Dr. Paul's Virtually Biology Show.

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Nastic. Undirected(usually) and reversible responses of plants to being touched. Nastic responses are undirected in the sense that the response does not bring about an orientation toward the touch. Examples of nastic response include leaf closings of mimosa plants when touched and the rapid closing of a Venus fly trap.

Natural selection. A mechanism of evolution in which different phenotypes have different survival and reproductive rates due to differing responses of the phenotypes to the environment. If phenotypic differences are at least partially genetically determined then natural selection can lead to evolution.

a. Observation: populations typically have the potential to grow faster than resources available to them.

b. Inference: Individuals of the population will end up competing for resources.

c. Observation: Individuals in a population vary in terms of phenotype.

d. Observation: The phenotype is typically at least partially heritable.

e.  Observation: Individuals vary, based on their phenotype in their ability to survive and leave offspring.

f. Inference: gene frequencies will change over time in response to the environment

g. Inference: Therefore the phenotypes found in the population will change over time.

Nephridium. A simple excretory system found in many invertebrates which often relies on diffusion combined with current flow set up be a series of ciliated cells called flame cells.
Nephron. The basic unit of the vertebrate kidney. The nephron in mammals consists of Bowman's capsule, the distal convoluted tubule, the Loop of Henle and the proximal convoluted tubule.
Neutral mutations. Mutations are called neutral if they have no obvious effect on an organism's survival and reproductive success. They are important because neutral mutations may accumulate in a population through time and provide a source of variation for natural selection to act on should the environment change.
Neutron. A neutron is a negatively charged particle in the nucleus of an atom. Two atoms of the same element can have different numbers of neutrons and hence are called isotopes of each other.
Neurula. In chordates, an embryonic stage after gastrulation that is characterized by a dorsal infolding of the ectoderm. This infolding develops into the nervous system and the hollow dorsal nerve cord characteristic of chordates.
Nicatinamide adenine dinucleotide(NAD+). An important electron and hydrogen ion acceptor in cellular respiration. This compound accepts electrons and transports them to electron transport systems involved in the production of ATP.
Nicatinamide adenine dinucleotide phosphate(NADP+). An important electron and hydrogen ion acceptor involved in transporting electrons used in biosynthetic pathways, for instance in the Calvin Benson cycle of photosynthesis.
Node. For plants, the area of the stem where the leaves arise.
Nondisjunction. The failure of homologous chromosomes to separate properly during meiosis resulting in a gamete with an excess or lack of one or more chromosomes. Disorders caused by nondisjunction include Down's syndrome in humans.

Nucleic acid. Any one of several polymers of nucleotides. Deoxyribonucleic acid(RNA) is the nucleic acid found in the chromosomes and that we know makes up the genetic material of organisms and many viruses. The DNA nitrogen bearing bases are adenine, guanine, cytosine and thymine and the sugar is deoxyribose. The various ribonucleic acids(RNA) are nucleic acids that serve a number of functions related to protein synthesis and for some viruses serve as the genetic material. Unlike DNA, the RNA's contain uracil instead of thymine and use the sugar ribose.
Nucleotide. In biochemistry, a molecule consisting of a phosphate group and a one or two ring nitrogen containing base, both of which are attached to a five carbon sugar. The five carbon sugar may be either ribose or deoxyribose depending on the nucleotide.
Nucleolus. A dark circular region inside the nucleus during interphase. This region consists of areas of duplicated DNA for rapid RNA transcription related to the production of ribosomal RNA.
Nucleosome. The basing level of coiling in a eukaryote chromosome, consisting of a large histone protein wrapped by several coils of DNA with the coils head in place by a smaller assembly of histone proteins.
Nucleus. An organelle in all eukaryotic cells that contains the bulk of the cell's DNA. The nucleus is surrounded by a double membrane envelope perforated with protein lined pores for the entrance and egress of large molecules.