chromosome, or they may be distributed among a
number of different chromosomes. (7.4)mutagen substance or event that increases the rate
of mutation in an organism. Mutagens can be
physical or chemical. (9.1)
mutation permanent change in the genetic material
of an organism. Mutations provide the genetic
variation within a population. (9.1, 11.1)
mutualism type of symbiotic relationship in which
both species benefit from the relationship and
growth in one population. (14.3)
myelin sheath the fatty layer around the axon of a
nerve cell, composed of Schwann cells. (5.2)
myopia near-sightedness or difficulty seeing things
that are far away. It is caused by a too-long
eyeball or too-strong ciliary muscles. (5.3)
NADH dehydrogenase complex a multienzyme
complex that oxidizes the coenzymes NADH
and FADH 2. (3.2)
natural killer cells white blood cells that devour
any of the body’s own cells that have become
cancerous or infected. (4.4)
natural selection process whereby the
characteristics of a population of organisms
change because individuals with certain heritable
traits survive specific local environmental
conditions. (10.1)
negative feedback loop homeostatic mechanism
that detects and reverses deviations from normal
homeostasis levels. The mechanism consists of a
receptor, an integrator, and an effector. (4.1)
negative gene regulation the process of shutting off
transcription by the use of a regulatory site. A
protein molecule interacts directly with the
genome to turn off gene expression. (8.4)
nephric filtrate urine. About twenty percent of the
blood plasma that enters the kidney becomes
nephric filtrate. (4.2)
nephron there are about 1 million of these filters in
each kidney, each filter consisting of a Bowman’s
capsule, the glomerulus, a proximal tubule, the
loop of Henle, the distal tubule, and a collecting
duct. (4.2)
nerve message pathway of the nervous system,
made up of many neurons connected together.
(5.1)
neuron the nerve cell that is the structural and
functional unit of the nervous system, consisting
of a nucleus, a cell body, dendrites, axons, and a
myelin sheath. (5.1)
neurotransmitters chemicals that are secreted by
neurons to stimulate motor neurons and central
nervous system neurons. (5.2)
neutral atoms atoms that have the same number of
protons and electrons, and that therefore have no
overall charge. (1.2)
neutralization chemical change in which one
compound in solution (a base) acquires a
hydrogen cation from another compound
(an acid). (1.3)
neutrons uncharged subatomic particles, found in
the nuclei of most atoms. The particles have
about the same mass as protons. (1.1)
neutrophils small white blood cells that engulf
bacteria through phagocytosis as an immediate
immune response. (4.4)
nitrogenous containing nitrogen. (7.1)
node of Ranvier the gap between Schwann cells
around the axon of a nerve cell. The membrane
of the axon is exposed and nerve impulses jump
from one node of Ranvier to the next. (5.2)
non-competitive inhibition binding of an inhibitor
molecule an enzyme at the allosteric site. (2.2)
non-cyclic photophosphorylation addition of a
phosphate group to ADP to form ATP. This
process uses energy from an electron transport
chain in which the electrons are not recycled in
the photosystem. (3.3)
non-overlapping generations populations in which
parents and offspring are never present in the
population at the same time. (14.2)
non-polar covalent bond chemical bond in which
electrons are shared equally between two bonded
atoms with the same electronegativity. (1.1)
non-random mating any situation in which
individuals do not choose mates on a random
basis, such as mating based on proximity,
relatedness, or similarity of phenotype. (11.3)
non-renewable resource resource, such as
petroleum, that cannot be replenished once it is
used up. (15.3)
nonsense mutation permanent change in the genetic
material of a cell that renders a gene unable to
code for any functional polypeptide product.
(9.1)
non-specific defences the ability to resist a disease,
even if there has been no prior exposure to it.
(4.4)
non-steroid hormones fat-insoluble hormones
composed proteins, peptides, or amino acids.
(6.1)
noradrenaline seenorepinephrine. (5.2)
norepinephrine the primary neurotransmitter of the
sympathetic nervous system. Also known as
noradrenaline. (5.2)
nucleic acid a polymer of nucleotides, such as DNA
and RNA. (1.3)N
Glossary • MHR 581