosomes
attached. The inner membrane lies next to a dense
filamentous network called the nuclear lamina and
surrounds the nucleus, except where there are nuclearnuclear envelope 247