328 ❯ Glossary
morula A structure formed during the cleavage divi-
sions of the zygote.
motor neurons Nerve cells that take the commands
from the central nervous system (CNS) and put
them into action as motor outputs.
M phase mitosis This is the stage during which the
cell separates into two new cells.
Müllerian mimicry Two species that are aposemati-
cally colored as an indicator of their chemical
defense mechanism; they mimic each other’s color
scheme in an effort to increase the speed with
which predators learn to avoid them.
mutant phenotypesCharacters that are not the
wild-type strain in fruit flies and other organisms.
mutationA random event that can cause changes in
allele frequencies. It is alwaysrandom with respect
to which genes are affected, although the changes
in allele frequencies that occur as a result of the
mutation may not be.
mutualism Scenario in which two organisms benefit
from an interaction or relationship.
myceliumMeshes of branching filaments formed
from hyphae that function as mouthlike structures
for fungi.
myelinated neurons Neurons with a layer of insula-
tion around the axon, allowing for faster transmis-
sion. They form the cable Internet of the body.
natural selection The process by which characters or
traits are maintained or eliminated in a population
based on their contribution to the differential sur-
vival and reproductive success of their “host”
organisms.
negative feedback Occurs when a hormone acts to
directly or indirectly inhibit further secretion of
the hormone of interest.
nephron The functional unit of the kidney.
net productivityDifference between the concentra-
tion of dissolved oxygen for the initial and light
bottle in a primary productivity experiment.
neural plate Structure that becomes the neural
groove, which eventually becomes the neural tube.
This neural tube later gives rise to the central ner-
vous system.
neural tube Embryonic structure that gives rise to
the central nervous system.
neuromuscular junction The space between the
motor neuron and the muscle cell.
neurotransmitter Chemical released by neurons that
functions as a messenger, causing a nearby cell to
react and continue the nervous impulse.
niche Term used to describe all the biotic and abiotic
resources used by the organism.
nitrogen cycle The shuttling of nitrogen from the
atmosphere, to living organisms, and back to the
atmosphere in a continuous cycle.
nitrogen fixation The conversion of N 2 to NH 3
(ammonia).
nitrogenous basesMonomers such as adenine,
guanine, cytosine, thymine, and uracil out of
which DNA and RNA are constructed.
noncompetitive inhibition Condition in which an
inhibitor molecule binds to an enzyme away from
the active site, causing a change in the shape of the
active site so that it can no longer interact with the
substrate.
noncyclic light reactions Pathway that produces
ATP, NADPH, and O 2. Uses both photosystem I
and II.
nondisjunction The improper separation of chromo-
somes during meiosis, which leads to an abnormal
number of chromosomes in offspring. Examples
include Down syndrome, Turner syndrome, and
Klinefelter’s syndrome.
nonsense mutation Substitution of the wrong
nucleotides into the DNA sequence. These substi-
tutions lead to premature stoppage of protein syn-
thesis by the early placement of a stop codon. This
type of mutation usually leads to a nonfunctional
protein.
nonspecific immunity The nonspecific prevention of
the entrance of invaders into the body.
notochordStructure that serves to support the body.
Found in the embryos of chordates.
nucleic acidMacromolecule composed of nucleotides,
sugars, and phosphates that serves as genetic mater-
ial of living organisms (DNA and RNA).
nucleoid Region of a prokaryotic cell that contains
the genetic material.
nucleolus Eukaryotic structure in which ribosomes
are constructed.
nucleus The control center of eukaryotic cells that is
the storage site of the genetic material (DNA). It is
the site of replication, transcription, and posttran-
scriptional modification of RNA.
obligate aerobeOrganism that requires oxygen for
respiration.