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ice age Interval of geologic time between 2 million and 10,000 years ago during which the northern hemisphere experienced several episodes of continental glacial advance and retreat along with a climatic cooling. The icing over of Antarctica was also completed during this time.
ileum The third and last section of the small intestine.
immovable joint A joint in which the bones interlock and are held together by Þbers or bony processes that prevent the joint from moving; e.g., the bones of the cranium.
immune system One of the eleven major body organ systems in vertebrates; defends the internal environment against invading microorganisms and viruses and provides defense against the growth of cancer cells.
immunoglobulins The Þve classes of protein to which antibodies belong (IgD, IgM, IgG, IgA, IgE).
implantation The process in which the blastocyst embeds in the endometrium.
incomplete dominance has a A type of inheritance in which the heterozygotephenotype intermediate to those of the homozygous parents.
incomplete flower Condition in which one or more "typical" flower parts are absent. Example: grass flowers such as corn tassels which are male.
incus One of the three bones comprising the middle ear of mammals.
inflammation A reaction to the invasion of microorganisms through the skin or through the epithelial layers of the respiratory, digestive, or urinary system; characterized by four signs: redness, swelling, heat, and pain.
inflammatory response The body's reaction to invading infectious microorganisms; includes an increase in blood þow to the affected area, the release of chemicals that draw white blood cells, an increased þow of plasma, and the arrival of monocytes to clean up the debris.
ingestive feeders Animals that ingest food through a mouth.
inheritance of acquired characteristics Lamarck's view that features acquired during an organism's lifetime would be passed on to succeeding generations, leading to inheritable change in species over time.
initiation molecule, a The Þrst step in translation; occurs when a messenger RNAribosomal subunit, and a transfer RNA molecule carrying the Þrst amino acid bind together to form a complex; begins at the start codon on mRNA.
initiation codon (AUG) Three-base sequence on the messenger RNA that codes for the amino acid methionine; the start command for protein synthesis.
insertion A type of mutation in which a new DNA base is inserted into an existing sequence of DNA bases. This shifts the reference frame in protein synthesis, resulting (sometimes) in altered amino acid sequences.
insulin A hormone secreted by the pancreas that stimulates the uptake of glucose by body cells. Insulin works antagonistically with glucagon to control blood sugar levels.
integration The process of combining incoming information; one of the functions of the nervous system.
integument Something that covers or encloses, e.g., the skin.
integumentary system The skin and its derivatives (hair, nails, feathers, horns, antlers, and glands), which in multicellular animals protect against invading foreign microorganisms and prevent the loss or exchange of internal þuids.
interferons Proteins released by cells in response to viral infection; activate the synthesis and secretion of antiviral proteins.
internal environment In multicellular organisms, the aqueous environment that is outside the cells but inside the body.
interneurons Neurons that process signals from one or more sensory neurons and relay signals to motor neurons. Aka connector neurons.
internodes The stem regions between nodes in plants.
interphase The period between cell divisions when growth and replacement occur in preparation for the next division; consists of gap 1 (G1), synthesis (S), and gap 2 (G2).
interstitial Being situated within a particular organ or tissue.
interstitial fluid Fluid surrounding the cells in body tissues; provides a path through which nutrients, gases, and wastes can travel between the capillaries and the cells.
intracellular digestion A form of digestion in which food is taken into cells by phagocytosis; found in sponges and most protozoa and coelenterates.
intracellular parasites Viruses that enter a host cell and take over the host's cellular machinery to produce new viral particles.
intracellular route Path taken by water through the cells of the root between the epidermis and the xylem, moving through plasmodesmata.
intron In eukaryotes, bases of a gene transcribed but later excised from the mRNA prior to exporting from the nucleus and subsequent translation of the message into a polypeptide.
inversion A reversal in the order of genes on a chromosome segment.
ion An atom that has lost or gained electrons from its outer shell and therefore has a positive or negative charge, respectively; symbolized by a superscript plus or minus sign and sometimes a number, e.g., H+, Na+1, Cl-2.
ionic bond A chemical bond in which atoms of opposite charge are held together by electrostatic attraction.
isotonic Term applied to two solutions with equal solute concentrations.
isotopes Atoms with the same atomic number but different numbers of neutrons; indicated by adding the mass number to the element's name, e.g., carbon 12 or 12C.